Top JUnit Interview Questions

Q – 1 How Do You Launch a Debugger When a Test Fails?

Ans- Configure the debugger to catch the java.lang.AssertionError exception and suspend the execution.

Call the TestRunner to run the test under the debugger.

When the test fails again, The debugger will suspend the execution and start the debug mode.

Now you are ready to use debugger commands to find the code issue that causing the fail.

How you configure this depends on the debugger you prefer to use.
Most Java debuggers provide support to stop the program when a specific exception is raised.

Q – 2 When Should Unit Tests Should Be Written In Development Cycle?

Ans- If you are a TDD (Test-Driven Development) believer, you should write unit test before writing the code. Test-first programming is practiced by only writing new code when an automated test is failing.

Good tests tell you how to best design the system for its intended use. They effectively communicate in an executable format how to use the software. They also prevent tendencies to over-build the system based on speculation. When all the tests pass, you know you’re done!

Whenever a customer test fails or a bug is reported, first write the necessary unit test(s) to expose the bug(s), then fix them. This makes it almost impossible for that particular bug to resurface later.

Test-driven development is a lot more fun than writing tests after the code seems to be working. Give it a try!

Q – 3 How Do You Test a Method That Does not Return Anything?

Ans- You need to follow the logic below to answer this question:

* If a method is not returning anything through the “return” statement (void method), it may return data through its arguments. In this case, you can test the data returned in any argument.
* Else if a method is not returning any data through its arguments, it may change values of its instance variables. In this case, you can test changes of any instance variables.
* Else if a method is not changing any instance variable, it may change values of its class variables. In this case, you can test changes of any class variables.
* Else if a method is not changing any class variable, it may change external resources. In this case, you can test changes of any external resources.
* Else if a method is not changing any external resources, it may just doing nothing but holding the thread in a waiting status. In this case, you can test this waiting condition.
* Else if a method is not holding the thread in waiting status, then this method is really doing nothing. In this case, there is no need to test this method. 🙂

Q – 4 When Objects Are Garbage Collected After a Test Is Executed?

Ans- My guess would be that all objects used in a test will be ready for Java garbage collector to release them immediately after the test has been executed.

By design, the tree of Test instances is built in one pass, then the tests are executed in a second pass. The test runner holds strong references to all Test instances for the duration of the test execution. This means that for a very long test run with many Test instances, none of the tests may be garbage collected until the end of the entire test run.

Therefore, if you allocate external or limited resources in a test, you are responsible for freeing those resources. Explicitly setting an object to null in the tearDown() method, for example, allows it to be garbage collected before the end of the entire test run.

If this is true, the JUnit runner should be improved to stop building all test instances before executing any tests. Instead, the JUnit runner should just take one test at a time, build an instance of this test, execute the test, and release the test when the execution is done.

Q – 5 Do You Have To Write a Test for Everything?

Ans- No, just test everything that could reasonably break.

Be practical and maximize your testing investment. Remember that investments in testing are equal investments in design. If defects aren’t being reported and your design responds well to change, then you’re probably testing enough. If you’re spending a lot of time fixing defects and your design is difficult to grow, you should write more tests.

If something is difficult to test, it’s usually an opportunity for a design improvement. Look to improve the design so that it’s easier to test, and by doing so a better design will usually emerge.

Q – 6 How Often Should You Run Your JUnit Tests?

Ans- Run all your unit tests as often as possible, ideally every time the code is changed. Make sure all your unit tests always run at 100%. Frequent testing gives you confidence that your changes didn’t break anything and generally lowers the stress of programming in the dark.

For larger systems, you may just run specific test suites that are relevant to the code you’re working on.

Run all your acceptance, integration, stress, and unit tests at least once per day (or night).

If you’re using Eclipse, be sure to check out David Saff’s continuous testing plug-in.

Q – 7 What Do You Do When a Defect Is Reported?

Ans- Test-driven development generally lowers the defect density of software. But we’re all fallible, so sometimes a defect will slip through. When this happens, write a failing test that exposes the defect. When the test passes, you know the defect is fixed!

