Declarations, Initialization, and Scoping - SCJP Sun Certified Programmer for Java Platform SE6 - page 87

Until x is assigned a value, it cannot appear within an expression, and the compiler will
gladly remind you of this rule.
The following Mouse class is another example of using local variables. Examine the code
and see if you can distinguish the local variables from the instance variables. Does the
Mouse class compile successfully?
1. public class Mouse {
2. public boolean hasWheel;
3. private int clickCount;
4.
5. public int rightClick(double d) {
6. int response = (int) d;
7. return response;
8. }
9.
10. public String wheelClick() {
11. if(hasWheel) {
12. double pi = 3.14159;
13. String greeting = “The mouse ate the “ + pi;
14. return greeting;
15. } else {
16. String error = “No wheel found”;
17. return error;
18. }
19. }
20.
21. public void leftClick(int clickCount) {
22. System.out.println(“Left click “ + clickCount + “ times”);
23. this.clickCount = clickCount;
24. }
25.}
Although there may be some confusion about clickCount in the leftClick method, this
class compiles fi ne. The Mouse class has two instance variables: hasWheel and clickCount .
The rightClick method has two local variables: d and response . When the rightClick
method is invoked, d and response get allocated in memory. When response is returned on
line 7, a copy of response is sent to the calling method and both d and response go out of scope.
The wheelClick method has three local variables: pi , greeting , and error . If hasWheel
is true, this method returns greeting , at which point pi and greeting go out of scope.
The String object “The mouse ate the 3.14159” is on the heap, as shown in Figure 2.2,
so it is not destroyed when the method returns. The same scenario happens when error is
returned: error goes out of scope but the String “No wheel found” is on the heap and still
exists (for as long as it is reachable by a reference).