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Understanding Variable Scope and Class Construction
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the instance of the class exists. Instance variables store the state of the object. They are not within the scope of any one particular method, instead they are in scope for the entire class. They exist and retain their value from the time a class is initialized until that class is either reinitialized or no longer referenced. The following example demonstrates two instance variables:
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public class Television { int channel = 0; boolean on = false; void setChannel(int channelValue) { this.channel = channelValue; } int getChannel() { return this.channel; } void setOn(boolean on) { this.on = on; } boolean isOn() { return this.on; } }
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In this example, channel is declared as an int, and on is declared as a boolean. These are both instance variables. It is important to understand that instance variables must be declared in the class, not in a method. The four methods in this class each access one of the instance variables. The setChannel method is used to set the instance variable channel to the value of the int that was passed to it as an argument. The getChannel and isOn methods return the values that are stored in the two instance variables, respectively. Notice that the setOn method has a parameter that is the same name as an instance variable. This is valid code. In a method that has these conditions, if the variable is referenced, it will be the method argument. To reference the instance variable, use the this keyword. The following code segment demonstrates the Television class in use:
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Television tv1 = new Television(); Television tv2 = new Television(); tv1.setChannel(2);
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Understanding Variable Scope
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: tv 1 channel = 2 on = false
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: tv 2 channel = 7 on = false
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Two instances of the tv class
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tv2.setChannel(7); System.out.println("Television channel for tv1: " + tv1.getChannel()); System.out.println("Television channel for tv2: " + tv2.getChannel());
The first two lines of this example create two unique instances of the Television class. When the instance of the class is created, each object gets an instance variable that will store a channel. The next two lines use the setChannel method to set the channel to 2 and 7, respectively. Figure 5-2 represents the two objects that have been created and the value of both of their instance variables. The last two lines of code use the getChannel method to retrieve the value stored in the channel instance variable.
Television channel for tv1: 2 Television channel for tv2: 7
If the code were executed, the preceding would be the output. It is important to remember that each tv object has a unique set of instance variables. The following Scenario & Solution covers each type of variable scope and a likely use for it.
SCenario & SolUTion
What variable scope would be best suited for a counter in a loop What variable scope must be used to store information about the state of an object What variable scope must be used to pass information to a method Local variable Instance variable Method parameter
5:
Understanding Variable Scope and Class Construction
CERTIFICATION OBJECTIVE
Constructing Methods
Exam Objective 4.4 Given an algorithm with multiple inputs and an output, develop method code that implements the algorithm using method parameters, a return type, and the return statement, and recognize the effects when object references and primitives are passed into methods that modify them. This section will explore the construction of methods. Methods are able to accept many different arguments or none at all. They are able to return one variable, but may also not return any data. The SCJA exam will ask questions that expect the test taker to understand how the input and output of a method works. Scenarios for a method will be presented with a list of code segments. The correct code segment must be matched to the given scenario. The following topics are covered in this section:
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