Java Fundamentals

1. Loop Control

A loop is used when a block of code needs to be executed several times. Programming languages provide control structures that allow more complicated execution paths.

A loop statement allows a statement or group of statements to be executed multiple times.

Types of Loops in Java

2. while Loop

A while loop repeatedly executes a target statement as long as a given condition is true.

Syntax

while(Boolean_expression) {
    // Statements
}

The statement may be a single statement or a block of statements. When the Boolean expression is true, the loop body executes. This continues until the expression becomes false, after which control moves to the statement immediately following the loop.

3. while Loop Example

public class Test {
    public static void main(String args[]) {
        int x = 10;

        while( x < 20 ) {
            System.out.print("value of x : " + x );
            x++;
            System.out.print("\n");
        }
    }
}

Output

value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19

4. for Loop

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to be executed a specific number of times.

It is useful when you know how many times a task is to be repeated.

Syntax

for(initialization; Boolean_expression; update) {
    // Statements
}

Flow of Control

1. Initialization: Executed first and only once. It declares and initializes loop-control variables.
2. Boolean expression: Evaluated next. If true, the loop body executes; if false, control moves past the loop.
3. Loop body: Executes when the condition is true.
4. Update: Executes after the body and updates loop-control variables.
5. The Boolean expression is evaluated again and the process repeats until it becomes false.

5. for Loop Example

public class Test {
    public static void main(String args[]) {
        for(int x = 10; x < 20; x = x + 1) {
            System.out.print("value of x : " + x );
            System.out.print("\n");
        }
    }
}

Output

value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19

6. do...while Loop

A do...while loop is similar to a while loop, except that it is guaranteed to execute at least one time.

Syntax

do {
    // Statements
} while(Boolean_expression);

The Boolean expression appears at the end of the loop, so the statements execute once before the condition is tested. If the expression is true, control jumps back to the do statement and the loop repeats.

7. do...while Example and Output

public class Test {
    public static void main(String args[]) {
        int x = 10;

        do {
            System.out.print("value of x : " + x );
            x++;
            System.out.print("\n");
        } while( x < 20 );
    }
}

Output

value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19

8. break Statement

The break statement has two main usages in the source:

• When encountered inside a loop, it immediately terminates the loop and program control resumes at the next statement following the loop.
• It can be used to terminate a case in a switch statement.

Syntax

break;

Example

public class Test {
    public static void main(String args[]) {
        int [] numbers = {10, 20, 30, 40, 50};

        for(int x : numbers ) {
            if( x == 30 ) {
                break;
            }
            System.out.print( x );
            System.out.print("\n");
        }
    }
}

Output

10
20

9. continue Statement

The continue keyword can be used in any loop-control structure. It causes the loop to immediately jump to the next iteration.

• In a for loop, control jumps to the update statement.
• In a while or do/while loop, control jumps to the Boolean expression.

Syntax

continue;

Example

public class Test {
    public static void main(String args[]) {
        int [] numbers = {10, 20, 30, 40, 50};

        for(int x : numbers ) {
            if( x == 30 ) {
                continue;
            }
            System.out.print( x );
            System.out.print("\n");
        }
    }
}

Output

10
20
40
50

10. Enhanced for Loop

As of Java 5, the enhanced for loop was introduced. It is mainly used to traverse collections of elements, including arrays.

Syntax

for(declaration : expression) {
    // Statements
}

Declaration

The newly declared block variable must have a type compatible with the elements of the array being accessed. It is available within the for block and contains the current array element.

Expression

The expression evaluates to the array to be traversed. It can be an array variable or a method call that returns an array.

Example

public class Test {
    public static void main(String args[]) {
        int [] numbers = {10, 20, 30, 40, 50};

        for(int x : numbers ) {
            System.out.print( x );
            System.out.print(",");
        }
        System.out.print("\n");

        String [] names = {"James", "Larry", "Tom", "Lacy"};

        for( String name : names ) {
            System.out.print( name );
            System.out.print(",");
        }
    }
}

Output

10, 20, 30, 40, 50,
James, Larry, Tom, Lacy,

11. Quick Revision