How to Test Java List Interface Methods using Mockito?

Mockito is an open-source testing framework used for unit testing of Java applications. It plays a vital role in developing testable applications. Mockito is used to mock interfaces so that a dummy functionality can be added to a mock interface that can be used in Unit Testing. The List interface in Java provides a way to store the ordered collection. It is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. There are many methods available in Java List Interface like 

• add()
• set()
• remove()
• size()
• get() and many more.

So in this article, we are going to test some of the methods with the help of the Mockito and Junit framework. 

Step by Step Implementation

Step 1: Creating a Maven project 

Create a maven project in your favorite Java IDE (In this article we are using IntelliJ IDEA)

👁 Image

Step 2: When you have successfully created a maven project you have to add some dependencies in your pom.xml file. You have to add the following dependency in your pom.xml file.

Dependency for Mockito is as follows:

Dependency for Junit is as follows: 

Implementation: Below is the complete code for the pom.xml file

Step 3: Testing the Java List Interface Methods

In order to test the Java List Interface Methods we have to create a test class inside the src > test > java > right-click > New > Java Class folder as shown in the below image. Here we have named the class as JavaListMock.

👁 Image

Step 4:  Now we are going to write the test cases for Java List Interface methods.

Test list.size() Method

4.1: Mockito provides us mock() method to mock the List Class.

4.2: Mockito provides us two other methods when() and thenReturn(). when() is used when we want to mock to return specific values when particular methods are called and thenReturn() methods lets you define the return value when a particular method of the mocked object is been called. 

4.3: Junit provides us assertEquals() which is used to check if two objects are equally defined or not.

4.4: One has to write the upper 3 lines inside a public void method and you have to annotate the method with @Test. @Test annotation tells the JUnit that the public void method in which it is used can run as a test case.

Example: 

By now, we have successfully tested the list.size() method, so moving onto next 

Test list.size() Method With Multiple Values

We can also test the list.size() method with multiple value.

Example

Test list.get() Method

Illustration A:

One can also throw an exception and test your method.

Illustration B:  Throw an Exception

Implementation: 

Step 5: Run test cases

Now we are going to run our test cases. To run your test cases you may click on the testing al methods button o test all the methods in one click. And if you want to test each method separately then you may click on the testing a single method button. In the Run tab, you can see all of your test cases have been passed. 

👁 Image