7.10. Passing Objects to Methods

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So far, you have learned how to pass arguments of primitive types and array types to methods. You can also pass objects to methods. Like passing an array, passing an object is actually passing the reference of the object. The following code passes the myCircle object as an argument to the printCircle method:

   public class   TestPassObject {   public static void   main(String[] args) { Circle3 myCircle =   new   Circle3(   5.0   );  printCircle(myCircle);  }   public static void    printCircle(Circle3 c)  { System.out.println(   "The area of the circle of radius "   + c.getRadius() +   " is "   + c.getArea()); } }

Java uses exactly one mode of passing arguments: pass- by-value . In the preceding code, the value of myCircle is passed to the printCircle method. This value is a reference to a Circle3 object.

Let us demonstrate the difference between passing a primitive type value and passing a reference value with the program in Listing 7.6.

Listing 7.6. TestPassObject.java

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 1   public class   TestPassObject { 2  /** Main method */  3   public static void   main(String[] args) { 4  // Create a Circle object with radius 1  5 Circle3 myCircle =   new   Circle3(1); 6 7  // Print areas for radius 1, 2, 3, 4, and 5.  8   int   n =   5   ; 9  printAreas(myCircle, n);  10 11  // See myCircle.radius and times  12 System.out.println(   "\n"   +   "Radius is "   + myCircle.getRadius()); 13 System.out.println(   "n is "   + n); 14 } 15

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 16  /** Print a table of areas for radius */  17   public static void    printAreas(Circle3 c,   int   times)  { 18 System.out.println(   "Radius \t\tArea"   ); 19   while   (times >=   1   ) { 20 System.out.println(c.getRadius() +   "\t\t"   + c.getArea()); 21 c.setRadius(c.getRadius() +   1   ); 22 times ” ”; 23 } 24 } 25 }

The program passes a Circle3 object myCircle and an integer value from n to invoke printAreas(myCircle, n) (line 19), which prints a table of areas for radii 1 , 2 , 3 , 4 , and 5 , as shown in Figure 7.16.

Figure 7.16. The program passes a Circle object myCircle and an integer value n as arguments to the printAreas method, which displays a table of the areas for radii 1 , 2 , 3 , 4 , and 5 .

Figure 7.17 shows the call stack for executing the methods in the program. Note that the objects are stored in a heap.

Figure 7.17. The value of n is passed to times , and the reference of myCircle is passed to c in the printAreas method.

When passing an argument of a primitive data type, the value of the argument is passed. In this case, the value of n ( 5 ) is passed to times . Inside the printAreas method, the content of times is changed; this does not affect the content of n . When passing an argument of a reference type, the reference of the object is passed. In this case, c contains a reference for the object that is also referenced via myCircle . Therefore, changing the properties of the object through c inside the printAreas method has the same effect as doing so outside the method through the variable myCircle .