IllegalArgumentException in Java

 In this article, we will learn what is IllegalArgumentException in Java, its common causes, practical examples, solutions, and tips to avoid IllegalArgumentException.

What is IllegalArgumentException? 

The IllegalArgumentException is a runtime exception thrown by the Java system (or by applications) to indicate that a method has received an argument that is invalid or inappropriate in context. 

Common Causes 

Passing Out-of-Range Values: For instance, passing a negative size to a method expecting a positive size. 

Inappropriate Null Arguments: Passing null to a method that explicitly requires a non-null argument. 

Incompatible Data Types: For instance, passing a string representing a date in the wrong format.

Out-of-bounds Values: Providing a value outside an expected range.

 Practical Example 

Let's dive into the IllegalArgumentException with a hands-on example:

public class Person {
    private int age;

    public void setAge(int age) {
        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative.");
        this.age = age;

    public static void main(String[] args) {
        Person person = new Person();
        person.setAge(-5);  // This will throw an IllegalArgumentException


Exception in thread "main" java.lang.IllegalArgumentException: Age cannot be negative.
	at Person.setAge(
	at Person.main(

Solutions and Best Practices 

1. Validation Before Processing 

Always validate method arguments before processing. This ensures that if an IllegalArgumentException is to be thrown, it happens immediately, making it easier to identify and fix.

public void setTemperature(float temperature) {
    if (temperature < -273.15) { // absolute zero in Celsius
        throw new IllegalArgumentException("Temperature below absolute zero is not possible.");
    // continue processing...

2. Provide Detailed Error Messages 

When throwing an IllegalArgumentException, always provide a detailed error message. This not only helps during debugging but also makes your code more understandable.

throw new IllegalArgumentException("Expected a non-negative value, but got: " + value);

3. Leverage Java's Built-in Objects' 

Methods Many built-in Java objects, like Objects.requireNonNull(), are designed to automatically throw IllegalArgumentException or related exceptions under certain conditions. Make use of these to simplify your validation process.

import java.util.Objects;

public void setName(String name) { = Objects.requireNonNull(name, "Name cannot be null");

Tips to Avoid IllegalArgumentException 

Unit Testing: Write unit tests that test boundary conditions and potential illegal arguments. 

Consistent Validation: Be consistent in how you validate method arguments across your codebase. 

Use Enums: For methods that accept only a limited set of values, consider using enums.

Related Exceptions Posts

Java built-in checked exceptions:
Java built-in unchecked exceptions:
Java built-in errors: