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.

3.2 Passing Information to an Object

Arguments and Parameters

The new class definition for OneRowNim is given in Figure 3.1. Note that now that we have a single method, takeSticks(), that can be used to take away a variable number of sticks, we have removed the three methods we wrote in the previous chapter, takeOne(), takeTwo(), and takeThree(), from OneRowNim. Using a single method, with a parameter, is clearly a better design. To see this, just imagine what we would have to do if we didn’t use a parameter and we wanted to be able to take away four sticks, or five, or more. If we didn’t have parameters, we’d have to write a separate method for each case, which is clearly a bad idea. Using parameters in this way leads to a more general useful method and thus is an example of the generality principle.

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Now let’s consider how we would create a OneRowNim instance and use the new method in the main() method or in a different class. If we want to have an instance of OneRowNim object to remove 3 sticks on the first move by using the takeSticks() method, we need to pass the int value 3 to the method. In order to effect this action, we would use the following statements:

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Because the definition of takeSticks() includes a single int parameter, we must supply a single int value (such as 3), when we invoke it. When the method is invoked, its formal parameter (num) will be set to the value we supply (3). The value we supply does not have to be a literal int value. We can supply any expression or variable that evaluates to an int value. For example:

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In this case, the value being passed to takeSticks() is 2, the value that val has at the time the method call is made

It would be an error to try to pass a value that was not a int to takeSticks(). For example, each of the following invocations of takeSticks() results in a syntax error:

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As you recall from Chapter 0, the value that is passed to a method when it is invoked is called an argument. Even though the terms argument and parameter are sometimes used interchangeably, it will be useful to observe a distinction. We will use the term parameter to refer to the formal parameter—the variable used to pass data to a method—that occurs in the method definition. We use the term argument to refer to the actual value that is supplied when the method is invoked.

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The distinction between parameter and argument is related to the difference between defining a method and invoking a method. Defining a method is a matter of writing a method definition, such as

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This definition defines a method that takes a single String parameter, and simply prints the value of its parameter. On the other hand, invoking a method is a matter of writing a method call statement, such as

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This statement calls the printStr() method and passes it the string “HelloWorld”. This notation assumes that the call to the instance method printStr() is made within the body of another instance method of the same class.