Implementing Classes - Java Concepts - page 147

3.8 Implicit and Explicit Method Parameters

In Section 2.4 , you learned that a method has an implicit parameterȌthe object on
which the method is invokedȌand explicit parameters, which are enclosed in
parentheses. In this section, we will examine these parameters in greater detail.

Have a look at a particular invocation of the deposit method:
momsSavings. deposit( 500 );

Now look again at the code of the deposit method:

public void deposit(double amount)
{
double newBalance = balance + amount;
balance = newBalance;
}

The parameter variable amount is set to 500 when the deposit method starts. But
what does balance mean exactly? After all, our program may have multiple
Bank-Account objects, and each of them has its own balance.

Of course, since we deposit the money into momsSavings , balance must mean
momsSavings.balance . In general, when you refer to an instance field inside a
method, it means the instance field of the object on which the method was called.

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Thus, the call to the deposit method depends on two values: the object to which
momsSavings refers, and the value 500 . The amount parameter inside the
parentheses is called an explicit parameter, because it is explicitly named in the
method definition. However, the reference to the bank account object is not explicit in
the method definitionȌit is called the implicit parameter of the method.

The implicit parameter of a method is the object on which the method is invoked.
The this reference denotes the implicit parameter.

If you need to, you can access the implicit parameterȌthe object on which the
method is calledȌwith the keyword this . For example, in the preceding method
invocation, this was set to momsSavings and amount was set to 500 (see Figure
8 ).