# Examples

## Introduction

This chapter consists entirely of examples of AspectJ use.

The examples can be grouped into four categories:

- technique
- Examples which illustrate how to use one or more features of the language.
- development
- Examples of using AspectJ during the development phase of a project.
- production
- Examples of using AspectJ to provide functionality in an application.
- reusable
- Examples of reuse of aspects and pointcuts.

## Obtaining, Compiling and Running the Examples

The examples source code is part of the AspectJ distribution which may be downloaded from the AspectJ project page ( ).

Compiling most examples is straightforward. Go the `InstallDir/examples` directory, and look for a `.lst` file in one of the example subdirectories. Use the `-arglist` option to `ajc` to compile the example. For instance, to compile the telecom example with billing, type

    ajc -argfile telecom/billing.lst

To run the examples, your classpath must include the AspectJ run-time Java archive (`aspectjrt.jar`). You may either set the `CLASSPATH` environment variable or use the `-classpath` command line option to the Java interpreter:

    (In Unix use a : in the CLASSPATH)
    java -classpath ".:InstallDir/lib/aspectjrt.jar" telecom.billingSimulation

    (In Windows use a ; in the CLASSPATH)
    java -classpath ".;InstallDir/lib/aspectjrt.jar" telecom.billingSimulation

## Basic Techniques

This section presents two basic techniques of using AspectJ, one each from the two fundamental ways of capturing crosscutting concerns: with dynamic join points and advice, and with static introduction. Advice changes an application's behavior. Introduction changes both an application's behavior and its structure.

The first example, , is about gathering and using information about the join point that has triggered some advice. The second example, , concerns a crosscutting view of an existing class hierarchy.

### Join Points and `thisJoinPoint`

(The code for this example is in `InstallDir/examples/tjp`.)

A join point is some point in the execution of a program together with a view into the execution context when that point occurs. Join points are picked out by pointcuts. When a program reaches a join point, advice on that join point may run in addition to (or instead of) the join point itself.

When using a pointcut that picks out join points of a single kind by name, typicaly the the advice will know exactly what kind of join point it is associated with. The pointcut may even publish context about the join point. Here, for example, since the only join points picked out by the pointcut are calls of a certain method, we can get the target value and one of the argument values of the method calls directly.

    before(Point p, int x): target(p)
                         && args(x)
                         && call(void setX(int)) {
        if (!p.assertX(x)) {
            System.out.println("Illegal value for x"); return;
        }
    }

But sometimes the shape of the join point is not so clear. For instance, suppose a complex application is being debugged, and we want to trace when any method of some class is executed. The pointcut

    pointcut execsInProblemClass(): within(ProblemClass)
                                 && execution(* *(..));

will pick out each execution join point of every method defined within `ProblemClass`. Since advice executes at each join point picked out by the pointcut, we can reasonably ask which join point was reached.

Information about the join point that was matched is available to advice through the special variable `thisJoinPoint`, of type `org.aspectj.lang.JoinPoint`. Through this object we can access information such as

- the kind of join point that was matched
- the source location of the code associated with the join point
- normal, short and long string representations of the current join point
- the actual argument values of the join point
- the signature of the member associated with the join point
- the currently executing object
- the target object
- an object encapsulating the static information about the join point. This is also available through the special variable `thisJoinPointStaticPart`.

#### The `Demo` class

The class `tjp.Demo` in `tjp/Demo.java` defines two methods `foo` and `bar` with different parameter lists and return types. Both are called, with suitable arguments, by `Demo`'s `go` method which was invoked from within its `main` method.

    public class Demo {
        static Demo d;

        public static void main(String[] args){
            new Demo().go();
        }

        void go(){
            d = new Demo();
            d.foo(1,d);
            System.out.println(d.bar(new Integer(3)));
        }

        void foo(int i, Object o){
            System.out.println("Demo.foo(" + i + ", " + o + ")\n");
        }

        String bar (Integer j){
            System.out.println("Demo.bar(" + j + ")\n");
            return "Demo.bar(" + j  + ")";
        }
    }

#### The `GetInfo` aspect

This aspect uses around advice to intercept the execution of methods `foo` and `bar` in `Demo`, and prints out information garnered from `thisJoinPoint` to the console.

    aspect GetInfo {

       static final void println(String s){ System.out.println(s); }

       pointcut goCut(): cflow(this(Demo) && execution(void go()));

       pointcut demoExecs(): within(Demo) && execution(* *(..));

       Object around(): demoExecs() && !execution(* go()) && goCut() {
          println("Intercepted message: " +
              thisJoinPointStaticPart.getSignature().getName());
          println("in class: " +
              thisJoinPointStaticPart.getSignature().getDeclaringType().getName());
          printParameters(thisJoinPoint);
          println("Running original method: \n" );
          Object result = proceed();
          println("  result: " + result );
          return result;
       }

       static private void printParameters(JoinPoint jp) {
          println("Arguments: " );
          Object[] args = jp.getArgs();
          String[] names = ((CodeSignature)jp.getSignature()).getParameterNames();
          Class[] types = ((CodeSignature)jp.getSignature()).getParameterTypes();
          for (int i = 0; i < args.length; i++) {
             println("  "  + i + ". " + names[i] +
                 " : " +            types[i].getName() +
                 " = " +            args[i]);
          }
       }
    }

##### Defining the scope of a pointcut

The pointcut `goCut` is defined as

    cflow(this(Demo)) && execution(void go())

so that only executions made in the control flow of `Demo.go` are intercepted. The control flow from the method `go` includes the execution of `go` itself, so the definition of the around advice includes `!execution(* go())` to exclude it from the set of executions advised.

##### Printing the class and method name

The name of the method and that method's defining class are available as parts of the org.aspectj.lang.Signature object returned by calling `getSignature()` on either `thisJoinPoint` or `thisJoinPointStaticPart`.

##### Printing the parameters

The static portions of the parameter details, the name and types of the parameters, can be accessed through the `org.aspectj.lang.reflect.CodeSignature` associated with the join point. All execution join points have code signatures, so the cast to `CodeSignature` cannot fail.

The dynamic portions of the parameter details, the actual values of the parameters, are accessed directly from the execution join point object.

### Roles and Views

(The code for this example is in `InstallDir/examples/introduction`.)

Like advice, inter-type declarations are members of an aspect. They declare members that act as if they were defined on another class. Unlike advice, inter-type declarations affect not only the behavior of the application, but also the structural relationship between an application's classes.

This is crucial: Publically affecting the class structure of an application makes these modifications available to other components of the application.

