Topic 8424 min readStructured Tutorial

Java For Loop Tutorial

Quick Answer

For Loop explains the for loop is a fundamental control structure in Java used to repeat a block of code a specific number of times.

Learning Objectives

Explain the purpose of For Loop in a practical learning context.
Identify the main ideas, terms, and decisions involved in For Loop.
Apply For Loop in a simple real-world scenario or practice task.

Introduction to Java For Loop

The for loop is a fundamental control structure in Java used to repeat a block of code a specific number of times.

It is especially useful when the number of iterations is known before entering the loop.

Understanding the for loop is essential for writing efficient and readable Java programs.

Control your loops, control your program.

Basic Syntax of the For Loop

The for loop in Java has a concise syntax that includes initialization, condition, and update expressions.

These three parts control the loop execution and are separated by semicolons.

Initialization: executed once before the loop starts.
Condition: evaluated before each iteration; if true, the loop continues.
Update: executed after each iteration to modify loop variables.

For Loop Syntax Components
Part	Description	Example
Initialization	Set starting point	int i = 0
Condition	Loop continuation test	i < 5
Update	Change loop variable	i++

Example of a For Loop in Java

Here is a simple example that prints numbers from 0 to 4 using a for loop.

Common Variations of For Loops

For loops can be customized in many ways to suit different programming needs.

Counting up or down by changing the update expression.
Using multiple variables in the initialization and update sections.
Omitting parts of the for loop syntax for more control.

Counting Down Example

You can count down by decreasing the loop variable in the update expression.

Multiple Variables in For Loop

Java allows initializing and updating multiple variables separated by commas.

When to Use For Loops

For loops are ideal when the number of iterations is predetermined or easily calculated.

They are commonly used for iterating over arrays, collections, or ranges of numbers.

Iterating through arrays or lists.
Repeating a task a fixed number of times.
Generating sequences of values.

Practical Example

This loop prints numbers from 0 to 4 by initializing i to 0, checking if i is less than 5, and incrementing i by 1 after each iteration.

This loop counts down from 5 to 1 by decrementing i in each iteration.

This loop uses two variables, i and j, updating both in each iteration.

Examples

Simple Java For Loop Example

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

This loop prints numbers from 0 to 4 by initializing i to 0, checking if i is less than 5, and incrementing i by 1 after each iteration.

Counting Down For Loop Example

for (int i = 5; i > 0; i--) {
    System.out.println(i);
}

This loop counts down from 5 to 1 by decrementing i in each iteration.

For Loop with Multiple Variables

for (int i = 0, j = 10; i < j; i++, j--) {
    System.out.println("i = " + i + ", j = " + j);
}

This loop uses two variables, i and j, updating both in each iteration.

Best Practices

Always ensure the loop condition will eventually become false to avoid infinite loops.
Use meaningful variable names for clarity.
Keep the loop body concise and focused on a single task.
Avoid modifying the loop variable inside the loop body to prevent unexpected behavior.
Use enhanced for loops (for-each) when iterating over collections or arrays for better readability.

Common Mistakes

Forgetting to update the loop variable, causing infinite loops.
Using incorrect loop conditions that never become false.
Modifying the loop variable inside the loop body leading to unpredictable results.
Declaring the loop variable outside the loop when not necessary, reducing code clarity.
Confusing the semicolons in the for loop syntax.

Hands-on Exercise

Print Even Numbers

Write a for loop that prints all even numbers from 2 to 20.

Expected output: 2 4 6 8 10 12 14 16 18 20

Hint: Use the modulus operator (%) to check if a number is even.

Sum of First N Numbers

Use a for loop to calculate the sum of the first 100 natural numbers.

Expected output: 5050

Hint: Initialize a sum variable before the loop and add each number inside the loop.

Interview Questions

What are the three parts of a for loop in Java?

Interview

The three parts are initialization, condition, and update.

How can you create an infinite loop using a for loop?

Interview

By omitting the condition or using a condition that always evaluates to true, such as for(;;).

When should you prefer a for loop over a while loop?

Interview

When the number of iterations is known beforehand or can be determined easily.