Compound Assignment Operators

Let's get right to it. The code we saw in our basketball example, score = score + 2;, feels a little clunky. You have to type the variable name score twice. Java gives us a much cleaner way to do this.

Combining Math and Assignment

The line score = score + 2; can be rewritten using the addition assignment operator, +=:

score += 2;

These two lines do the exact same thing! The += operator tells Java:

1. Take the value on the right (2).
2. Add it to the current value of the variable on the left (score).
3. Store the final result back into the variable on the left (score).

Think of it like making a deposit at the bank. If you have $100 in your account and you deposit $20, you don't say "my new balance is my old balance of $100 plus $20." You just say you're adding $20. The += operator is that direct.

This pattern works for all the basic arithmetic operations:

Let's see them in action with a few quick examples:

• Tracking Game Health: Your character in a video game, Marcus, has 100 health points. He takes 15 points of damage.

  
  int health = 100;
  health -= 15; // health is now 85

```

• Calculating a Sale
  An item's price is being doubled for a special promotion, then a coupon is applied.

  double price = 25.50;
  price *= 2;   // price is now 51.00
  price -= 10;  // price is now 41.00

• Integer Division
  Remember how integer division works in Java—it drops the decimal. This is a place students often get tripped up.

  int totalCookies = 25;
  totalCookies /= 4; // 25 divided by 4 is 6.25. As an int, it's 6.
                     // totalCookies is now 6.

• Using the Modulo Operator
  The modulo operator (%) gives you the remainder of a division. It's incredibly useful. Imagine you have 125 cents and you want to find out how many cents are left after you take out all the whole dollars.

  int cents = 125;
  cents %= 100; // 125 divided by 100 is 1 with a remainder of 25.
                // cents is now 25.

The Ultimate Shortcut: Increment and Decrement

The single most common operation in programming is adding or subtracting 1. You might be counting button clicks, loop iterations, or items in a shopping cart.

You could write: count = count + 1;

Or, using our new operator: count += 1;

But it's so common that Java has an even shorter way: the post-increment operator ++.

count++;

All three of these lines accomplish the exact same thing: they increase the value of count by 1. The ++ is just the fastest and most common way to write it.

Similarly, there's a post-decrement operator -- for subtracting 1.

lives--; // is the same as lives = lives - 1;

On the AP exam, you'll only see the post-increment (x++) and post-decrement (x--) versions. This means the ++ or -- comes after the variable. For now, when you see x++; or x--; on a line by itself, just think of it as a simple command: "add 1 to x" or "subtract 1 from x."

Using these operators makes your code cleaner, easier to read for other programmers, and shows you have a solid grasp of Java fundamentals.

Let's walk through a few problems to make sure these concepts are crystal clear. I'll show you my thought process, step-by-step.

int numBoxes = 20;
numBoxes -= 8;
numBoxes /= 2;
numBoxes++;

double itemCost = 50.0;
itemCost /= 2.0;
itemCost += 5.0;

Ready to try a couple on your own? Think through the steps just like we did in the worked examples.

Problem 1: The T-Shirt Inventory

You're tracking inventory for an online store based in Seattle. You start with 50 T-shirts. A new shipment arrives, doubling your stock. Over the day, you sell 15 shirts. Finally, you want to see how many shirts will be left over if you package the rest into boxes that hold 4 shirts each.

Declare an int variable shirts and apply the operations. Then, use the modulo assignment operator (%=) to find the leftovers. What is the final value of shirts after all operations?

Hint: You'll need to use `=,-=, and finally%=`.*

Problem 2: Score Drill

A variable int score = 0; is put through the following sequence. Trace the value of score after each line. What is its final value?

score++;
score += 9;
score--;
score /= 4;

Hint: Remember that integer division drops the decimal.