Aspects can declare inter-type

- fields
- methods
- constructors

and can also declare that target types

- implement new interfaces
- extend new classes

This example provides three illustrations of the use of inter-type declarations to encapsulate roles or views of a class. The class our aspect will be dealing with, `Point`, is a simple class with rectangular and polar coordinates. Our inter-type declarations will make the class `Point`, in turn, cloneable, hashable, and comparable. These facilities are provided by AspectJ without having to modify the code for the class `Point`.

#### The `Point` class

The `Point` class defines geometric points whose interface includes polar and rectangular coordinates, plus some simple operations to relocate points. `Point`'s implementation has attributes for both its polar and rectangular coordinates, plus flags to indicate which currently reflect the position of the point. Some operations cause the polar coordinates to be updated from the rectangular, and some have the opposite effect. This implementation, which is in intended to give the minimum number of conversions between coordinate systems, has the property that not all the attributes stored in a `Point` object are necessary to give a canonical representation such as might be used for storing, comparing, cloning or making hash codes from points. Thus the aspects, though simple, are not totally trivial.

The diagram below gives an overview of the aspects and their interaction with the class `Point`.

#### The `CloneablePoint` aspect

This first aspect is responsible for `Point`'s implementation of the `Cloneable` interface. It declares that `Point implements Cloneable` with a `declare parents` form, and also publically declares a specialized `Point`'s `clone()` method. In Java, all objects inherit the method `clone` from the class `Object`, but an object is not cloneable unless its class also implements the interface `Cloneable`. In addition, classes frequently have requirements over and above the simple bit-for-bit copying that `Object.clone` does. In our case, we want to update a `Point`'s coordinate systems before we actually clone the `Point`. So our aspect makes sure that `Point` overrides `Object.clone` with a new method that does what we want.

We also define a test `main` method in the aspect for convenience.

    public aspect CloneablePoint {

       declare parents: Point implements Cloneable;

       public Object Point.clone() throws CloneNotSupportedException {
          // we choose to bring all fields up to date before cloning.
          makeRectangular();
          makePolar();
          return super.clone();
       }

       public static void main(String[] args){
          Point p1 = new Point();
          Point p2 = null;

          p1.setPolar(Math.PI, 1.0);
          try {
             p2 = (Point)p1.clone();
          } catch (CloneNotSupportedException e) {}
          System.out.println("p1 =" + p1 );
          System.out.println("p2 =" + p2 );

          p1.rotate(Math.PI / -2);
          System.out.println("p1 =" + p1 );
          System.out.println("p2 =" + p2 );
       }
    }

#### The `ComparablePoint` aspect

`ComparablePoint` is responsible for `Point`'s implementation of the `Comparable` interface.

The interface `Comparable` defines the single method `compareTo` which can be use to define a natural ordering relation among the objects of a class that implement it.

`ComparablePoint` uses `declare parents` to declare that `Point implements Comparable`, and also publically declares the appropriate `compareTo(Object)` method: A `Point` `p1` is said to be less than another `Point` `p2` if `p1` is closer to the origin.

We also define a test `main` method in the aspect for convenience.

    public aspect ComparablePoint {

       declare parents: Point implements Comparable;

       public int Point.compareTo(Object o) {
          return (int) (this.getRho() - ((Point)o).getRho());
       }

       public static void main(String[] args){
          Point p1 = new Point();
          Point p2 = new Point();

          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

          p1.setRectangular(2,5);
          p2.setRectangular(2,5);
          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

          p2.setRectangular(3,6);
          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

          p1.setPolar(Math.PI, 4);
          p2.setPolar(Math.PI, 4);
          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

          p1.rotate(Math.PI / 4.0);
          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

          p1.offset(1,1);
          System.out.println("p1 =?= p2 :" + p1.compareTo(p2));
       }
    }

#### The `HashablePoint` aspect

Our third aspect is responsible for `Point`'s overriding of `Object`'s `equals` and `hashCode` methods in order to make `Point`s hashable.

The method `Object.hashCode` returns an unique integer, suitable for use as a hash table key. Different implementations are allowed return different integers, but must return distinct integers for distinct objects, and the same integer for objects that test equal. But since the default implementation of `Object.equal` returns `true` only when two objects are identical, we need to redefine both `equals` and `hashCode` to work correctly with objects of type `Point`. For example, we want two `Point` objects to test equal when they have the same `x` and `y` values, or the same `rho` and `theta` values, not just when they refer to the same object. We do this by overriding the methods `equals` and `hashCode` in the class `Point`.

So `HashablePoint` declares `Point`'s `hashCode` and `equals` methods, using `Point`'s rectangular coordinates to generate a hash code and to test for equality. The `x` and `y` coordinates are obtained using the appropriate get methods, which ensure the rectangular coordinates are up-to-date before returning their values.

And again, we supply a `main` method in the aspect for testing.

    public aspect HashablePoint {

       public int Point.hashCode() {
          return (int) (getX() + getY() % Integer.MAX_VALUE);
       }

       public boolean Point.equals(Object o) {
          if (o == this) { return true; }
          if (!(o instanceof Point)) { return false; }
          Point other = (Point)o;
          return (getX() == other.getX()) && (getY() == other.getY());
       }

       public static void main(String[] args) {
          Hashtable h = new Hashtable();
          Point p1 = new Point();

          p1.setRectangular(10, 10);
          Point p2 = new Point();

          p2.setRectangular(10, 10);

          System.out.println("p1 = " + p1);
          System.out.println("p2 = " + p2);
          System.out.println("p1.hashCode() = " + p1.hashCode());
          System.out.println("p2.hashCode() = " + p2.hashCode());

          h.put(p1, "P1");
          System.out.println("Got: " + h.get(p2));
       }
    }

## Development Aspects

### Tracing using aspects

(The code for this example is in `InstallDir/examples/tracing`.)

Writing a class that provides tracing functionality is easy: a couple of functions, a boolean flag for turning tracing on and off, a choice for an output stream, maybe some code for formatting the output -- these are all elements that `Trace` classes have been known to have. `Trace` classes may be highly sophisticated, too, if the task of tracing the execution of a program demands it.

But developing the support for tracing is just one part of the effort of inserting tracing into a program, and, most likely, not the biggest part. The other part of the effort is calling the tracing functions at appropriate times. In large systems, this interaction with the tracing support can be overwhelming. Plus, tracing is one of those things that slows the system down, so these calls should often be pulled out of the system before the product is shipped. For these reasons, it is not unusual for developers to write ad-hoc scripting programs that rewrite the source code by inserting/deleting trace calls before and after the method bodies.

