Declarations, Initialization, and Scoping - SCJP Sun Certified Programmer for Java Platform SE6 - page 88

FIGURE 2.2 The local variable greeting points to an object on the heap.

Call stack memory
for wheelClick.

Heap Memory

pi

3.14159

greeting

“The mouse ate the 3.14159”

The String object remains on the
heap and wheelClick
returns a reference to it.

pi and greeting go out of
scope when wheelClick returns.

On line 21, the leftClick method has one local variable: clickCount . The clickCount
parameter just happens to match the identifi er of the clickCount fi eld. In these situations,
the local variable is seen fi rst by the method and you must use the this reference to
distinguish between the instance and local variable. The clickCount displayed on line 22 is
the value of the parameter. To assign the clickCount parameter to the clickCount fi eld, we
must use this.clickCount on line 23 to refer to the fi eld.
Examine the following statements and try to determine the output:

4. Mouse m = new Mouse();
5. m.clickCount = 2;
6. System.out.println(m.wheelClick());
7. m.leftClick(1);
8. System.out.println(m.clickCount);

The fi eld hasWheel initializes to false , so calling wheelClick on line 6 causes “No
wheel found” to be returned. Calling leftClick with 1 as the argument causes the 1 to be
displayed and also assigned to the fi eld clickCount . Therefore, the output is

No wheel found
Left click 1 times
1

Declaring Arrays

The exam objectives state that you should be able to “develop code that declares, initializes,
and uses arrays.” An array is a contiguous chunk of memory on the heap representing
a fi xed-size collection of values that all have the same data type. An array in Java is an