Lesson 21. Guide to Using the For Loop in Java | Learn Basic Java

What is the for Loop?

In programming, loops are an essential technique that allows you to execute a block of code repeatedly without having to write it multiple times. Java provides several types of loops, and the for loop is one of the most commonly used.

The for loop is used when you know in advance how many times the loop should run, making your code cleaner, more readable, and easier to maintain.

Syntax of the for Loop

The general syntax of the for loop in Java is as follows:

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

Where:

• initialization: Initializes the loop control variable (executed only once before the loop starts).
• condition: Checked before each iteration; if true, the loop continues; if false, the loop stops.
• update: Updates the loop control variable after each iteration.

Simple Example:

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

Output:

Iteration: 1
Iteration: 2
Iteration: 3
Iteration: 4
Iteration: 5

In this example, the loop starts with i = 1, checks the condition i <= 5, and increments i by 1 after each iteration until the condition becomes false.

Variations of the for Loop

Using Multiple Control Variables

You can declare multiple control variables in the for loop, as long as they have the same data type.

public class MultiVariableForLoop {
    public static void main(String[] args) {
        for (int i = 1, j = 10; i <= 5; i++, j--) {
            System.out.println("i = " + i + ", j = " + j);
        }
    }
}

Output:

i = 1, j = 10
i = 2, j = 9
i = 3, j = 8
i = 4, j = 7
i = 5, j = 6

Nested Loops

A nested loop is a loop inside another loop. The inner loop runs completely for each iteration of the outer loop.

Example: Printing the Multiplication Table Using the for Loop

public class MultiplicationTable {
    public static void main(String[] args) {
        for (int i = 2; i <= 9; i++) { // Outer loop: Iterates through tables from 2 to 9
            System.out.println("Multiplication Table of " + i + ":");
            for (int j = 1; j <= 10; j++) { // Inner loop: Multiplies from 1 to 10
                System.out.println(i + " x " + j + " = " + (i * j));
            }
            System.out.println(); // Adds a blank line between tables
        }
    }
}

Output:

Multiplication Table of 2:
2 x 1 = 2
2 x 2 = 4
...
2 x 10 = 20

Multiplication Table of 3:
3 x 1 = 3
3 x 2 = 6
...
3 x 10 = 30

Explanation:

• The outer loop (i) iterates through numbers from 2 to 9, representing different multiplication tables.
• The inner loop (j) runs from 1 to 10, performing multiplication for each number.
• Each multiplication table is printed separately with a blank line in between.

Nested loops are very useful when working with matrices, tables, or multi-level data processing!

Omitting One or More Components

You can omit the initialization, condition, or update in the for loop.

Omitting Initialization

If the loop control variable is declared beforehand, you can skip the initialization part.

public class SkipInitialization {
    public static void main(String[] args) {
        int i = 1;
        for (; i <= 5; i++) {
            System.out.println("i = " + i);
        }
    }
}

Omitting the Update Step

You can update the loop control variable inside the loop body instead of in the update section.

public class SkipUpdate {
    public static void main(String[] args) {
        for (int i = 1; i <= 5;) {
            System.out.println("i = " + i);
            i++; // Update inside the loop body
        }
    }
}

Infinite for Loop

If you omit the condition, the loop will run indefinitely.

public class InfiniteLoop {
    public static void main(String[] args) {
        for (;;) {
            System.out.println("Infinite loop!");
        }
    }
}

Note: You should include a stopping condition (e.g., break) to prevent the loop from running forever.

Iterating Through an Array Using the for Loop

The for loop is commonly used to iterate through array elements.

public class ArrayIteration {
    public static void main(String[] args) {
        int[] numbers = {2, 4, 6, 8, 10};

        for (int i = 0; i < numbers.length; i++) {
            System.out.println("Element at index " + i + ": " + numbers[i]);
        }
    }
}

The for-each Loop (Enhanced For Loop)

Java provides the for-each loop, which simplifies iterating through arrays or collections without needing an index variable.

Syntax of the for-each Loop

for (DataType variable : collection) {
    // Code block to be executed
}

Example:

public class ForEachExample {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};

        for (int num : numbers) {
            System.out.println("Value: " + num);
        }
    }
}

Output:

Value: 1
Value: 2
Value: 3
Value: 4
Value: 5

• Advantages of for-each: Clean, easy to read, and avoids index-related errors.
• Disadvantages: You cannot access the index of the current element.

Using for with break and continue

Using break to Exit the Loop Early

public class BreakExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            if (i == 6) {
                break; // Stops the loop when i == 6
            }
            System.out.println(i);
        }
    }
}

Output:

1
2
3
4
5

Using continue to Skip an Iteration

public class ContinueExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            if (i == 3) {
                continue; // Skips i == 3
            }
            System.out.println(i);
        }
    }
}

Output:

1
2
4
5

Conclusion

The for loop is a powerful tool for repeating a block of code in a controlled manner. You can use various variations of the for loop, combined with break and continue, to optimize your program.

Next Lesson: Lesson 22. How to Use the for-each Loop in Java