AspectJ can be used for some of these tracing concerns in a less ad-hoc way. Tracing can be seen as a concern that crosscuts the entire system and as such is amenable to encapsulation in an aspect. In addition, it is fairly independent of what the system is doing. Therefore tracing is one of those kind of system aspects that can potentially be plugged in and unplugged without any side-effects in the basic functionality of the system.

#### An Example Application

Throughout this example we will use a simple application that contains only four classes. The application is about shapes. The `TwoDShape` class is the root of the shape hierarchy:

    public abstract class TwoDShape {
        protected double x, y;
        protected TwoDShape(double x, double y) {
            this.x = x; this.y = y;
        }
        public double getX() { return x; }
        public double getY() { return y; }
        public double distance(TwoDShape s) {
            double dx = Math.abs(s.getX() - x);
            double dy = Math.abs(s.getY() - y);
            return Math.sqrt(dx*dx + dy*dy);
        }
        public abstract double perimeter();
        public abstract double area();
        public String toString() {
            return (" @ (" + String.valueOf(x) + ", " + String.valueOf(y) + ") ");
        }
    }

`TwoDShape` has two subclasses, `Circle` and `Square`:

    public class Circle extends TwoDShape {
        protected double r;
        public Circle(double x, double y, double r) {
            super(x, y); this.r = r;
        }
        public Circle(double x, double y) { this(  x,   y, 1.0); }
        public Circle(double r)           { this(0.0, 0.0,   r); }
        public Circle()                   { this(0.0, 0.0, 1.0); }
        public double perimeter() {
            return 2 * Math.PI * r;
        }
        public double area() {
            return Math.PI * r*r;
        }
        public String toString() {
            return ("Circle radius = " + String.valueOf(r) + super.toString());
        }
    }

    public class Square extends TwoDShape {
        protected double s;    // side
        public Square(double x, double y, double s) {
            super(x, y); this.s = s;
        }
        public Square(double x, double y) { this(  x,   y, 1.0); }
        public Square(double s)           { this(0.0, 0.0,   s); }
        public Square()                   { this(0.0, 0.0, 1.0); }
        public double perimeter() {
            return 4 * s;
        }
        public double area() {
            return s*s;
        }
        public String toString() {
            return ("Square side = " + String.valueOf(s) + super.toString());
        }
    }

To run this application, compile the classes. You can do it with or without ajc, the AspectJ compiler. If you've installed AspectJ, go to the directory `InstallDir/examples` and type:

    ajc -argfile tracing/notrace.lst

To run the program, type

    java tracing.ExampleMain

(we don't need anything special on the classpath since this is pure Java code). You should see the following output:

    c1.perimeter() = 12.566370614359172
    c1.area() = 12.566370614359172
    s1.perimeter() = 4.0
    s1.area() = 1.0
    c2.distance(c1) = 4.242640687119285
    s1.distance(c1) = 2.23606797749979
    s1.toString(): Square side = 1.0 @ (1.0, 2.0)

#### Tracing—Version 1

In a first attempt to insert tracing in this application, we will start by writing a `Trace` class that is exactly what we would write if we didn't have aspects. The implementation is in `version1/Trace.java`. Its public interface is:

    public class Trace {
        public static int TRACELEVEL = 0;
        public static void initStream(PrintStream s) {...}
        public static void traceEntry(String str) {...}
        public static void traceExit(String str) {...}
    }

If we didn't have AspectJ, we would have to insert calls to `traceEntry` and `traceExit` in all methods and constructors we wanted to trace, and to initialize `TRACELEVEL` and the stream. If we wanted to trace all the methods and constructors in our example, that would amount to around 40 calls, and we would hope we had not forgotten any method. But we can do that more consistently and reliably with the following aspect (found in `version1/TraceMyClasses.java`):

    aspect TraceMyClasses {
        pointcut myClass(): within(TwoDShape) || within(Circle) || within(Square);
        pointcut myConstructor(): myClass() && execution(new(..));
        pointcut myMethod(): myClass() && execution(* *(..));

        before (): myConstructor() {
            Trace.traceEntry("" + thisJoinPointStaticPart.getSignature());
        }
        after(): myConstructor() {
            Trace.traceExit("" + thisJoinPointStaticPart.getSignature());
        }

        before (): myMethod() {
            Trace.traceEntry("" + thisJoinPointStaticPart.getSignature());
        }
        after(): myMethod() {
            Trace.traceExit("" + thisJoinPointStaticPart.getSignature());
        }
    }

This aspect performs the tracing calls at appropriate times. According to this aspect, tracing is performed at the entrance and exit of every method and constructor defined within the shape hierarchy.

What is printed at before and after each of the traced join points is the signature of the method executing. Since the signature is static information, we can get it through `thisJoinPointStaticPart`.

To run this version of tracing, go to the directory `InstallDir/examples` and type:

      ajc -argfile tracing/tracev1.lst

Running the main method of `tracing.version1.TraceMyClasses` should produce the output:

      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Circle(double, double, double)
      <-- tracing.Circle(double, double, double)
      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Circle(double, double, double)
      <-- tracing.Circle(double, double, double)
      --> tracing.Circle(double)
      <-- tracing.Circle(double)
      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Square(double, double, double)
      <-- tracing.Square(double, double, double)
      --> tracing.Square(double, double)
      <-- tracing.Square(double, double)
      --> double tracing.Circle.perimeter()
      <-- double tracing.Circle.perimeter()
    c1.perimeter() = 12.566370614359172
      --> double tracing.Circle.area()
      <-- double tracing.Circle.area()
    c1.area() = 12.566370614359172
      --> double tracing.Square.perimeter()
      <-- double tracing.Square.perimeter()
    s1.perimeter() = 4.0
      --> double tracing.Square.area()
      <-- double tracing.Square.area()
    s1.area() = 1.0
      --> double tracing.TwoDShape.distance(TwoDShape)
        --> double tracing.TwoDShape.getX()
        <-- double tracing.TwoDShape.getX()
        --> double tracing.TwoDShape.getY()
        <-- double tracing.TwoDShape.getY()
      <-- double tracing.TwoDShape.distance(TwoDShape)
    c2.distance(c1) = 4.242640687119285
      --> double tracing.TwoDShape.distance(TwoDShape)
        --> double tracing.TwoDShape.getX()
        <-- double tracing.TwoDShape.getX()
        --> double tracing.TwoDShape.getY()
        <-- double tracing.TwoDShape.getY()
      <-- double tracing.TwoDShape.distance(TwoDShape)
    s1.distance(c1) = 2.23606797749979
      --> String tracing.Square.toString()
        --> String tracing.TwoDShape.toString()
        <-- String tracing.TwoDShape.toString()
      <-- String tracing.Square.toString()
    s1.toString(): Square side = 1.0 @ (1.0, 2.0)

When `TraceMyClasses.java` is not provided to `ajc`, the aspect does not have any affect on the system and the tracing is unplugged.

