Loops in Java

 

Loops in Java

Java - Loops

Introduction

In Java, code execution generally follows a sequential flow, with each block executing once. However, there are scenarios where we need to repeat a particular block multiple times. Instead of rewriting the same code, we use loops, which allow us to execute a block of code multiple times efficiently.

Java provides three types of loops:

• while loop
• do-while loop
• for loop
  
  

1. While Loop

The while loop in Java continuously executes a block of code as long as the given condition remains true. It is similar to Python's while loop, but Java uses curly braces {} instead of indentation.

Syntax:

while (condition) {
    // loop body
}

Example:

class Main {
    public static void main(String[] args) {
        int num = 4;
        int counter = 0;
        while (counter < 3) {
            num++;
            System.out.println(num);
            counter++;
        }
    }
}

Output:

5
6
7

In this example, the loop runs three times, incrementing num and printing its value.

Common Mistakes in While Loop:

Incorrect Termination Condition :

class Main {

    public static void main(String[] args) {

        int num = 4;

        int counter = 0;

        boolean condition = (counter < 3);

        while (condition) {

            num++;

            System.out.println(num);

            counter++;

        }

    }

}

Error: Infinite loop because the condition variable does not update within the loop.

Not Updating Counter Variable :

class Main {

    public static void main(String[] args) {

        int n = 4;

        int counter = 0;

        while (counter < 3) {

            n++;

            System.out.println(n);

        }

    }

}

Error: Infinite loop as the counter variable is not incremented, causing the condition to always remain true.

2. Do-While Loop

The do-while loop is similar to while, but it executes at least once before checking the condition.

Syntax :

do {

    // loop body

} while (condition);

Example :

class Main {

    public static void main(String[] args) {

        int n = 4;

        int counter = 0;

        do {

            n++;

            System.out.println(n);

            counter++;

        } while (counter < 3);

    }

}

Output:

5  

6  

7 

Key Difference from While Loop:

Even if the condition is false, the loop body executes at least once.

Example:

class Main {

    public static void main(String[] args) {

        int n = 3;

        boolean condition = (n > 10);

        do {

            System.out.println("Number is greater than 10");

        } while (condition);

    }

}

Output:

Number is greater than 10  

Even though n > 10 is false, the message prints once before exiting.

3. For Loop

The for loop is widely used when the number of iterations is known in advance. It consists of three parts:

• Initialization: Declares and initializes the loop variable.
• Condition: Checks whether the loop should continue.
• Update Expression: Modifies the loop variable after each iteration.

Syntax:

for (initialExpression; condition; updateExpression) {

    // loop body

}

Example:

class Main {

    public static void main(String[] args) {

        int num = 4;

        for (int counter = 0; counter < 3; counter++) {

            num++;

            System.out.println(num);

        }

    }

}

Output:

5  

6  

7  

Common Mistakes in For Loop:

Incorrect Syntax (Missing Semicolons) :

class Main {

    public static void main(String[] args) {

        int num = 4;

        for (int counter = 0, counter < 3; counter++) { // Incorrect syntax

            num++;

            System.out.println(num);

        }

    }

}

Error: Compilation error due to misplaced commas instead of semicolons.

Summary

• In while and for loops, the condition is checked before executing the loop body.
• In do-while, the condition is checked after executing the loop body at least once.
• The for loop is preferred when the number of iterations is known, while while and do-while loops are useful for indefinite loops with dynamic conditions.