Loops in Java – for, while, do-while, break and continue

Table of Contents

1. Introduction to Loops
2. The for Loop
3. The while Loop
4. The do-while Loop
5. Loop Control Statements: break and continue
6. Infinite Loops
7. Nested Loops
8. Real-World Examples
9. Summary and What’s Next

1. Introduction to Loops

Loops are used in programming to execute a block of code repeatedly until a certain condition is met. Java supports three main types of loops:

• for loop
• while loop
• do-while loop

Each loop type has its specific use case, but all can be used to achieve similar outcomes. Loops help reduce redundancy, automate repetitive tasks, and improve code readability.

2. The for Loop

The for loop is best when you know in advance how many times you want to iterate.

Syntax:

for (initialization; condition; update) {
    // code block to be executed
}

Example:

for (int i = 1; i <= 5; i++) {
    System.out.println("Iteration: " + i);
}

Explanation:

• Initialization: Runs once at the beginning, e.g., int i = 1.
• Condition: Checked before each iteration; loop runs if it’s true.
• Update: Executed after each iteration, e.g., i++.

3. The while Loop

The while loop is used when the number of iterations is not known in advance. It checks the condition before the loop body.

Syntax:

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

Example:

int i = 1;
while (i <= 5) {
    System.out.println("Count: " + i);
    i++;
}

The loop continues as long as i <= 5 is true.

4. The do-while Loop

The do-while loop is similar to the while loop, but it guarantees at least one execution of the loop body, since the condition is evaluated after the loop body.

Syntax:

do {
    // code block to be executed
} while (condition);

Example:

int i = 1;
do {
    System.out.println("Running: " + i);
    i++;
} while (i <= 5);

This loop runs at least once, even if the condition is initially false.

5. Loop Control Statements: break and continue

The break Statement

Used to exit the loop prematurely when a certain condition is met.

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;
    }
    System.out.println(i);
}

This loop prints numbers 1 to 4 and then exits when i becomes 5.

The continue Statement

Skips the current iteration and jumps to the next one.

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }
    System.out.println(i);
}

This loop prints 1, 2, 4, 5 (skips 3).

6. Infinite Loops

An infinite loop keeps running forever unless forcibly terminated. Can be useful in event-driven or server applications.

Example:

while (true) {
    // keep listening for connections
}

Use infinite loops with care, ensuring there’s a break condition or manual stop mechanism.

7. Nested Loops

You can place a loop inside another loop to perform complex iterations like matrix processing or pattern printing.

Example:

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        System.out.println("i = " + i + ", j = " + j);
    }
}

8. Real-World Examples

Printing a Table of a Number

int number = 7;
for (int i = 1; i <= 10; i++) {
    System.out.println(number + " * " + i + " = " + (number * i));
}

Counting User Inputs Until a Sentinel Value

Scanner scanner = new Scanner(System.in);
int input;
do {
    System.out.print("Enter number (0 to quit): ");
    input = scanner.nextInt();
} while (input != 0);

Searching for an Element

int[] arr = {3, 5, 7, 9};
int target = 7;
boolean found = false;

for (int i = 0; i < arr.length; i++) {
    if (arr[i] == target) {
        found = true;
        break;
    }
}
System.out.println("Element found: " + found);

9. Summary and What’s Next

In this module, we learned:

• The structure and use cases of for, while, and do-while loops
• How to control loop flow using break and continue
• Common looping patterns including nested loops and infinite loops

Understanding loops is essential to solving repetitive tasks and optimizing code efficiency.