5.3.1Call Stacks

FIGURE 5.3 When the max method is invoked, the flow of control transfers to the max method. Once the max method is finished, it returns control back to the caller.

6System.out.print("The grade is ");

10public static void printGrade(double score) {

11if (score >= 90.0) {

12System.out.println('A');

13}

14else if (score >= 80.0) {

15System.out.println('B');

16}

17else if (score >= 70.0) {

18System.out.println('C');

19}

20else if (score >= 60.0) {

21System.out.println('D');

22}

23else {

24System.out.println('F');

25}

26}

27}

The grade is C

The grade is F

5.5 Passing Parameters by Values 163

and prints "Hello" three times. However, the statement nPrintln(3, "Hello") would be wrong. The data type of 3 does not match the data type for the first parameter, message, nor does the second parameter, "Hello", match the second parameter, n.

Caution

The arguments must match the parameters in order, number, and compatible type, as defined in the method signature. Compatible type means that you can pass an argument to a parameter without explicit casting, such as passing an int value argument to a double value parameter.

When you invoke a method with a parameter, the value of the argument is passed to the para-

meter. This is referred to as pass-by-value. If the argument is a variable rather than a literal pass-by-value value, the value of the variable is passed to the parameter. The variable is not affected, regard-

less of the changes made to the parameter inside the method. As shown in Listing 5.4, the value of x (1) is passed to the parameter n to invoke the increment method (line 5). n is incremented by 1 in the method (line 10), but x is not changed no matter what the method does.

Listing 5.5 gives another program that demonstrates the effect of passing by value. The program creates a method for swapping two variables. The swap method is invoked by passing two arguments. Interestingly, the values of the arguments are not changed after the method is invoked.

LISTING 5.5 TestPassByValue.java

1 public class TestPassByValue {

2/** Main method */

164 Chapter 5 Methods

13

14System.out.println("After invoking the swap method, num1 is " +

15num1 + " and num2 is " + num2);

16}

17

18/** Swap two variables */

19public static void swap(int n1, int n2) {

20System.out.println("\tInside the swap method");

21System.out.println("\t\tBefore swapping n1 is " + n1

22+ " n2 is " + n2);

23

24// Swap n1 with n2

25int temp = n1;

26n1 = n2;

27n2 = temp;

28

29System.out.println("\t\tAfter swapping n1 is " + n1

30+ " n2 is " + n2);

31}

32}

Before the swap method is invoked (line 12), num1 is 1 and num2 is 2. After the swap method is invoked, num1 is still 1 and num2 is still 2. Their values have not been swapped. As shown in Figure 5.4, the values of the arguments num1 and num2 are passed to n1 and n2, but n1 and n2 have their own memory locations independent of num1 and num2. Therefore, changes in n1 and n2 do not affect the contents of num1 and num2.

Another twist is to change the parameter name n1 in swap to num1. What effect does this have? No change occurs, because it makes no difference whether the parameter and the argument have the same name. The parameter is a variable in the method with its own memory space. The variable is allocated when the method is invoked, and it disappears when the method is returned to its caller.

The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2.

FIGURE 5.4 The values of the variables are passed to the parameters of the method.