#### Tracing—Version 2

Another way to accomplish the same thing would be to write a reusable tracing aspect that can be used not only for these application classes, but for any class. One way to do this is to merge the tracing functionality of `Trace—version1` with the crosscutting support of `TraceMyClasses—version1`. We end up with a `Trace` aspect (found in `version2/Trace.java`) with the following public interface

    abstract aspect Trace {

        public static int TRACELEVEL = 2;
        public static void initStream(PrintStream s) {...}
        protected static void traceEntry(String str) {...}
        protected static void traceExit(String str) {...}
        abstract pointcut myClass();
    }

In order to use it, we need to define our own subclass that knows about our application classes, in `version2/TraceMyClasses.java`:

    public aspect TraceMyClasses extends Trace {
        pointcut myClass(): within(TwoDShape) || within(Circle) || within(Square);

        public static void main(String[] args) {
            Trace.TRACELEVEL = 2;
            Trace.initStream(System.err);
            ExampleMain.main(args);
        }
    }

Notice that we've simply made the pointcut `classes`, that was an abstract pointcut in the super-aspect, concrete. To run this version of tracing, go to the directory `examples` and type:

      ajc -argfile tracing/tracev2.lst

The file tracev2.lst lists the application classes as well as this version of the files Trace.java and TraceMyClasses.java. Running the main method of `tracing.version2.TraceMyClasses` should output exactly the same trace information as that from version 1.

The entire implementation of the new `Trace` class is:

    abstract aspect Trace {

        // implementation part

        public static int TRACELEVEL = 2;
        protected static PrintStream stream = System.err;
        protected static int callDepth = 0;

        public static void initStream(PrintStream s) {
            stream = s;
        }
        protected static void traceEntry(String str) {
            if (TRACELEVEL == 0) return;
            if (TRACELEVEL == 2) callDepth++;
            printEntering(str);
        }
        protected static void traceExit(String str) {
            if (TRACELEVEL == 0) return;
            printExiting(str);
            if (TRACELEVEL == 2) callDepth--;
        }
        private static void printEntering(String str) {
            printIndent();
            stream.println("--> " + str);
        }
        private static void printExiting(String str) {
            printIndent();
            stream.println("<-- " + str);
        }
        private static void printIndent() {
            for (int i = 0; i < callDepth; i++)
                stream.print("  ");
        }

        // protocol part

        abstract pointcut myClass();

        pointcut myConstructor(): myClass() && execution(new(..));
        pointcut myMethod(): myClass() && execution(* *(..));

        before(): myConstructor() {
            traceEntry("" + thisJoinPointStaticPart.getSignature());
        }
        after(): myConstructor() {
            traceExit("" + thisJoinPointStaticPart.getSignature());
        }

        before(): myMethod() {
            traceEntry("" + thisJoinPointStaticPart.getSignature());
        }
        after(): myMethod() {
            traceExit("" + thisJoinPointStaticPart.getSignature());
        }
    }

This version differs from version 1 in several subtle ways. The first thing to notice is that this `Trace` class merges the functional part of tracing with the crosscutting of the tracing calls. That is, in version 1, there was a sharp separation between the tracing support (the class `Trace`) and the crosscutting usage of it (by the class `TraceMyClasses`). In this version those two things are merged. That's why the description of this class explicitly says that "Trace messages are printed before and after constructors and methods are," which is what we wanted in the first place. That is, the placement of the calls, in this version, is established by the aspect class itself, leaving less opportunity for misplacing calls.

A consequence of this is that there is no need for providing `traceEntry` and `traceExit` as public operations of this class. You can see that they were classified as protected. They are supposed to be internal implementation details of the advice.

The key piece of this aspect is the abstract pointcut classes that serves as the base for the definition of the pointcuts constructors and methods. Even though `classes` is abstract, and therefore no concrete classes are mentioned, we can put advice on it, as well as on the pointcuts that are based on it. The idea is "we don't know exactly what the pointcut will be, but when we do, here's what we want to do with it." In some ways, abstract pointcuts are similar to abstract methods. Abstract methods don't provide the implementation, but you know that the concrete subclasses will, so you can invoke those methods.

## Production Aspects

### A Bean Aspect

(The code for this example is in `InstallDir/examples/bean`.)

This example examines an aspect that makes Point objects into Java beans with bound properties.

Java beans are reusable software components that can be visually manipulated in a builder tool. The requirements for an object to be a bean are few. Beans must define a no-argument constructor and must be either `Serializable` or `Externalizable`. Any properties of the object that are to be treated as bean properties should be indicated by the presence of appropriate `get` and `set` methods whose names are `get` property and `set` property where property is the name of a field in the bean class. Some bean properties, known as bound properties, fire events whenever their values change so that any registered listeners (such as, other beans) will be informed of those changes. Making a bound property involves keeping a list of registered listeners, and creating and dispatching event objects in methods that change the property values, such as setproperty methods.

`Point` is a simple class representing points with rectangular coordinates. `Point` does not know anything about being a bean: there are set methods for `x` and `y` but they do not fire events, and the class is not serializable. Bound is an aspect that makes `Point` a serializable class and makes its `get` and `set` methods support the bound property protocol.

#### The `Point` class

The `Point` class is a very simple class with trivial getters and setters, and a simple vector offset method.

    class Point {

      protected int x = 0;
      protected int y = 0;

      public int getX() {
        return x;
      }

      public int getY() {
        return y;
      }

      public void setRectangular(int newX, int newY) {
        setX(newX);
        setY(newY);
      }

      public void setX(int newX) {
        x = newX;
      }

      public void setY(int newY) {
        y = newY;
      }

      public void offset(int deltaX, int deltaY) {
        setRectangular(x + deltaX, y + deltaY);
      }

      public String toString() {
        return "(" + getX() + ", " + getY() + ")" ;
      }
    }

#### The `BoundPoint` aspect

The `BoundPoint` aspect is responsible for `Point`'s "beanness". The first thing it does is privately declare that each `Point` has a `support` field that holds reference to an instance of `PropertyChangeSupport`.

      private PropertyChangeSupport Point.support = new PropertyChangeSupport(this);

The property change support object must be constructed with a reference to the bean for which it is providing support, so it is initialized by passing it `this`, an instance of `Point`. Since the `support` field is private declared in the aspect, only the code in the aspect can refer to it.

