Loops

While Loop

The while loop loops through a block of code as long as a specified condition is true:

while (condition) {
  // Code block to be executed
}

In this example, the code in the loop will run again and again, as long as variable i is still less than 10:

int i = 0;

while (i < 10) {
  System.out.println(i);
  i = i + 2;
}

The output would be:

0
2
4
6
8

For Loop

A for loop iterates over a range of values. Its declaration is made up of the following three parts, each separated by a semicolon:

1. The initialization of the loop control variable.
2. A conditional expression.
3. An expression that modifies the loop control variable.

In this example, the loop control variable starts at 3 and decrements. The loop will continue iterating until the conditional expression is no longer true:

for (int i = 3; i > 0; i--) {
  System.out.println(i);
}

System.out.println("Liftoff!");

The output would be:

3
2
1
Liftoff!

For-Each Loop

In Java, the for-each loop allows you to directly loop through each item in an array or ArrayList and perform some action with each item.

When creating a for-each loop, you must include the for keyword and two expressions inside of parentheses, separated by a colon. These include:

• The data type and handle for an element we’re currently iterating over.
• The source array or ArrayList we’re iterating over.

// Array of numbers
int[] numbers = {1, 2, 3, 4, 5};

// For-each loop that prints each number in numbers
// int num is the handle while numbers is the source array
for (int num : numbers) {
    System.out.println(num);
}