Java Control Statement

2. Looping Statements

In Java, loops are used to runs a block of code repeatedly until a given condition evaluates to false. The same code can be reused and executed multiple times, which reduces code duplication and improving code reusability.

• while loop
• do-while loop
• for loop
• Nested loop
• Enhanceh for loop

1. while loop statement:

The while loop in Java first checks a condition and then continues executing the block of code until the condition becomes false.

Syntax:

while (condition/boolean expression) {
  // block of code to be executed
  // The loop will continue as long as the condition is true
}

Example:

public class HelloWorld {
    public static void main(String[] args) {
        int number = 1;
 
        while (number <= 5) {
            System.out.println("Number: " + number);
            number++;
        }
    }
}
 
output: 
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Explanation:

• The variable number is initialized to 1.
• The while loop runs as long as number <= 5.
• During each iteration, it prints the value of number and increments it by 1.
• The loop stops when number becomes 6

Note: If the number is not increased by 1, the condition will remain true, and the loop will run forever.

2. do-while loop statement:

A do-while loop in Java runs a block of code at least once, then keeps repeating it as long as the condition is true.

Syntax:

    do {
    //block of code to be executed
    //The loop will continue as long as the condition is true
    }
    while (condition);

Example:

public class HelloWorld {
    public static void main(String[] args) {
        int number = 1;
 
        do {
            System.out.println("Number: " + number);
            number++;
        } while (number <= 5);
    }
}
output:
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Explanation:

• The variable number is initialized to 1.
• The do block executes first, so it prints the value of number at least once.
• After printing, number is increased by 1.
• The condition number <= 5 is checked after each iteration.
• The loop continues until number becomes 6, at which point the condition becomes false and the loop stops.

Note: It runs the block of code at least once, even if the condition is false.

3. for loop statement:

It is used to execute a block of code repeatedly for a specific number of times, based on an initialization, condition, and update expression.

Syntax:

for (initialization; condition; update) {
    // code to be executed
}
 
//Initialization : sets the starting value of the loop variable.
//Condition : is checked before each iteration to decide whether the loop should continue or stop.
//Update : changes the loop variable after each iteration to move the loop forward.

Example:

public class HelloWorld {
    public static void main(String[] args) {
        for (int count = 1; count <= 5; count++) {
            System.out.println("Number: " + count);
        }
    }
}
output:
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Explanation:

• The variable count is initialized to 1.
• The loop runs as long as count <= 5.
• In each iteration, it prints the value of count.
• After each iteration, count is increased by 1 using count++.
• The loop stops when count becomes 6, because the condition becomes false.

4. Nested loop statement:

It is a loop inside another loop. It is used when one loop depends on another loop to repeat a task multiple times.

Syntax:

for (initialization; condition; update) {
    // outer loop code
    for (initialization; condition; update) {
        // inner loop code
    }
}

Example:

public class HelloWorld {
    public static void main(String[] args) {
        for (int row = 1; row <= 3; row++) {
            for (int column = 1; column <= 3; column++) {
                System.out.println("Row: " + row + ", Column: " + column);
            }
        }
    }
}
output
Row: 1, Column: 1
Row: 1, Column: 2
Row: 1, Column: 3
Row: 2, Column: 1
Row: 2, Column: 2
Row: 2, Column: 3
Row: 3, Column: 1
Row: 3, Column: 2
Row: 3, Column: 3

Explanation:

• The outer loop (row) runs 3 times.
• For each value of row, the inner loop (column) also runs 3 times.
• The inner loop completes all its iterations for every single iteration of the outer loop.
• This is why nested loops are useful for working with rows and columns (like tables or patterns).

5. Enhanced for loop statement:

It is a control statement used to iterate each elements of an array or collection one by one without using an index.

Syntax:

for (dataType variable : arrayOrCollection) {
    // code to be executed
}

Example:

public class HelloWorld {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40, 50};
 
        for (int number : numbers) {
            System.out.println("Number: " + number);
        }
    }
}
output:
Number: 10
Number: 20
Number: 30
Number: 40
Number: 50
 

Explanation:

• The array numbers stores multiple integer values.
• The enhanced for loop takes each element from the array one by one.
• The variable number temporarily holds each value during iteration.
• It prints each value without using an index.
• The loop automatically stops after all elements are processed.

Difference between for loop and enhanced for loop:

A for loop uses an index and allows full control over initialization, condition, and update, which helps in accessing elements by their position.

An enhanced for loop (for-each loop) goes through each element directly without using an index, making it easier and more readable for arrays and collections.