JUnit Expected Exception Test: @Test(expected)

JUnit provides the facility to trace the exception and also to check whether the code is throwing expected exception or not.

Junit4 provides an easy and readable way for exception testing, you can use

• Optional parameter (expected) of @test annotation and
• To trace the information ,"fail()" can be used

While Testing exception, you need to ensure that exception class you are providing in that optional parameter of @test annotation is the same. This is because you are expecting an exception from the method you are Unit Testing, otherwise our JUnit test would fail.

Example@Test(expected=IllegalArgumentException.class)

By using "expected" parameter, you can specify the exception name our test may throw. In above example, you are using "IllegalArgumentException" which will be thrown by the test if a developer uses an argument which is not permitted.

Example using @test(expected)

Let's understand exception testing by creating a Java class with a method throwing an exception. You will handle it and test it in a test class. Consider JUnitMessage.java having a method which simply do a mathematical operation based on input received by the user. If any illegal argument would be entered, it will throw "ArithmeticException". See below:

package guru99.junit;

public class JUnitMessage{

	private String message;

	public JUnitMessage(String message) {
		this.message = message;
	}

public void printMessage(){

	System.out.println(message); 
	int divide=1/0;

}

public String printHiMessage(){ 

	message="Hi!" + message;
	
	System.out.println(message);

	return message;
}

}

Code Explanation:

• Code Line 7: Creating a parameterized constructor with field initialization.
• Code Line 11-14: Creating a method for mathematical operation.
• Code Line 18: Creating another method to print a message.
• Code Line 20: Creating a new string to print a message.
• Code Line 21: Printing new message created in line 20.

Let's create a test class for above java class to verify exception.

See below test class to unit test exception (ArithmeticException here) throwing from above java class:

AirthematicTest.java

package guru99.junit;

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class AirthematicTest {

	public String message = "Saurabh";
	
	JUnitMessage junitMessage = new JUnitMessage(message);
	
	@Test(expected = ArithmeticException.class)
	public void testJUnitMessage(){

		System.out.println("Junit Message is printing ");
		junitMessage.printMessage();

	}

	@Test
	public void testJUnitHiMessage(){ 
		message="Hi!" + message;
		System.out.println("Junit Message is printing ");
		assertEquals(message, junitMessage.printMessage());
	
	}
}

Code Explanation:

• Code Line 13: Using @Test annotation to create our test. As you execute above classes method, it will invoke a mathematical operation. Here Arithmetic Exception is expected, so you are listing it out as a parameter in @Test.
• Code Line 17: Invoking printMessage() JUnitMessage.java
• Code Line 18: Creating another method to print HI message.

If you execute this test class, the test method is executed with each defined parameter. In the above example, the test method is executed five times.

Let's execute it and verify the result. See below the test runner class to execute JunitTestExample.java

Output:

Here is the output which shows successful test with no failure trace as given below:

Summary:

• Exception testing is a special feature introduced in JUnit4. In this tutorial, you have learned how to test exception in JUnit using @test(excepted)
• Junit provides the facility to trace the exception and also to check whether the code is throwing exception or not
• For exception testing, you can use
  • Optional parameter (expected) of @test annotation and
  • To trace the information ,"fail()" can be used