• Learn about compound assignment operators.
• Show how they work within memory using the browser-based Java Visualizer.

A compound assignment operator combines both an arithmetic operator, like the + operator and the - operator, with the assignment operator =. It is a shortcut way of writing certain expressions. Consider the following table of compound assignment operators:

These operators can be applied to numerical data types, like int and double. Let's look at the increment and decrement operators first. But to help us, consider the following variables when reading about each operator:

int firstNumber = 20;
int secondNumber = 10;

• ++ is the increment operator. The increment operator is a unary operator and increases the value of the single operand by 1:
  • firstNumber++ returns 21.
  • Notice that the increment operator is placed directly after the variable name with no space between the variable name and the operand.
• -- is the decrement operator. The decrement operator is a unary operator and decreases the value of the single operand by 1:
  • secondNumber-- returns 9
  • Notice that the decrement operator is placed directly after the variable name with no spaces between the variable name and the operand.

Note that both the increment and decrement operators operate on the operand and assign it the new value. For example:

firstNumber++; 

Has the exact same result as:

firstNumber = firstNumber + 1; 

Another thing we should note about the increment and decrement operators is that the operator could be placed directly before or after the variable. In the examples above, we placed the operators after the variable. But we could have also done this:

++firstNumber;
--secondNumber;

When we place the operator before the variable, it will still have the same result as before: ++firstNumber will return 21 and --secondNumber will return 9. But order still matters! Let's take a look at an example of when the placement of the increment or decrement operator would make a difference:

int result = ++firstNumber;
System.out.println(result);

// Reset firstNumber to 20
firstNumber = 20;
result = firstNumber++;
System.out.println(result);

Now let's look at the output of the code above:

21
20

So why are the outputs different? When the first line is run with the ++ preceding the variable, Java will first perform the operation of firstNumber = firstNumber + 1; before assigning the value of firstNumber to result. The next time when the increment operator is used is in the format of firstNumber++. When the ++ follows the variable, Java will access the value of firstNumber before completing the increment. This is because:

result = firstNumber++;

is equivalent to

result = firstNumber;
firstNumber = firstNumber + 1;

Therefore, result will be assigned the value 20 before firstNumber is incremented.

To avoid these types of issues, it is best to not combine the assignment operator, =, with the increment or decrement operators in the same statement.

Now let's discuss the other compound assignment operators:

• += is the addition compound assignment operator. It is a binary operator where the value of the expression on the right-hand side will be added to the variable on the left-hand side of the operator and then stored in that variable. Consider the example:

  • firstNumber += secondNumber; is the same as firstNumber = firstNumber + secondNumber;

  • In the example above, the addition compound operator will first add the value of secondNumber to the value stored in firstNumber and then re-assign the result of that expression to firstNumber. So after the statement has been executed,firstNumber will have a value of 30.

  • The addition compound assignment operator is an abbreviated statement of variable = variable + right-hand expression.

  • To see this played out in memory, consider the following:

    <iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=public%20class%20Main%20%7B%0A%20%20%20%20public%20static%20void%20main%28String%5B%5D%20args%29%20%7B%0A%20%20%20%20%20%20int%20firstNumber%20%3D%2020%3B%0A%20%20%20%20%20%20int%20secondNumber%20%3D%2010%3B%0A%20%20%20%20%20%20%0A%20%20%20%20%20%20firstNumber%20%2B%3D%20secondNumber%3B%0A%20%20%20%20%20%20System.out.println%28firstNumber%29%3B%0A%20%20%20%20%7D%0A%7D&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=3&heapPrimitives=nevernest&origin=opt-frontend.js&py=java&rawInputLstJSON=%5B%5D&textReferences=false"> </iframe>

• -= is the subtraction compound assignment operator. It is a binary operator where the value of the expression on the right-hand side will be subtracted from the variable on the left-hand side of the operator and then stored in that variable. Consider the example:

  • firstNumber -= secondNumber; is the same as firstNumber = firstNumber - secondNumber;

  • In the example above, the subtraction compound operator will first subtract the value of secondNumber from the value stored in firstNumber and then re-assign the result of that expression to firstNumber. So after the statement has been executed,firstNumber will have a value of 10.

  • The subtraction compound assignment operator is an abbreviated statement of variable = variable - right-hand expression.