The aspect also declares `Point`'s methods for registering and managing listeners for property change events, which delegate the work to the property change support object:

      public void Point.addPropertyChangeListener(PropertyChangeListener listener){
        support.addPropertyChangeListener(listener);
      }
      public void Point.addPropertyChangeListener(String propertyName,
                                                  PropertyChangeListener listener){

        support.addPropertyChangeListener(propertyName, listener);
      }
      public void Point.removePropertyChangeListener(String propertyName,
                                                     PropertyChangeListener listener) {
        support.removePropertyChangeListener(propertyName, listener);
      }
      public void Point.removePropertyChangeListener(PropertyChangeListener listener) {
        support.removePropertyChangeListener(listener);
      }
      public void Point.hasListeners(String propertyName) {
        support.hasListeners(propertyName);
      }

The aspect is also responsible for making sure `Point` implements the `Serializable` interface:

      declare parents: Point implements Serializable;

Implementing this interface in Java does not require any methods to be implemented. Serialization for `Point` objects is provided by the default serialization method.

The `setters` pointcut picks out calls to the `Point`'s `set` methods: any method whose name begins with "`set`" and takes one parameter. The around advice on `setters()` stores the values of the `X` and `Y` properties, calls the original `set` method and then fires the appropriate property change event according to which set method was called.

    aspect BoundPoint {
      private PropertyChangeSupport Point.support = new PropertyChangeSupport(this);

      public void Point.addPropertyChangeListener(PropertyChangeListener listener){
        support.addPropertyChangeListener(listener);
      }
      public void Point.addPropertyChangeListener(String propertyName,
                                                  PropertyChangeListener listener){

        support.addPropertyChangeListener(propertyName, listener);
      }
      public void Point.removePropertyChangeListener(String propertyName,
                                                     PropertyChangeListener listener) {
        support.removePropertyChangeListener(propertyName, listener);
      }
      public void Point.removePropertyChangeListener(PropertyChangeListener listener) {
        support.removePropertyChangeListener(listener);
      }
      public void Point.hasListeners(String propertyName) {
        support.hasListeners(propertyName);
      }

      declare parents: Point implements Serializable;

      pointcut setter(Point p): call(void Point.set*(*)) && target(p);

      void around(Point p): setter(p) {
            String propertyName =
          thisJoinPointStaticPart.getSignature().getName().substring("set".length());
            int oldX = p.getX();
            int oldY = p.getY();
            proceed(p);
            if (propertyName.equals("X")){
          firePropertyChange(p, propertyName, oldX, p.getX());
            } else {
          firePropertyChange(p, propertyName, oldY, p.getY());
            }
      }

      void firePropertyChange(Point p,
                              String property,
                              double oldval,
                              double newval) {
            p.support.firePropertyChange(property,
                                     new Double(oldval),
                                     new Double(newval));
      }
    }

#### The Test Program

The test program registers itself as a property change listener to a `Point` object that it creates and then performs simple manipulation of that point: calling its set methods and the offset method. Then it serializes the point and writes it to a file and then reads it back. The result of saving and restoring the point is that a new point is created.

      class Demo implements PropertyChangeListener {

        static final String fileName = "test.tmp";

        public void propertyChange(PropertyChangeEvent e){
          System.out.println("Property " + e.getPropertyName() + " changed from " +
             e.getOldValue() + " to " + e.getNewValue() );
        }

        public static void main(String[] args){
          Point p1 = new Point();
          p1.addPropertyChangeListener(new Demo());
          System.out.println("p1 =" + p1);
          p1.setRectangular(5,2);
          System.out.println("p1 =" + p1);
          p1.setX( 6 );
          p1.setY( 3 );
          System.out.println("p1 =" + p1);
          p1.offset(6,4);
          System.out.println("p1 =" + p1);
          save(p1, fileName);
          Point p2 = (Point) restore(fileName);
          System.out.println("Had: " + p1);
          System.out.println("Got: " + p2);
          }
        ...
      }

#### Compiling and Running the Example

To compile and run this example, go to the examples directory and type:

    ajc -argfile bean/files.lst
    java bean.Demo

### The Subject/Observer Protocol

(The code for this example is in `InstallDir/examples/observer`.)

This demo illustrates how the Subject/Observer design pattern can be coded with aspects.

The demo consists of the following: A colored label is a renderable object that has a color that cycles through a set of colors, and a number that records the number of cycles it has been through. A button is an action item that records when it is clicked.

With these two kinds of objects, we can build up a Subject/Observer relationship in which colored labels observe the clicks of buttons; that is, where colored labels are the observers and buttons are the subjects.

The demo is designed and implemented using the Subject/Observer design pattern. The remainder of this example explains the classes and aspects of this demo, and tells you how to run it.

#### Generic Components

The generic parts of the protocol are the interfaces `Subject` and `Observer`, and the abstract aspect `SubjectObserverProtocol`. The `Subject` interface is simple, containing methods to add, remove, and view `Observer` objects, and a method for getting data about state changes:

        interface Subject {
          void addObserver(Observer obs);
          void removeObserver(Observer obs);
          Vector getObservers();
          Object getData();
      }

The `Observer` interface is just as simple, with methods to set and get `Subject` objects, and a method to call when the subject gets updated.

      interface Observer {
          void setSubject(Subject s);
          Subject getSubject();
          void update();
      }

The `SubjectObserverProtocol` aspect contains within it all of the generic parts of the protocol, namely, how to fire the `Observer` objects' update methods when some state changes in a subject.

      abstract aspect SubjectObserverProtocol {

          abstract pointcut stateChanges(Subject s);

          after(Subject s): stateChanges(s) {
              for (int i = 0; i < s.getObservers().size(); i++) {
                  ((Observer)s.getObservers().elementAt(i)).update();
              }
          }

          private Vector Subject.observers = new Vector();
          public void   Subject.addObserver(Observer obs) {
              observers.addElement(obs);
              obs.setSubject(this);
          }
          public void   Subject.removeObserver(Observer obs) {
              observers.removeElement(obs);
              obs.setSubject(null);
          }
          public Vector Subject.getObservers() { return observers; }

          private Subject Observer.subject = null;
          public void     Observer.setSubject(Subject s) { subject = s; }
          public Subject  Observer.getSubject() { return subject; }

      }

Note that this aspect does three things. It define an abstract pointcut that extending aspects can override. It defines advice that should run after the join points of the pointcut. And it declares an inter-tpye field and two inter-type methods so that each `Observer` can hold onto its `Subject`.

