JavaLoops (for, while, do-while)

Java Loops: for, while, do-while

What Are Loops?

In programming, loops are used to repeat a block of code multiple times.

Keep doing something until a condition is false.

For example: printing numbers 1 to 10, repeating a game level, checking user input until correct.

Types of Loops in Java

Loop Type

When to Use

for

When you know how many times to repeat

while

When you don’t know how many times, but have a condition

do-while

When the loop should run at least once, no matter what

for Loop Syntax

Java
for (initialization; condition; update) {
    // code to repeat
}

  • Initialization: start value (e.g. int i = 1;)

  • Condition: loop runs while this is true (i <= 10)

  • Update: value change after each loop (i++)

Example – Print Numbers 1 to 5

Java
for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}
1
2
3
4
5
Note
Step-by-step: i = 1 → print → +1 → check again → until i = 6 (then stops).
Example – Sum of Numbers

Java
int sum = 0;

for (int i = 1; i <= 5; i++) {
    sum = sum + i;
}

System.out.println("Sum = " + sum);
Sum = 15
while Loop Syntax

Java
while (condition) {
    // code to repeat
}

  • Checks the condition before running the loop.

  • If condition is false at the start, the loop doesn’t run at all.

Example – while Loop

Java
int i = 1;

while (i <= 5) {
    System.out.println(i);
    i++;
}
1
2
3
4
5
Warning
Common Mistake: If you forget to update the variable (like i++), the loop becomes infinite and never stops.
do-while Loop Syntax

Java
do {
    // code to repeat
} while (condition);

  • This loop runs at least once, even if the condition is false.

  • The condition is checked after the loop runs once.

Example – do-while Loop

Java
int i = 1;

do {
    System.out.println(i);
    i++;
} while (i <= 5);
1
2
3
4
5
Example – Condition False but Still Runs Once

Java
int i = 6;

do {
    System.out.println("Number: " + i);
    i++;
} while (i <= 5);
Number: 6
Note
Even though i <= 5 is false, the loop runs once before checking the condition.
Comparison Table

Feature

for

while

do-while

Condition checked

Before loop

Before loop

After loop

Executes at least once?

No

No

Yes

When to use

When loop count known

When condition-based repetition

When you want code to run once minimum

Common use

Counting, arrays

Waiting for input

Menu-based programs

Nested Loops

A loop inside another loop is called a nested loop. Useful for printing patterns or tables.

Multiplication Table

Java
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.println(i + " × " + j + " = " + (i * j));
    }
}
Infinite Loops

If you forget to change the variable or use the wrong condition, the loop never ends.

Java
while (true) {
    System.out.println("This will run forever!");
}
Warning
Be careful — this can crash your program.
Practice Exercises

  1. Print numbers 1–10 using a for loop.

  2. Print even numbers between 1 and 20 using while.

  3. Find the factorial of a number using a for loop.

  4. Use do-while to ask a user for input until they type "exit".

  5. Print a simple triangle pattern using nested loops.