  • To see this played out in memory, consider the following:

    <iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=public%20class%20Main%20%7B%0A%20%20%20%20public%20static%20void%20main%28String%5B%5D%20args%29%20%7B%0A%20%20%20%20%20%20int%20firstNumber%20%3D%2020%3B%0A%20%20%20%20%20%20int%20secondNumber%20%3D%2010%3B%0A%20%20%20%20%20%20%0A%20%20%20%20%20%20firstNumber%20-%3D%20secondNumber%3B%0A%20%20%20%20%20%20System.out.println%28firstNumber%29%3B%0A%20%20%20%20%7D%0A%7D&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=3&heapPrimitives=nevernest&origin=opt-frontend.js&py=java&rawInputLstJSON=%5B%5D&textReferences=false"> </iframe>

• *= is the multiplication compound assignment operator. It is a binary operator that multiples the value of the expression on the right-hand side with the variable on the left-hand side of the operator and then stores the result in that variable. Consider the example:

  • firstNumber *= secondNumber; is the same as firstNumber = firstNumber * secondNumber;

  • In the example above, the multiplication compound operator will first multiply the value of secondNumber with the value stored in firstNumber and then re-assign the product of that expression to firstNumber. So after the statement has been executed,firstNumber will have a value of 200.

  • The multiplication compound assignment operator is an abbreviated statement of variable = variable * right-hand expression.

  • To see this played out in memory, consider the following:

    <iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=public%20class%20Main%20%7B%0A%20%20%20%20public%20static%20void%20main%28String%5B%5D%20args%29%20%7B%0A%20%20%20%20%20%20int%20firstNumber%20%3D%2020%3B%0A%20%20%20%20%20%20int%20secondNumber%20%3D%2010%3B%0A%20%20%20%20%20%20%0A%20%20%20%20%20%20firstNumber%20*%3D%20secondNumber%3B%0A%20%20%20%20%20%20System.out.println%28firstNumber%29%3B%0A%20%20%20%20%7D%0A%7D&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=3&heapPrimitives=nevernest&origin=opt-frontend.js&py=java&rawInputLstJSON=%5B%5D&textReferences=false"> </iframe>

• /= is the division compound assignment operator. It is a binary operator that divides the variable on the left-hand side of the operator with the value of the expression on the right-hand side of the operator. It then will store the result in the variable on the left-hand side. Consider the example:

  • firstNumber /= secondNumber; is the same as firstNumber = firstNumber / secondNumber;

  • In the example above, the division compound operator will first perform the division of firstNumber / secondNumber and then re-assign the quotient of that expression to firstNumber. So after the statement has been executed, firstNumber will have a value of 2.

  • The division compound assignment operator is an abbreviated statement of variable = variable / right-hand expression.

  • To see this played out in memory, consider the following:

    <iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=public%20class%20Main%20%7B%0A%20%20%20%20public%20static%20void%20main%28String%5B%5D%20args%29%20%7B%0A%20%20%20%20%20%20int%20firstNumber%20%3D%2020%3B%0A%20%20%20%20%20%20int%20secondNumber%20%3D%2010%3B%0A%20%20%20%20%20%20%0A%20%20%20%20%20%20firstNumber%20/%3D%20secondNumber%3B%0A%20%20%20%20%20%20System.out.println%28firstNumber%29%3B%0A%20%20%20%20%7D%0A%7D&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=3&heapPrimitives=nevernest&origin=opt-frontend.js&py=java&rawInputLstJSON=%5B%5D&textReferences=false"> </iframe>

• %= is the modulus compound assignment operator. It is a binary operator that takes the variable on the left-hand side of the operator mod the value of the expression the right-hand side of the operator. Remember, the modulus or mod operator will return the remainder value of dividing two numbers. Once the expression has been evaluated, it will be stored in the variable on the left-hand side of the operator. Consider the example:

  • firstNumber %= secondNumber; is the same as firstNumber = firstNumber % secondNumber;

  • In the example above, the modulus compound operator will first perform firstNumber % secondNumber and then re-assign the result of that expression to firstNumber. So after the statement has been executed, firstNumber will have a value of 0.

    • It is 0 because 20 / 10 is 2 with no (or 0) remainder.

  • The modulus compound assignment operator is an abbreviated statement of variable = variable % right-hand expression.

  • To see this played out in memory, consider the following:

    <iframe width="800" height="500" frameborder="0" src="https://pythontutor.com/iframe-embed.html#code=public%20class%20Main%20%7B%0A%20%20%20%20public%20static%20void%20main%28String%5B%5D%20args%29%20%7B%0A%20%20%20%20%20%20int%20firstNumber%20%3D%2020%3B%0A%20%20%20%20%20%20int%20secondNumber%20%3D%2010%3B%0A%20%20%20%20%20%20%0A%20%20%20%20%20%20firstNumber%20%25%3D%20secondNumber%3B%0A%20%20%20%20%20%20System.out.println%28firstNumber%29%3B%0A%20%20%20%20%7D%0A%7D&codeDivHeight=400&codeDivWidth=350&cumulative=false&curInstr=3&heapPrimitives=nevernest&origin=opt-frontend.js&py=java&rawInputLstJSON=%5B%5D&textReferences=false"> </iframe>

• Java, How To Program (Early Objects), Tenth Edition