Examples

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 <literal>thisJoinPoint</literal> (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. 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 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 k

import org.aspectj.testing.Tester;

import java.awt.Component;

public class VariousConstructors {
    public static void main(String[] args) {
	Component c = new Component() {};
	AbstractC ac = new AbstractC() {};
    }
}

abstract class AbstractC {}

aving to modify the code for the class Point. The <classname>Point</classname> 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 <classname>CloneablePoint</classname> 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. The <classname>ComparablePoint</classname> 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. The <classname>HashablePoint</classname> aspect Our third aspect is responsible for Point's overriding of Object's equals and hashCode methods in order to make Points hashable. The method Object.hashCode returns an integer, suitable for use as a hash table key. It is not required that two objects which are not equal (according to the equals method) return different integer results from hashCode but it can improve performance when the integer is used as a key into a data structure. However, any two objects which are equal must return the same integer value from a call to hashCode. Since the default implementation of Object.equals 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. 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: TwoDShape has two subclasses, Circle and Square: 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: 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: 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): 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: 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 In order to use it, we need to define our own subclass that knows about our application classes, in version2/TraceMyClasses.java: 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: 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: " + 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 getproperty 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 <classname>Point</classname> class The Point class is a very simple class with trivial getters and setters, and a simple vector offset method. The <classname>BoundPoint</classname> 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. 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: The aspect is also responsible for making sure Point implements the Serializable interface: 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. 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. Compiling and Running the Example To compile and run this example, go to the examples directory and type: 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: 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. 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. 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. 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. Finally, the SubjectObserverProtocolImpl implements the subject/observer protocol, with Button objects as subjects and ColorLabel objects as observers: 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: 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 <classname>Customer</classname> class Customer has methods call, pickup, hangup and merge for managing calls. The <classname>Call</classname> 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 <classname>Connection</classname> 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. The <literal>Local</literal> and <literal>LongDistance</literal> classes The two kinds of connections supported by our simulation are Local and LongDistance connections. 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: 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 <classname>Timer</classname> 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. The <classname>TimerLog</classname> aspect The TimerLog aspect can be included in a build to get the timer to announce when it is started and stopped. The <classname>Timing</classname> 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. 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. The <literal>Billing</literal> 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. 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. 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: To build and run the application with the timing and billing features, go to the directory examples and type: 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, 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: Another way is to enforce the requirement with a second argument in the trace operations, e.g. 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: 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: 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 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: 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 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) ]]>