Parameter Passing in Java
Parameters of a method are specified in the method declaration. When we call a method, we must provide values that match with the types of the parameters. In the following example, the sum method accepts two integers and returns their summation.
public static int sum(int a, int b)
{
return a + b;
}
public static void main(String[] args)
{
int c = sum(1, 2); // c is 3
int x = 3, y = 4;
int z = sum(x, y); // z is 7
}
Parameter variables work as local variables in the method. They are initialized by the values passed from the calling code. But note that, parameter variables are different from the variables in the calling code. This approach is known as pass by value. (In some other languages, there is an alternative approach known as pass by reference, in which parameter variables could be the same variables from the calling code.)
In the next example, the parameter variable a in dummyIncrease is only visible inside the method. Although it is increased by 1, this will affect the variables x or a in the main method; they are difference variables with different memory locations. Note that the parameter variable a in dummyIncrease and the local variable a in main are two different variables.
public static void dummyIncrease(int a)
{
a++;
}
public static void main(String[] args)
{
int x = 3;
dummyIncrease(x); // x is still 3
int a = 4;
dummyIncrease(a); // a is still 4
}
Object variables in parameters are more complicated than primitive variables. Since object variables are essentially references to objects, the values passed to object variables in parameters are references (not objects themselves). In the next example, dummyIncrease accepts two parameters: one is an array, and the other is an integer. Since arrays are objects, the parameter variable x refers to the same array as the variable arr from the calling code. Thus, the increase of the first element of the array is still effective after the method returns.
public static void dummyIncrease(int[] x, int y)
{
x[0]++;
y++;
}
public static void main(String[] args)
{
int[] arr = {3, 4, 5};
int b = 1;
dummyIncrease(arr, b);
// arr[0] is 4, but b is still 1
}
main()
arr +---+ +---+---+---+
| # | ----> | 3 | 4 | 5 |
+---+ +---+---+---+
b +---+ ^
| 1 | |
+---+ |
|
dummyIncrease() |
x +---+ |
| # | ------------+
+---+
y +---+
| 1 |
+---+
As another example, the following code defines a method swap which exchanges two elements of an array.
public static void swap(int[] arr, int i, int j)
{
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
public static void main(String[] args)
{
int[] arr = {3, 4, 5, 6};
swap(arr, 1, 3);
// arr becomes {3, 6, 5, 4}
}