#### Application Classes

`Button` objects extend `java.awt.Button`, and all they do is make sure the `void click()` method is called whenever a button is clicked.

      class Button extends java.awt.Button {

          static final Color  defaultBackgroundColor = Color.gray;
          static final Color  defaultForegroundColor = Color.black;
          static final String defaultText = "cycle color";

          Button(Display display) {
              super();
              setLabel(defaultText);
              setBackground(defaultBackgroundColor);
              setForeground(defaultForegroundColor);
              addActionListener(new ActionListener() {
                      public void actionPerformed(ActionEvent e) {
                          Button.this.click();
                      }
                  });
              display.addToFrame(this);
          }

          public void click() {}

      }

Note that this class knows nothing about being a Subject.

ColorLabel objects are labels that support the void colorCycle() method. Again, they know nothing about being an observer.

      class ColorLabel extends Label {

          ColorLabel(Display display) {
              super();
              display.addToFrame(this);
          }

          final static Color[] colors = {Color.red, Color.blue,
                                         Color.green, Color.magenta};
          private int colorIndex = 0;
          private int cycleCount = 0;
          void colorCycle() {
              cycleCount++;
              colorIndex = (colorIndex + 1) % colors.length;
              setBackground(colors[colorIndex]);
              setText("" + cycleCount);
          }
      }

Finally, the `SubjectObserverProtocolImpl` implements the subject/observer protocol, with `Button` objects as subjects and `ColorLabel` objects as observers:

    package observer;

    import java.util.Vector;

    aspect SubjectObserverProtocolImpl extends SubjectObserverProtocol {

        declare parents: Button implements Subject;
        public Object Button.getData() { return this; }

        declare parents: ColorLabel implements Observer;
        public void    ColorLabel.update() {
            colorCycle();
        }

        pointcut stateChanges(Subject s):
            target(s) &&
            call(void Button.click());

    }

It does this by assuring that `Button` and `ColorLabel` implement the appropriate interfaces, declaring that they implement the methods required by those interfaces, and providing a definition for the abstract `stateChanges` pointcut. Now, every time a `Button` is clicked, all `ColorLabel` objects observing that button will `colorCycle`.

#### Compiling and Running

`Demo` is the top class that starts this demo. It instantiates a two buttons and three observers and links them together as subjects and observers. So to run the demo, go to the `examples` directory and type:

      ajc -argfile observer/files.lst
      java observer.Demo

### A Simple Telecom Simulation

(The code for this example is in `InstallDir/examples/telecom`.)

This example illustrates some ways that dependent concerns can be encoded with aspects. It uses an example system comprising a simple model of telephone connections to which timing and billing features are added using aspects, where the billing feature depends upon the timing feature.

#### The Application

The example application is a simple simulation of a telephony system in which customers make, accept, merge and hang-up both local and long distance calls. The application architecture is in three layers.

- The basic objects provide basic functionality to simulate customers, calls and connections (regular calls have one connection, conference calls have more than one).

- The timing feature is concerned with timing the connections and keeping the total connection time per customer. Aspects are used to add a timer to each connection and to manage the total time per customer.

- The billing feature is concerned with charging customers for the calls they make. Aspects are used to calculate a charge per connection and, upon termination of a connection, to add the charge to the appropriate customer's bill. The billing aspect builds upon the timing aspect: it uses a pointcut defined in Timing and it uses the timers that are associated with connections.

The simulation of system has three configurations: basic, timing and billing. Programs for the three configurations are in classes `BasicSimulation`, `TimingSimulation` and `BillingSimulation`. These share a common superclass `AbstractSimulation`, which defines the method run with the simulation itself and the method wait used to simulate elapsed time.

#### The Basic Objects

The telecom simulation comprises the classes `Customer`, `Call` and the abstract class `Connection` with its two concrete subclasses `Local` and `LongDistance`. Customers have a name and a numeric area code. They also have methods for managing calls. Simple calls are made between one customer (the caller) and another (the receiver), a `Connection` object is used to connect them. Conference calls between more than two customers will involve more than one connection. A customer may be involved in many calls at one time.

#### The `Customer` class

`Customer` has methods `call`, `pickup`, `hangup` and `merge` for managing calls.

    public class Customer {

          private String name;
          private int areacode;
          private Vector calls = new Vector();

          protected void removeCall(Call c){
              calls.removeElement(c);
          }

          protected void addCall(Call c){
              calls.addElement(c);
          }

          public Customer(String name, int areacode) {
              this.name = name;
              this.areacode = areacode;
          }

          public String toString() {
              return name + "(" + areacode + ")";
          }

          public int getAreacode(){
              return areacode;
          }

          public boolean localTo(Customer other){
              return areacode == other.areacode;
          }

          public Call call(Customer receiver) {
              Call call = new Call(this, receiver);
              addCall(call);
              return call;
          }

          public void pickup(Call call) {
              call.pickup();
              addCall(call);
          }

          public void hangup(Call call) {
              call.hangup(this);
              removeCall(call);
          }

          public void merge(Call call1, Call call2){
              call1.merge(call2);
              removeCall(call2);
          }
      }

#### The `Call` class

Calls are created with a caller and receiver who are customers. If the caller and receiver have the same area code then the call can be established with a `Local` connection (see below), otherwise a `LongDistance` connection is required. A call comprises a number of connections between customers. Initially there is only the connection between the caller and receiver but additional connections can be added if calls are merged to form conference calls.