Don’t forget to use this as a learning opportunity. Perhaps the defect could have been prevented by being more aggressive about testing everything that could reasonably break.

Or perhaps there are other places in the application that have the similar code that might break too.

Q – 8 How simple is too simple to break?

Ans- The general philosophy is this: if it can’t break on its own, it’s too simple to break.

First example is the getX() method. Suppose the getX() method only answers the value of an instance variable. In that case, getX() cannot break unless either the compiler or the interpreter is also broken. For that reason, don’t test getX(); there is no benefit. The same is true of the setX() method, although if your setX() method does any parameter validation or has any side effects, you likely need to test it.

Q – 9 How To Create Test Class in Eclipse?

Ans- There are five ways to create a JUnit test case class in Eclipse with JUnit plugin: org.junit_3.8.1. First, select the directory (usually unittests/) that you wish to create the test case class in.

1. Select File > New > JUnit Test Case.

2. Select the arrow of the button in the upper left of the toolbar. Select JUnit Test Case.

3. Right click on a package in the Package Explorer view in the Java Perspective, and select JUnitTestCase.

4. Click on the arrow of the icon in the toolbar. Select JUnit Test Case.

5. You can create a normal Java class as shown in the Eclipse tutorial, but include junit.framework.TestCase as the super class of the test class you are creating.

Q – 10 How to Run a JUnit Test Case in Eclipse?

Ans- There are three ways to run JUnit Test Cases or Test Suites in Eclipse with JUnit plugin: org.junit_3.8.1.

1. You can right click on the test case class or test suite class and select Run As > JUnit Test.

2. You can select a test case or suite and click the arrow on the icon or select Run from the toolbar, and select Run As > JUnit Test.

3. You can select a test case or suite and click the arrow on the icon or select Run from the toolbar, and select Run… From here you will create a new JUnit test configuration, and name it. You can choose to run a single test case, or run all test cases in a project or folder.

Q – 11 Can You Describe Steps of Creating Test Case Classes in Eclipse?

Ans- There are detailed steps to create a test case class in Eclipse with JUnit plugin: org.junit_3.8.1.

1. Use the Browse button to search for a different super class. The default super class is junit.framework.TestCase.

2. Check which method stubs you would like to create. You can create a main method, setUp(), tearDown(), or a constructor(), but all of these are optional. A constructor is only run when the test case class is first instantiated, but the setUp() and tearDown() methods are run before and after, respectively, each test case is run.

3. You can browse the application that you are creating for a class that you wish to test, or this could be left blank if you will generate the class while creating while creating the test.

– If you selected a “Class Under Test” you can click the Next button, otherwise click Finish. You will be able to select which methods in the class under test that you want to write test cases for. The method signatures will be created for you. Click Finish. The new test case class will be open in the editor.

– This test class demonstrates the basic functionality of the setUp() and tearDown() methods, and gives example test cases. The testForException() method demonstrates how to test that an exception is properly thrown.

Note:

All source methods in the class under test must be public or protected, not private, in order to be tested by JUnit. If the method in the class under test is private, the test class must be in the same package.

Q – 12 How to creating a Test Suite using JUnit in Eclipse?

Ans- There are four ways to create a JUnit test suite class in Eclipse with JUnit plugin: org.junit_3.8.1. First, select the directory (usually unittests/) that you wish to create the test suite class in.

1. Select File > New > Other… > Java > JUnit > JUnit Test Suite.

2. Select the arrow of the button in the upper left of the toolbar. Select Other… > Java > JUnit > JUnit Test Suite,

3. Right click on a package in the Package Explorer view in the Java Perspective, and select Other… > Java > JUnit > JUnit Test Suite, or

4. You can create a normal Java class as shown in the Eclipse tutorial, but include junit.framework.TestSuite as the super class of the test class you are creating.

Q – 13 How Do You Test Classes That Must Be Run in a J2EE Container?

Ans- To test classes that must be run in a J2EE container (e.g. servlets, EJBs), you should refactor J2EE components to delegate functionality to other objects that don’t have to be run in a J2EE container will improve the design and testability of the software.

