Methods: Communicating with Objects

We communicate with objects using methods. Methods are executable code within each object, for which an interface has been established. Sometimes the interface is only for the object itself. Other times it is an interface accessible by other objects. This chapter discusses that topic in detail.

3.5 Passing a Value and Passing a Reference

The effect of passing arguments to a method differs depending on whether you are passing a value of primitive type (such as 5 or true) or a value of reference type (such as “Hello” or game1). When an argument of primiPassing a primitive value tive type is passed to a method, a copy of the argument is passed to the formal parameter. For example, consider the PrimitiveCall class shown

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in Figure 3.7. Note that we have an int variable k, which initially stores the value 5, and a method myMethod(), which takes an int parameter n. In this case, when we invoke myMethod(k), k’s value (5) is copied into n and stored there during the method.

One implication of passing a copy of a primitive value to a method is that the original value of k in main() cannot be altered from inside the method. Thus, the output generated by PrimitiveCall would be

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Note that in main(), k’s value is printed both before and after myMethod() is called, but that its value remains unaffected even though n’s value is changed within the method. This is because myMethod() contains just a copy of k’s value, not k itself. Any changes to the copy within myMethod() leave k unaltered (See Fig. 3.8).

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In contrast to this, when an argument of a reference type is passed to a method, a copy of the reference to the object itself is assigned to the parameter. For example, in the case of a String parameter or a OneRowNim parameter, the method would be given a reference to the object–that is, the address of the object. The object itself is not passed, because it would be too inefficient to copy the entire object with all its data and methods. However, because the object’s reference gives the object’s location in memory, the method will have access to the object and can make changes to the original object from within the method.

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Figure 3.8: Tracing the state of variables k and n in PrimitiveCall (a) Just before calling myMethod(k) in main. (b) Just before executing the body of myMethod(). (c) Just after executing the body of myMethod(). (d) After flow of control returns to main().

For example, consider the ReferenceCall class (Fig. 3.9). In this case, myMethod() takes a parameter g of type OneRowNim. Because

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a OneRowNim instance is an object, g is a reference variable. So when myMethod(game) is invoked in main(), a reference to game is passed to myMethod(). Note that in myMethod(), we use takeSticks(3) to change the number of sticks of g from 10 to 7 and that this change persists even after the method returns control to main(). The reason is that during the method’s execution, both game and g refer to the exact same object (see Fig. 3.10). The output generated by ReferenceCall would be

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This illustrates that when passing a reference variable to a method, it is possible for the method to change the state of the object associated with

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Figure 3.10: Tracing the state of OneRowNim object in ReferenceCall (a) Just before calling myMethod(game). (b) Just before executing the body of myMethod(). (c) Just after executing the body of myMethod(). (d) After flow of control returns to main().

the reference variable. In subsequent chapters we will see ways to make use of this feature of reference parameters.

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