Java Lesson — For Loops in Detail

Why do we need loops?

Suppose you want to print the word Hello five times. Without a loop, you would write the same line again and again.

System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");
System.out.println("Hello");

This works, but it is repetitive, long, and hard to maintain. A loop solves this problem by repeating a block of code automatically.

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

What is a for loop?

A for loop is used when you know how many times a block of code should run, or when the loop follows a clear counting pattern.

for (initialization; condition; update) {
    // code to repeat
}

This loop has three important parts:

• Initialization: runs once at the beginning.
• Condition: checked before each repetition.
• Update: runs after each repetition.

Basic syntax explained

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

So this loop prints 1, 2, 3, 4, and 5.

Step-by-step flow of a for loop

Let us understand exactly how Java runs a for loop.

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

1. Java runs int i = 1 once.
2. It checks i <= 3. This is true.
3. It executes the body and prints i = 1.
4. It runs i++. Now i becomes 2.
5. It checks the condition again.
6. The loop continues until the condition becomes false.

Printing numbers with for loops

From 1 to 10

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

From 10 down to 1

for (int i = 10; i >= 1; i--) {
    System.out.println(i);
}

Even numbers from 2 to 20

for (int i = 2; i <= 20; i += 2) {
    System.out.println(i);
}

Odd numbers from 1 to 19

for (int i = 1; i < 20; i += 2) {
    System.out.println(i);
}

Using for loops for calculations

Sum from 1 to 5

int sum = 0;

for (int i = 1; i <= 5; i++) {
    sum = sum + i;
}

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

Here the loop adds each value of i into sum.

Factorial of a number

int n = 5;
int fact = 1;

for (int i = 1; i <= n; i++) {
    fact = fact * i;
}

System.out.println("Factorial = " + fact);

Count digits from repeated division idea

int num = 12345;
int count = 0;

for (; num != 0; num = num / 10) {
    count++;
}

System.out.println("Digits = " + count);

Multiplication table using for loop

int n = 7;

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

This is one of the most common and useful beginner examples.

Using a for loop with strings

You can use a for loop to go through each character of a string.

String word = "JAVA";

for (int i = 0; i < word.length(); i++) {
    System.out.println(word.charAt(i));
}

This prints J, A, V, and A one by one.

Using a for loop with arrays

int[] arr = {10, 20, 30, 40, 50};

for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

Here i is used as an index. It starts at 0 because array positions begin from 0.

Sum of array elements

int[] arr = {10, 20, 30, 40, 50};
int sum = 0;

for (int i = 0; i < arr.length; i++) {
    sum += arr[i];
}

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

Nested for loops

A nested for loop means a for loop inside another for loop. This is extremely useful for patterns, tables, grids, and matrix-like work.

Simple nested loop

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

For each value of i, the inner loop runs completely.

Multiplication grid

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.print((i * j) + "\t");
    }
    System.out.println();
}

Pattern printing with for loops

Pattern 1: stars in one row

for (int i = 1; i <= 5; i++) {
    System.out.print("*");
}

Pattern 2: square of stars

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= 4; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

Pattern 3: right triangle

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("* ");
    }
    System.out.println();
}

Pattern 4: number triangle

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

break in for loops

The break statement stops the loop immediately.

for (int i = 1; i <= 10; i++) {
    if (i == 6) {
        break;
    }
    System.out.println(i);
}

This prints 1 to 5 only, because the loop stops when i becomes 6.

continue in for loops

The continue statement skips the current iteration and moves to the next one.

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

This prints all numbers from 1 to 10 except 5.

Infinite for loop

If you remove the condition or make it always true, the loop may run forever.

for (;;) {
    System.out.println("Running forever");
}

This is called an infinite loop. It can be useful in special cases, but beginners should use it carefully.

Different valid forms of for loop

Initialization outside the loop

int i = 1;

for (; i <= 5; i++) {
    System.out.println(i);
}

Update inside the loop body

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

Multiple variables

for (int i = 1, j = 10; i <= 5; i++, j--) {
    System.out.println(i + " " + j);
}

Scope of loop variables

A variable declared in the loop header usually exists only inside that loop.

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

// System.out.println(i); // not allowed here

If you need the variable after the loop, declare it before the loop.

int i;
for (i = 1; i <= 5; i++) {
    System.out.println(i);
}
System.out.println("After loop, i = " + i);

Common mistakes with for loops

• Using the wrong condition, causing the loop to run too many or too few times.
• Forgetting to update the loop variable, causing an infinite loop.
• Using <= when < is needed for arrays.
• Changing the loop variable in confusing ways inside the body.
• Using the wrong starting value.

Array mistake example

int[] arr = {10, 20, 30};

// Wrong:
// for (int i = 0; i <= arr.length; i++) {
//     System.out.println(arr[i]);
// }

This is wrong because the last valid index is arr.length - 1. The correct condition is i < arr.length.

When should you use a for loop?

• When you know how many times something should repeat.
• When you need counting logic.
• When working with array indices.
• When building tables and patterns.

If the number of repetitions is unknown and depends on a condition, a while loop may sometimes be a better fit. But for counting and indexed work, for loops are excellent.

Complete example from beginner to advanced

public class Main {
    public static void main(String[] args) {
        // 1. Basic counting
        for (int i = 1; i <= 5; i++) {
            System.out.println("Count: " + i);
        }

        // 2. Sum
        int sum = 0;
        for (int i = 1; i <= 5; i++) {
            sum += i;
        }
        System.out.println("Sum = " + sum);

        // 3. Table of 3
        for (int i = 1; i <= 10; i++) {
            System.out.println("3 x " + i + " = " + (3 * i));
        }

        // 4. Nested loop pattern
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }

        // 5. Continue example
        for (int i = 1; i <= 5; i++) {
            if (i == 3) continue;
            System.out.println("After continue check: " + i);
        }

        // 6. Break example
        for (int i = 1; i <= 10; i++) {
            if (i == 6) break;
            System.out.println("Before break point: " + i);
        }
    }
}

Assignments

Interactive MCQ Quiz

Choose one answer for each question and click Check Answers.

Quick revision

• A for loop is best when repetition follows a counting pattern.
• A for loop has initialization, condition, and update.
• The condition is checked before every iteration.
• Nested for loops are useful for patterns and tables.
• break stops the loop immediately.
• continue skips the current iteration.
• Array loops usually use i < arr.length.