#### The `Connection` class

The class `Connection` models the physical details of establishing a connection between customers. It does this with a simple state machine (connections are initially `PENDING`, then `COMPLETED` and finally `DROPPED`). Messages are printed to the console so that the state of connections can be observed. Connection is an abstract class with two concrete subclasses: `Local` and `LongDistance`.

      abstract class Connection {

          public static final int PENDING = 0;
          public static final int COMPLETE = 1;
          public static final int DROPPED = 2;

          Customer caller, receiver;
          private int state = PENDING;

          Connection(Customer a, Customer b) {
              this.caller = a;
              this.receiver = b;
          }

          public int getState(){
              return state;
          }

          public Customer getCaller() { return caller; }

          public Customer getReceiver() { return receiver; }

          void complete() {
              state = COMPLETE;
              System.out.println("connection completed");
          }

          void drop() {
              state = DROPPED;
              System.out.println("connection dropped");
          }

          public boolean connects(Customer c){
              return (caller == c || receiver == c);
          }

      }

#### The `Local` and `LongDistance` classes

The two kinds of connections supported by our simulation are `Local` and `LongDistance` connections.

      class Local extends Connection {
          Local(Customer a, Customer b) {
              super(a, b);
              System.out.println("[new local connection from " +
                 a + " to " + b + "]");
          }
      }

      class LongDistance extends Connection {
          LongDistance(Customer a, Customer b) {
              super(a, b);
              System.out.println("[new long distance connection from " +
                  a + " to " + b + "]");
          }
      }

#### Compiling and Running the Basic Simulation

The source files for the basic system are listed in the file `basic.lst`. To build and run the basic system, in a shell window, type these commands:

    ajc -argfile telecom/basic.lst
    java telecom.BasicSimulation

#### The Timing aspect

The `Timing` aspect keeps track of total connection time for each `Customer` by starting and stopping a timer associated with each connection. It uses some helper classes:

##### The `Timer` class

A `Timer` object simply records the current time when it is started and stopped, and returns their difference when asked for the elapsed time. The aspect `TimerLog` (below) can be used to cause the start and stop times to be printed to standard output.

      class Timer {
          long startTime, stopTime;

          public void start() {
              startTime = System.currentTimeMillis();
              stopTime = startTime;
          }

          public void stop() {
              stopTime = System.currentTimeMillis();
          }

          public long getTime() {
              return stopTime - startTime;
          }
      }

#### The `TimerLog` aspect

The `TimerLog` aspect can be included in a build to get the timer to announce when it is started and stopped.

    public aspect TimerLog {

        after(Timer t): target(t) && call(* Timer.start())  {
          System.err.println("Timer started: " + t.startTime);
        }

        after(Timer t): target(t) && call(* Timer.stop()) {
          System.err.println("Timer stopped: " + t.stopTime);
        }
    }

#### The `Timing` aspect

The `Timing` aspect is declares an inter-type field `totalConnectTime` for `Customer` to store the accumulated connection time per `Customer`. It also declares that each `Connection` object has a timer.

        public long Customer.totalConnectTime = 0;
        private Timer Connection.timer = new Timer();

Two pieces of after advice ensure that the timer is started when a connection is completed and and stopped when it is dropped. The pointcut `endTiming` is defined so that it can be used by the `Billing` aspect.

    public aspect Timing {

        public long Customer.totalConnectTime = 0;

        public long getTotalConnectTime(Customer cust) {
            return cust.totalConnectTime;
        }
        private Timer Connection.timer = new Timer();
        public Timer getTimer(Connection conn) { return conn.timer; }

        after (Connection c): target(c) && call(void Connection.complete()) {
            getTimer(c).start();
        }

        pointcut endTiming(Connection c): target(c) &&
            call(void Connection.drop());

        after(Connection c): endTiming(c) {
            getTimer(c).stop();
            c.getCaller().totalConnectTime += getTimer(c).getTime();
            c.getReceiver().totalConnectTime += getTimer(c).getTime();
        }
    }

#### The `Billing` aspect

The Billing system adds billing functionality to the telecom application on top of timing.

The `Billing` aspect declares that each `Connection` has a `payer` inter-type field to indicate who initiated the call and therefore who is responsible to pay for it. It also declares the inter-type method `callRate` of `Connection` so that local and long distance calls can be charged differently. The call charge must be calculated after the timer is stopped; the after advice on pointcut `Timing.endTiming` does this, and `Billing` is declared to be more precedent than `Timing` to make sure that this advice runs after `Timing`'s advice on the same join point. Finally, it declares inter-type methods and fields for `Customer` to handle the `totalCharge`.

    public aspect Billing {
        // precedence required to get advice on endtiming in the right order
        declare precedence: Billing, Timing;

        public static final long LOCAL_RATE = 3;
        public static final long LONG_DISTANCE_RATE = 10;

        public Customer Connection.payer;
        public Customer getPayer(Connection conn) { return conn.payer; }

        after(Customer cust) returning (Connection conn):
            args(cust, ..) && call(Connection+.new(..)) {
            conn.payer = cust;
        }

        public abstract long Connection.callRate();

        public long LongDistance.callRate() { return LONG_DISTANCE_RATE; }
        public long Local.callRate() { return LOCAL_RATE; }

        after(Connection conn): Timing.endTiming(conn) {
            long time = Timing.aspectOf().getTimer(conn).getTime();
            long rate = conn.callRate();
            long cost = rate * time;
            getPayer(conn).addCharge(cost);
        }

        public long Customer.totalCharge = 0;
        public long getTotalCharge(Customer cust) { return cust.totalCharge; }

        public void Customer.addCharge(long charge){
            totalCharge += charge;
        }
    }

#### Accessing the inter-type state

Both the aspects `Timing` and `Billing` contain the definition of operations that the rest of the system may want to access. For example, when running the simulation with one or both aspects, we want to find out how much time each customer spent on the telephone and how big their bill is. That information is also stored in the classes, but they are accessed through static methods of the aspects, since the state they refer to is private to the aspect.

Take a look at the file `TimingSimulation.java`. The most important method of this class is the method `report(Customer)`, which is used in the method run of the superclass `AbstractSimulation`. This method is intended to print out the status of the customer, with respect to the `Timing` feature.

      protected void report(Customer c){
          Timing t = Timing.aspectOf();
          System.out.println(c + " spent " + t.getTotalConnectTime(c));
      }

#### Compiling and Running

The files timing.lst and billing.lst contain file lists for the timing and billing configurations. To build and run the application with only the timing feature, go to the directory examples and type:

      ajc -argfile telecom/timing.lst
      java telecom.TimingSimulation

To build and run the application with the timing and billing features, go to the directory examples and type:

      ajc -argfile telecom/billing.lst
      java telecom.BillingSimulation

#### Discussion

There are some explicit dependencies between the aspects Billing and Timing:

- Billing is declared more precedent than Timing so that Billing's after advice runs after that of Timing when they are on the same join point.

- Billing uses the pointcut Timing.endTiming.

- Billing needs access to the timer associated with a connection.

## Reusable Aspects

### Tracing using Aspects, Revisited

(The code for this example is in `InstallDir/examples/tracing`.)

#### Tracing—Version 3

One advantage of not exposing the methods traceEntry and traceExit as public operations is that we can easily change their interface without any dramatic consequences in the rest of the code.