Cactus is an open source JUnit extension that can be used to test J2EE components in their natural environment.

Q – 14 Can You Write a Simple Class for JUnit Testing in 1 Minute?

Ans- You need to write a class with less than 10 lines. Here is a nice example provided in the article “Writing a JUnit Test in 5 minutes” by Kamal Mettananda with some minor changes:

package com.parcelhouse.myproj;
public class Calc {
public int add(int a, int b) {
// errorneous method
return a+b+1;
}
public int multiply(int a, int b) {
return a*b;
}
}

Q – 15 How Do You Test a protected Method?

Ans- When a method is declared as “protected”, it can only be accessed within the same package where the class is defined.

In order to test a “protected” method of a target class, you need to define your test class in the same package as the target class.

Q – 16 How Do You Test a private Method?

Ans- When a method is declared as “private”, it can only be accessed within the same class. So there is no way to test a “private” method of a target class from any test class.

To resolve this problem, you have to perform unit testing manually. Or you have to change your method from “private” to “protected”.

Q – 17 How To Group Test Cases Class using JUnit TestSuite?

Ans- Usually, there are many classes to be tested in a Java application. For each Java class in the application you need to write a JUnit test case class comtaining multiple test methods.

You could call a JUnit runner to run each JUnit test case class manually. But you should group all JUnit test case clases into a test suite with JUnit TestSuite. The following code and notes are provided by Varun Chopra and valid for JUnit 3.8 with some corrections.

To group all test case classes together and run them a single unit, you should create a new class named like AllTests.java. In this class you must create a “public static Test suite()” method, which returns a TestSuite object as a container of test case classes.

Q – 18 Can You Explain the Life Cycle of a JUnit 4.4 Test Class?

Ans- A JUnit 4.4 test class contains a @Before method, an @After method and multiple @test methods. When calling a test runner to run this test class, the runner will execute those methods in a specific order giving the test class an execution life cycle like this:

@Before
@Test XXX1
@After

@Before
@Test XXX2
@After

@Before
@Test XXX3
@After

Q – 19 How To Write Setup Code to Run Once for All Tests in a Test Class?

Ans- From the previous question, you know that if you write a setup code under the “@Before” annotation, it will be executed many times: once per each test in the test class.

Is there any way to write a setup code that will be executed only once for all tests in a single test class? The answer is YES. Here is how the JUnit FAQ answers this question in details:

The desire to do this is usually a symptom of excessive coupling in your design. If two or more tests must share the same test fixture state, then the tests may be trying to tell you that the classes under test have some undesirable dependencies.

Refactoring the design to further decouple the classes under test and eliminate code duplication is usually a better investment than setting up a shared test fixture.

But if you must…

You can add a @BeforeClass annotation to a method to be run before all the tests in a class, and a @AfterClass annotation to a method to be run after all the tests in a class.

Q – 20 Can You Explain the Life Cycle of a JUnit 3.8 Test Case Class?

Ans- A JUnit 3.8 test case class contains a setUp() method, a tearDown() method and multiple testXXX() methods. When calling a test runner to run this test class, the runner will execute those methods in a specific order giving the test case class an execution life cycle like this:

setUp()
testXXX1()
tearDown()

setUp()
testXXX2()
tearDown()

setUp()
testXXX3()
tearDown()

Q – 21 What Is a JUnit Test Fixture?

Ans- A test fixture is a fixed state of a set of objects used as a baseline for running tests. The purpose of a test fixture is to ensure that there is a well known and fixed environment in which tests are run so that results are repeatable. Examples of fixtures:

* Loading a database with a specific, known set of data
* Copying a specific known set of files
* Preparation of input data and setup/creation of fake or mock objects

In other word, creating a test fixture is to create a set of objects initialized to certain states.

If a group of tests requires diferent test fixtures, you can write code inside the test method to create its own test fixture.

If a group of tests shares the same fixtures, you should write a separate setup code to create the common test fixture.