Java Loops

Loops repeat code. Printing 1,000 invoices, checking every item in a cart, retrying a failed login: all loops. Java has four kinds.

The for loop#

Use for when you know how many times to repeat:

public class Table {
    public static void main(String[] args) {
        int n = 7;
        for (int i = 1; i <= 10; i++) {
            System.out.println(n + " x " + i + " = " + (n * i));
        }
    }
}

A for has three parts: for (start; condition; update).

1. int i = 1 runs once at the beginning.
2. i <= 10 is checked before each round; if false, the loop ends.
3. i++ runs after each round.

Counting down or in steps:

for (int i = 10; i >= 1; i--) { System.out.print(i + " "); }       // 10 9 8 ... 1
for (int i = 0; i <= 20; i += 5) { System.out.print(i + " "); }    // 0 5 10 15 20

The while loop#

Use while when you don’t know how many rounds you’ll need, only the stopping condition:

public class DigitSum {
    public static void main(String[] args) {
        int n = 98765, sum = 0;
        while (n > 0) {
            sum += n % 10;   // take the last digit
            n /= 10;         // drop the last digit
        }
        System.out.println(sum);   // 35
    }
}

The do-while loop#

do-while checks the condition after the body, so it always runs at least once. Perfect for menus:

import java.util.Scanner;

public class Menu {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int choice;
        do {
            System.out.println("1. Start quiz  2. Scores  0. Exit");
            choice = Integer.parseInt(in.nextLine());
            System.out.println("You chose " + choice);
        } while (choice != 0);
    }
}

The enhanced for (for-each)#

To visit every item in an array or collection, for-each is simplest:

int[] marks = {78, 92, 65, 88};
int total = 0;
for (int m : marks) {
    total += m;
}
System.out.println("Average: " + (double) total / marks.length);   // 80.75

Read it as “for each m in marks“. You don’t get the index, and you can’t modify the array through m. Use a normal for when you need the index.

break and continue#

• break exits the loop immediately.
• continue skips the rest of this round and moves to the next.

public class FirstMultiple {
    public static void main(String[] args) {
        for (int i = 1; i <= 50; i++) {
            if (i % 3 != 0) continue;        // skip non-multiples of 3
            if (i % 7 == 0) {
                System.out.println("First multiple of 3 and 7: " + i);   // 21
                break;
            }
        }
    }
}

Nested loops#

A loop inside a loop. The inner loop finishes all its rounds for each round of the outer loop:

public class Stars {
    public static void main(String[] args) {
        int rows = 4;
        for (int r = 1; r <= rows; r++) {
            for (int c = 1; c <= r; c++) {
                System.out.print("* ");
            }
            System.out.println();
        }
    }
}

* 
* * 
* * * 
* * * * 

Pattern programs like this are very common in fresher interviews and college exams. Nested loops over n items run n × n times, which is O(n²). See the DSA course’s Big-O lesson for why that matters.

Labelled break#

To exit both loops at once, label the outer loop:

outer:
for (int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++) {
        if (i * j == 6) {
            System.out.println("Found " + i + " x " + j);
            break outer;
        }
    }
}

Infinite loops#

int i = 0;
while (i < 5) {
    System.out.println(i);
    // forgot i++ → runs forever!
}

Always make sure something inside the loop moves it towards the stopping condition. while (true) with a break inside is fine when used on purpose (servers, game loops).

Which loop when?#

Loop	Use when
for	you know the count, or need the index
for-each	you visit every item of an array/collection
while	you repeat until something happens
do-while	the body must run at least once (menus, input validation)

Common mistakes#

• Off-by-one: i <= arr.length instead of i < arr.length → ArrayIndexOutOfBoundsException.
• Semicolon after the loop header: for (...); makes an empty loop.
• Changing the loop variable inside a for body: confusing, avoid.

Exercise#

1. Print all even numbers from 1 to 50 in one line.
2. Calculate the factorial of 10 with a loop (use long).
3. Print the first 15 Fibonacci numbers.
4. Check whether a number is prime (loop up to the square root).
5. Print this pyramid for 4 rows:

   *
  ***
 *****
*******

Hint for 4

Loop i from 2 while i * i <= n. If n % i == 0, it’s not prime.

Hint for 5

Row r has rows - r spaces then 2 * r - 1 stars.

Interview corner#

• while vs do-while? do-while runs the body at least once; while may run zero times.
• Can you use for-each to remove items from a list? No: it throws ConcurrentModificationException. Use an Iterator’s remove() or list.removeIf(...).
• Print 1 to 100 without a loop? Recursion: void print(int n) { if (n > 100) return; System.out.println(n); print(n + 1); }.

Summary#

• for for counted loops, while for condition loops, do-while for at-least-once, for-each for collections.
• break exits, continue skips a round, labels break outer loops.
• Nested loops multiply work: n × n rounds.

Next: arrays, to store many values in one variable.