Java Interview Questions

In Java, several types of loops can be used to execute a block of code repeatedly. These loops include the for loop, the while loop, the do-while loop, and the enhanced for loop (also known as the "for-each" loop). Each type of loop has its specific use cases and benefits.

1. for Loop

Definition: The for loop is used when you know in advance how many times you want to execute a statement or a block of statements.

Syntax:

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

Example:

public class ForLoopExample {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            System.out.println("Iteration: " + i);
        }
    }
}

2. while Loop

Definition: The while loop is used to execute a block of statements repeatedly as long as the condition is true. It is generally used when the number of iterations is not known beforehand.

Syntax:

while (condition) {
    // statements to be executed
}

Example:

public class WhileLoopExample {
    public static void main(String[] args) {
        int i = 0;
        while (i < 5) {
            System.out.println("Iteration: " + i);
            i++;
        }
    }
}

3. do-while Loop

Definition: The do-while loop is similar to the while loop, but it guarantees that the block of statements will be executed at least once, regardless of whether the condition is true or false.

Syntax:

do {
    // statements to be executed
} while (condition);

Example:

public class DoWhileLoopExample {
    public static void main(String[] args) {
        int i = 0;
        do {
            System.out.println("Iteration: " + i);
            i++;
        } while (i < 5);
    }
}

4. Enhanced for Loop (for-each Loop)

Definition: The enhanced for loop is used to iterate over arrays or collections. It simplifies the code and eliminates the need for explicit counters.

Syntax:

for (type variable : collection) {
    // statements to be executed
}

Example:

public class EnhancedForLoopExample {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};
        for (int num : numbers) {
            System.out.println("Number: " + num);
        }
    }
}

Comparison and Use Cases

1. for Loop:

   • Use Case: When the number of iterations is known.
   • Example: Iterating through a sequence with a known length.

2. while Loop:

   • Use Case: When the number of iterations is unknown and depends on a condition evaluated before the loop.
   • Example: Reading data from a file until the end of the file.

3. do-while Loop:

   • Use Case: When the loop body must be executed at least once, and the condition is evaluated after the loop body.
   • Example: A menu-driven program that prompts the user at least once before checking if they want to continue.

4. Enhanced for Loop:

   • Use Case: When iterating over arrays or collections, particularly when you do not need the index.
   • Example: Iterating through elements of an array or a list to access and process each element.

Summary

Java provides several loops (for, while, do-while, and enhanced for loop) to handle different looping requirements. You can write more efficient and readable code by choosing the appropriate loop based on the specific scenario.