Consider, again, the program without AspectJ. Suppose, for example, that at some point later the requirements for tracing change, stating that the trace messages should always include the string representation of the object whose methods are being traced. This can be achieved in at least two ways. One way is keep the interface of the methods `traceEntry` and `traceExit` as it was before,

      public static void traceEntry(String str);
      public static void traceExit(String str);

In this case, the caller is responsible for ensuring that the string representation of the object is part of the string given as argument. So, calls must look like:

      Trace.traceEntry("Square.distance in " + toString());

Another way is to enforce the requirement with a second argument in the trace operations, e.g.

      public static void traceEntry(String str, Object obj);
      public static void traceExit(String str, Object obj);

In this case, the caller is still responsible for sending the right object, but at least there is some guarantees that some object will be passed. The calls will look like:

      Trace.traceEntry("Square.distance", this);

In either case, this change to the requirements of tracing will have dramatic consequences in the rest of the code -- every call to the trace operations traceEntry and traceExit must be changed!

Here's another advantage of doing tracing with an aspect. We've already seen that in version 2 `traceEntry` and `traceExit` are not publicly exposed. So changing their interfaces, or the way they are used, has only a small effect inside the `Trace` class. Here's a partial view at the implementation of `Trace`, version 3. The differences with respect to version 2 are stressed in the comments:

    abstract aspect Trace {

        public static int TRACELEVEL = 0;
        protected static PrintStream stream = null;
        protected static int callDepth = 0;

        public static void initStream(PrintStream s) {
            stream = s;
        }

        protected static void traceEntry(String str, Object o) {
            if (TRACELEVEL == 0) return;
            if (TRACELEVEL == 2) callDepth++;
            printEntering(str + ": " + o.toString());
        }

        protected static void traceExit(String str, Object o) {
            if (TRACELEVEL == 0) return;
            printExiting(str + ": " + o.toString());
            if (TRACELEVEL == 2) callDepth--;
        }

        private static void printEntering(String str) {
            printIndent();
            stream.println("Entering " + str);
        }

        private static void printExiting(String str) {
            printIndent();
            stream.println("Exiting " + str);
        }

        private static void printIndent() {
            for (int i = 0; i < callDepth; i++)
                stream.print("  ");
        }

        abstract pointcut myClass(Object obj);

        pointcut myConstructor(Object obj): myClass(obj) && execution(new(..));
        pointcut myMethod(Object obj): myClass(obj) &&
            execution(* *(..)) && !execution(String toString());

        before(Object obj): myConstructor(obj) {
            traceEntry("" + thisJoinPointStaticPart.getSignature(), obj);
        }
        after(Object obj): myConstructor(obj) {
            traceExit("" + thisJoinPointStaticPart.getSignature(), obj);
        }

        before(Object obj): myMethod(obj) {
            traceEntry("" + thisJoinPointStaticPart.getSignature(), obj);
        }
        after(Object obj): myMethod(obj) {
            traceExit("" + thisJoinPointStaticPart.getSignature(), obj);
        }
    }

As you can see, we decided to apply the first design by preserving the interface of the methods `traceEntry` and `traceExit`. But it doesn't matter—we could as easily have applied the second design (the code in the directory `examples/tracing/version3` has the second design). The point is that the effects of this change in the tracing requirements are limited to the `Trace` aspect class.

One implementation change worth noticing is the specification of the pointcuts. They now expose the object. To maintain full consistency with the behavior of version 2, we should have included tracing for static methods, by defining another pointcut for static methods and advising it. We leave that as an exercise.

Moreover, we had to exclude the execution join point of the method `toString` from the `methods` pointcut. The problem here is that `toString` is being called from inside the advice. Therefore if we trace it, we will end up in an infinite recursion of calls. This is a subtle point, and one that you must be aware when writing advice. If the advice calls back to the objects, there is always the possibility of recursion. Keep that in mind!

In fact, esimply excluding the execution join point may not be enough, if there are calls to other traced methods within it -- in which case, the restriction should be

    && !cflow(execution(String toString()))

excluding both the execution of toString methods and all join points under that execution.

In summary, to implement the change in the tracing requirements we had to make a couple of changes in the implementation of the `Trace` aspect class, including changing the specification of the pointcuts. That's only natural. But the implementation changes were limited to this aspect. Without aspects, we would have to change the implementation of every application class.

Finally, to run this version of tracing, go to the directory `examples` and type:

    ajc -argfile tracing/tracev3.lst

The file tracev3.lst lists the application classes as well as this version of the files `Trace.java` and `TraceMyClasses.java`. To run the program, type

    java tracing.version3.TraceMyClasses

The output should be:

      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Circle(double, double, double)
      <-- tracing.Circle(double, double, double)
      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Circle(double, double, double)
      <-- tracing.Circle(double, double, double)
      --> tracing.Circle(double)
      <-- tracing.Circle(double)
      --> tracing.TwoDShape(double, double)
      <-- tracing.TwoDShape(double, double)
      --> tracing.Square(double, double, double)
      <-- tracing.Square(double, double, double)
      --> tracing.Square(double, double)
      <-- tracing.Square(double, double)
      --> double tracing.Circle.perimeter()
      <-- double tracing.Circle.perimeter()
    c1.perimeter() = 12.566370614359172
      --> double tracing.Circle.area()
      <-- double tracing.Circle.area()
    c1.area() = 12.566370614359172
      --> double tracing.Square.perimeter()
      <-- double tracing.Square.perimeter()
    s1.perimeter() = 4.0
      --> double tracing.Square.area()
      <-- double tracing.Square.area()
    s1.area() = 1.0
      --> double tracing.TwoDShape.distance(TwoDShape)
        --> double tracing.TwoDShape.getX()
        <-- double tracing.TwoDShape.getX()
        --> double tracing.TwoDShape.getY()
        <-- double tracing.TwoDShape.getY()
      <-- double tracing.TwoDShape.distance(TwoDShape)
    c2.distance(c1) = 4.242640687119285
      --> double tracing.TwoDShape.distance(TwoDShape)
        --> double tracing.TwoDShape.getX()
        <-- double tracing.TwoDShape.getX()
        --> double tracing.TwoDShape.getY()
        <-- double tracing.TwoDShape.getY()
      <-- double tracing.TwoDShape.distance(TwoDShape)
    s1.distance(c1) = 2.23606797749979
      --> String tracing.Square.toString()
        --> String tracing.TwoDShape.toString()
        <-- String tracing.TwoDShape.toString()
      <-- String tracing.Square.toString()
    s1.toString(): Square side = 1.0 @ (1.0, 2.0)