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

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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

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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.

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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++.

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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.

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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.

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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).

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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.

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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);
    }
}

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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);

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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.