Overview of This Lesson

You're already familiar with the assignment operator (=) that we use to assign values to variables. There are also some augmented assignment operators that you can use to perform simple mathematical operations and assign the results, using a single operator.

Pre-Requisites

Before starting this tutorial, make sure you've completed Expressions and Statements and Arithmetic Expressions.

Augmented Assignment Operators

There are some special assignment operators you can use as shortcuts to statements that need to perform a simple mathematical operation and assign the result to a variable. To understand how these can be used, look at the program listing below. What is the output of this program? What is the value of the variable sum during execution of this program?

public class  AssignOps {
public static void main(String[] args) {

        int sum = 10;

        System.out.println("The value stored in sum is " + sum);
        sum = sum + 20;
        System.out.println("The value now stored in sum is " + sum);
    }
}

This small program demonstrates a situation where you can use the special += augmented assignment operator. This operator, called the "plus-equals" operator, adds the operand on the right to the operand on the left. If you rewrote the program using the += operator, you'd have:

public class  AssignOps {
    public static void main(String[] args) {

        int sum = 10;

        System.out.println("The value stored in sum is " + sum);
        sum += 20;
        System.out.println("The value now stored in sum is " + sum);
    }
}

There are also similar operators to perform subtraction, multiplication, division, and modulus:

Note that the augmented assignment operators will not work if the left-hand operand has not been initialized to a value. For example, the following is invalid and will not compile:

int x;
x += 2;

To fix this example, make sure the variable x is initialized to a value:

int x = 0;
  x += 2;

Exercises

1. Examine the code listing below. What do you think the output should be? Write this down. Then run the program and see if you're correct.

public class Question1
{
    public static void main(String[] args)
    {
        int factor = 2;
        int sum = 10;
        System.out.println("sum is " + sum);
        sum *= factor;
        System.out.println("sum is now " + sum);
        sum *= factor;
        System.out.println("sum is now " + sum);
        sum *= factor;
        System.out.println("sum is now " + sum);
    }
}

sum is 10
sum is now 20
sum is now 40
sum is now 80

2. How do you think the output would change if you wrote the program in question 2 like this:

public class Question2
{
    public static void main(String[] args)
    {
        int factor = 2;
        int sum = 10;
        System.out.println("sum is " + sum);
        sum *= factor;
        sum *= factor;
        sum *= factor;
        System.out.println("sum is now " + sum);
        System.out.println("sum is now " + sum);
        System.out.println("sum is now " + sum);
      }
}

sum is 10
sum is now 80
sum is now 80
sum is now 80

int counter = 1;
int increment = 2;
System.out.print(counter + " ");
counter += increment;
System.out.print(counter + " ");
counter *= increment;
System.out.print(counter + " ");
increment /= 2;
counter -= increment;
System.out.println(counter);
System.out.println("increment: " + increment);

1 3 6 5
increment: 1