Java for Loop

In computer programming, loops are used to repeat a block of code. For example, if you want to show a message 100 times, then rather than typing the same code 100 times, you can use a loop.

In Java, there are three types of loops:

• for loop
• while loop
• do...while loop

This tutorial focuses on the for loop.

1. Java for Loop

Java for loop is used to run a block of code for a certain number of times. The syntax of for loop is:

for (initialExpression; testExpression; updateExpression) {
    // body of the loop
}

Here,

1. The initialExpression initializes and/or declares variables and executes only once.
2. The condition is evaluated. If the condition is true, the body of the for loop is executed.
3. The updateExpression updates the value of initialExpression.
4. The condition is evaluated again. The process continues until the condition is false.

Working of for loop in Java with flowchart

graph TD
    A([Start]) --> B[Initialization]
    B --> C{Condition}
    C -- True --> D[Body]
    D --> E[Update]
    E --> C
    C -- False --> F([End])

    style A fill:#f9f,stroke:#333,stroke-width:2px
    style F fill:#f9f,stroke:#333,stroke-width:2px
    style C fill:#ffd700,stroke:#333,stroke-width:2px

Example 1: Display a Text Five Times

// Program to print a text 5 times
class Main {
  public static void main(String[] args) {

    int n = 5;
    // for loop
    for (int i = 1; i <= n; ++i) {
      System.out.println("Java is fun");
    }
  }
}

Output:

Java is fun
Java is fun
Java is fun
Java is fun
Java is fun

Example 2: Display Sum of n Natural Numbers

// Program to find the sum of natural numbers from 1 to 1000.
class Main {
  public static void main(String[] args) {

    int sum = 0;
    int n = 1000;

    // for loop
    for (int i = 1; i <= n; ++i) {
      // body inside for loop
      sum += i;     // sum = sum + i
    }

    System.out.println("Sum = " + sum);
  }
}

Output:

Sum = 500500

2. Java Nested for Loop

A for loop inside another for loop is called a nested for loop. It is often used to work with multidimensional arrays or print patterns.

class Main {
    public static void main(String[] args) {
        // outer loop
        for (int i = 1; i <= 3; ++i) {
            // inner loop
            for (int j = 1; j <= 3; ++j) {
                System.out.println(i + " " + j);
            }
        }
    }
}

Output:

1 1
1 2
1 3
2 1
...
3 3

3. Java Infinite for Loop

If we set the test expression in such a way that it never evaluates to false, the for loop will run forever. This is called infinite for loop.

// Infinite for Loop
class Infinite {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; --i) {
            System.out.println("Hello");
        }
    }
}

Here, the test expression i <= 10 is never false because i keeps decreasing.

Key Takeaways

• Structure: for (initialization; condition; update).
• Use Case: Best when you know exactly how many times to loop (e.g., iterating an array).
• Nested Loops: Use loops inside loops for grids or matrices.

Common Pitfalls

[!WARNING] Off-by-One: Looping from 0 to <= length instead of < length is a common error that causes "Index Out of Bounds" exceptions.

[!WARNING] Infinite Loops: If your condition never becomes false (e.g., i never increments), your program will freeze.

Challenge