Loops in Java
Loops in Java are used to repeat a block of code multiple times. They help automate repetitive tasks, reduce errors, and make your code efficient.
What Are Loops in Java?
A loop executes a block of code repeatedly until a condition becomes false.
Types of loops in Java:
1. for Loop
Used when you know how many times you want to repeat a block.
Syntax:
for (initialization; condition; update) {
// code to execute
}
Example: Print 1 to 5
for (int i = 1; i <= 5; i++) {
System.out.println(i);
}
Output:
1
2
3
4
5
2. while Loop
- Used when you don’t know the exact number of iterations in advance.
- Condition is checked before executing the block.
Syntax:
while (condition) {
// code to execute
}
Example: Sum numbers until total < 50
int total = 0;
int num = 1;
while (total < 50) {
total += num;
num++;
}
System.out.println("Total: " + total);
Output:
Total: 55
3. do-while Loop
- Executes the block at least once, then checks the condition.
- Useful when code must run at least once regardless of condition.
Syntax:
do {
// code to execute
} while (condition);
Example: Input validation
import java.util.Scanner;
Scanner sc = new Scanner(System.in);
int number;
do {
System.out.print("Enter a positive number: ");
number = sc.nextInt();
} while (number <= 0);
System.out.println("You entered: " + number);
sc.close();
4. for-each Loop
- Used to iterate through arrays or collections without worrying about index.
- Introduced in Java 5 (enhanced for loop).
Syntax:
for (type var : arrayOrCollection) {
// code
}
Example: Iterating through an array
int[] numbers = {10, 20, 30, 40};
for (int num : numbers) {
System.out.println(num);
}
Output:
10
20
30
40
Nested Loops
You can place one loop inside another. Useful for patterns, matrices, etc.
Example: Print 3x3 stars
for (int i = 1; i <= 3; i++) {
for (int j = 1; j <= 3; j++) {
System.out.print("* ");
}
System.out.println();
}
Output:
* * *
* * *
* * *
Example: Calculate Factorial Using Loops
import java.util.Scanner;
public class Factorial {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter a number: ");
int n = sc.nextInt();
int factorial = 1;
for (int i = 1; i <= n; i++) {
factorial *= i;
}
System.out.println("Factorial of " + n + " is " + factorial);
sc.close();
}
}
Output Example:
Enter a number: 5
Factorial of 5 is 120