Integer vs floating point division
Java uses the same operator, / for both integer and floating point (decimal, double) division. When at least 1 of the operands is of type double, the operator performs floating point division. When both operands are of type int, the operator performs integer division.
The result of integer divisions is an int. Any decimal portion of the quotient is discarded.
Examples
System.out.println(25 / 2); // prints 12
System.out.println(25.0 / 2); // prints 12.5
System.out.println(25 / 2.0); // prints 12.5
System.out.println(25.0 / 2.0); // prints 12.5
int a = 25;
int b = 2;
double c = 2; // stored as 2.0
System.out.println(a / b); // prints 12
System.out.println((double) (a / b)); // prints 12.0
// Each line below prints 12.5
System.out.println(a / c);
System.out.println((double) a / b);
System.out.println(a / (double) b);
System.out.println((double) a / (double) b);
The most commonly misunderstood example is (double) (a / b). This expression evaluates to 12.0. The division happens before the cast. Both operands are of type int so integer division is peformed. The result of integer division is cast to a double.
Modular division
Modular division in Java uses the % symbol. When both operands are of type int and are non-negative (>= 0), modular division returns the remainder from integer division.
(Modular division with floating point values and negative values is also possible. Many other resources discuss the behavior.)
Examples
System.out.println(22 % 5); // prints 2
System.out.println(20 % 5); // prints 0
System.out.println(20 % 20); // prints 0
System.out.println(45 % 1234); // prints 45
System.out.println(0 % 20); // prints 0
System.out.println(20 % 0); // throws ArithmeticException
When a number is modularly divided by a larger number, the result is the original (smaller) number.
As with regular division, 0 can be the left operand for modular division but it cannot be the right operand. Modularly dividing by 0 results in an ArithmeticException.
Determining divisiblity
Modular division has many uses, including determining if a number is divisible by another number. The expression a % b == 0 evaluates to true if and only if a is divisible by b.
Wrapping a value
int val = (int) (Math.random() * 26); // 0 <= val <= 25
int k = (int) (Math.random() * 100); // 0 <= k <= 99
int newVal = (val + k) % 26; // 0 <= newVal <= 25
In the example above, newVal will never exceed 25. This technique is commonly used to wrap a value when it may exceed a maximum.
val |
k |
val + k |
(val + k) % 26 |
|---|---|---|---|
23 |
1 |
24 |
24 |
23 |
2 |
25 |
25 |
23 |
3 |
26 |
0 |
23 |
4 |
27 |
1 |