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Method: example

You will have gained experience and understanding of the sending of messages between objects, and the invocation of methods corresponding to received messages. What you may have not realised is that an object can send a message to itself, simply by a statement that consists of a message, but no explicit object to send the message to.

We shall investigate such issues in detail in a later unit, however, for illustration we shall give a simple example to show the deviation from the default, sequential flow of statement execution.

Consider this pseudocode design for part of a class:

class UnconditionalBranch

{

  method2()

    {

    statementM1;

    statementM2;

    }

 method1()

  {

    statementA;

    method2();

   statementB;

  }

// other methods

...

}

When method1 is being executed, the order that statements will be executed will be as follows:

statementA;

statementM1;

statementM2;

statementB;

since statementA is executed first, followed by the sending of the message method2(). Since no object is specified for the object to be sent to, this message will be sent to the object that is the instance of the class currently having its method2() method executed. The run-time interpreter will see that this class defines a method2() that corresponds to the. The run-time interpreter then transfers control of execution by invoking method method2() ?i.e. method2() will now have its statements executed. Next statementM1 is executed, followed by statementM2. That ends execution of method2(), so control of execution returns to the place where the message was sent in method1(). Next statementB is executed. Then method2() completes its execution (and control of execution returns to whatever happened before method1() was invoked.

A program listing of a class UncontionalBranch (the listing is UnconditionalBranch.java) is available to examine, compile, modify etc. if you wish to explore these issues now. Although a later unit will present a more detailed investigation of the relationship between messages, classes, objects and methods.

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