Java - How to Solve IllegalArgumentException?

1. Overview

In this tutorial, We'll learn when IllegalArgumentException is thrown and how to solve IllegalArgumentException in java 8 programming.

This is a very common exception thrown by the java runtime for any invalid inputs. IllegalArgumentException is part of java.lang package and this is an unchecked exception.

IllegalArgumentException is extensively used in java api development and used by many classes even in java 8 stream api.

First, we will see when we get IllegalArgumentException in java with examples and next will understand how to troubleshoot and solve this problem?

2. Java.lang.IllegalArgumentException Simulation

In the below example, first crated the ArrayList instance and added few string values to it. 

package com.javaprogramto.exception.IllegalArgumentException;

import java.util.ArrayList;
import java.util.List;

public class IllegalArgumentExceptionExample {

	public static void main(String[] args) {
		
		// Example 1
		List<String> list = new ArrayList<>(-10);
		list.add("a");
		list.add("b");
		list.add("c");
	}
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: Illegal Capacity: -10
	at java.base/java.util.ArrayList.<init>(ArrayList.java:160)
	at com.javaprogramto.exception.IllegalArgumentException.IllegalArgumentExceptionExample.main(IllegalArgumentExceptionExample.java:11)

From the above output, we could see the illegal argument exception while creating the ArrayList instance.

Few other java api including java 8 stream api and custom exceptions.

3. Java IllegalArgumentException Simulation using Java 8 stream api

Java 8 stream api has the skip() method which is used to skip the first n objects of the stream.

public class IllegalArgumentExceptionExample2 {

	public static void main(String[] args) {
		
		// Example 2
		List<String> stringsList = new ArrayList<>();
		stringsList.add("a");
		stringsList.add("b");
		stringsList.add("c");
		
		stringsList.stream().skip(-100);
	}
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: -100
	at java.base/java.util.stream.ReferencePipeline.skip(ReferencePipeline.java:476)
	at com.javaprogramto.exception.IllegalArgumentException.IllegalArgumentExceptionExample2.main(IllegalArgumentExceptionExample2.java:16)

4. Java IllegalArgumentException - throwing from custom condition

In the below code, we are checking the employee age that should be in between 18 and 65. The remaining age groups are not allowed for any job.

Now, if we get the employee object below 18 or above 65 then we need to reject the employee request.

So, we will use the illegal argument exception to throw the error.

import com.javaprogramto.java8.compare.Employee;

public class IllegalArgumentExceptionExample3 {

	public static void main(String[] args) {

		// Example 3
		Employee employeeRequest = new Employee(222, "Ram", 17);

		if (employeeRequest.getAge() < 18 || employeeRequest.getAge() > 65) {
			throw new IllegalArgumentException("Invalid age for the emp req");
		}

	}
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: Invalid age for the emp req
	at com.javaprogramto.exception.IllegalArgumentException.IllegalArgumentExceptionExample3.main(IllegalArgumentExceptionExample3.java:13)

5. Solving IllegalArgumentException in Java

After seeing the few examples on IllegalArgumentException, you might have got an understanding on when it is thrown by the API or custom conditions based.

IllegalArgumentException is thrown only if any one or more method arguments are not in its range. That means values are not passed correctly.

If IllegalArgumentException is thrown by the java api methods then to solve, you need to look at the error stack trace for the exact location of the file and line number.

To solve IllegalArgumentException, we need to correct method values passed to it. But, in some cases it is completely valid to throw this exception by the programmers for the specific conditions. In this case, we use it as validations with the proper error message.

Below code is the solution for all java api methods. But for the condition based, you can wrap it inside try/catch block. But this is not recommended.

package com.javaprogramto.exception.IllegalArgumentException;

import java.util.ArrayList;
import java.util.List;

import com.javaprogramto.java8.compare.Employee;

public class IllegalArgumentExceptionExample4 {

	public static void main(String[] args) {

		// Example 1
		List<String> list = new ArrayList<>(10);
		list.add("a");
		list.add("b");
		list.add("c");

		// Example 2
		List<String> stringsList = new ArrayList<>();
		stringsList.add("a");
		stringsList.add("b");
		stringsList.add("c");

		stringsList.stream().skip(2);

		// Example 3
		Employee employeeRequest = new Employee(222, "Ram", 20);

		if (employeeRequest.getAge() < 18 || employeeRequest.getAge() > 65) {
			throw new IllegalArgumentException("Invalid age for the emp req");
		}
		
		System.out.println("No errors");

	}
}

Output:

No errors

6. Conclusion

In this article, We've seen how to solve IllegalArgumentException in java.

GitHub

IllegalArgumentException