Primitive variables

Every Java variable is one of two kinds, and the kind decides what = copies, what a method can change, and what == compares. This module is about the difference, starting with the simpler kind.

The primitive types

Java has eight primitive types. Four of them you've used all along:

You'll rarely need the other four: long holds whole numbers too big for an int, float is a less precise decimal number, and byte and short hold whole numbers with small ranges. All eight names start with a small letter.

Every other type is a reference type: String, Scanner, ArrayList, arrays such as int[], and every class you write. Those are the next step.

A primitive variable holds its value

A primitive variable holds the value itself. Picture a box with a number in it. Assigning one variable to another copies the value into the other box, and after that the two boxes have nothing to do with each other:

int planned = 12;
int done = planned;
done = done - 5;
planned++;
System.out.println("Planned: " + planned);
System.out.println("Done: " + done);

Planned: 13
Done: 7

• int done = planned; copies the value 12 into done. It doesn't tie done to planned in any way.
• Changing done doesn't touch planned, and changing planned doesn't touch done.

The same goes for a value worked out from other variables. int total = a + b; stores the sum as it is at that moment, and if a changes later, total keeps the old sum.

Swapping two values

Assigning puts a new value in the box, and the old one is gone. That's why swapping the values of two variables takes three steps and a third variable:

int left = 3;
int right = 8;
int temp = left;
left = right;
right = temp;
System.out.println(left + " " + right);

8 3

With only left = right; and right = left;, the first line overwrites the 3, and both variables end up 8. temp keeps a copy of the old value until it's needed.