FAQ : JUnit Interview Questions Page 1

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What is JUnit?
JUnit is a simple, open source framework to write and run repeatable tests. It is an instance of the xUnit architecture for unit testing frameworks. JUnit features include:
Assertions for testing expected results
Test fixtures for sharing common test data
Test runners for running tests

How do I install JUnit?
First, download the latest version of JUnit, referred to below as junit.zip.
Then install JUnit on your platform of choice:
Windows
To install JUnit on Windows, follow these steps:
Unzip the junit.zip distribution file to a directory referred to as %JUNIT_HOME%.
Add JUnit to the classpath:
set CLASSPATH=%CLASSPATH%;%JUNIT_HOME%\junit.jar

Unix (bash)
To install JUnit on Unix, follow these steps:
Unzip the junit.zip distribution file to a directory referred to as $JUNIT_HOME.
Add JUnit to the classpath:
export CLASSPATH=$CLASSPATH:$JUNIT_HOME/junit.jar
(Optional) Unzip the $JUNIT_HOME/src.jar file.

Test the installation by running the sample tests distributed with JUnit. Note that the sample tests are located in the installation directory directly, not the junit.jar file. Therefore, make sure that the JUnit installation directory is on your CLASSPATH. Then simply type:

java org.junit.runner.JUnitCore org.junit.tests.AllTests
All the tests should pass with an "OK" message.

If the tests don't pass, verify that junit.jar is in the CLASSPATH.

How do I write and run a simple test?  Create a class:
package junitfaq;
import org.junit.*;
import static org.junit.Assert.*;
import java.util.*;
 
public class SimpleTest {

Write a test method (annotated with @Test) that asserts expected results on the object under test:
@Test
public void testEmptyCollection() {
Collection collection = new ArrayList();
assertTrue(collection.isEmpty());
}

If you are running your JUnit 4 tests with a JUnit 3.x runner, write a suite() method that uses the JUnit4TestAdapter class to create a suite containing all of your test methods:
public static junit.framework.Test suite() {
return new junit.framework.JUnit4TestAdapter(SimpleTest.class);
}

Although writing a main() method to run the test is much less important with the advent of IDE runners, it's still possible:
public static void main(String args[]) {
org.junit.runner.JUnitCore.main("junitfaq.SimpleTest");
}
}
Run the test:

To run the test from the console, type:
java org.junit.runner.JUnitCore junitfaq.SimpleTest

To run the test with the test runner used in main(), type:
java junitfaq.SimpleTest

The passing test results in the following textual output:
.Time: 0
OK (1 tests)

How do I use a test fixture?
A test fixture is useful if you have two or more tests for a common set of objects. Using a test fixture avoids duplicating the code necessary to initialize (and cleanup) the common objects.

Tests can use the objects (variables) in a test fixture, with each test invoking different methods on objects in the fixture and asserting different expected results. Each test runs in its own test fixture to isolate tests from the changes made by other tests. That is, tests don't share the state of objects in the test fixture. Because the tests are isolated, they can be run in any order.

To create a test fixture, declare instance variables for the common objects. Initialize these objects in a public void method annotated with @Before. The JUnit framework automatically invokes any @Before methods before each test is run.

The following example shows a test fixture with a common Collection object.

package junitfaq;

import org.junit.*;
import static org.junit.Assert.*;
import java.util.*;

public class SimpleTest {

private Collection<Object> collection;

@Before
public void setUp() {
collection = new ArrayList<Object>();
}

@Test
public void testEmptyCollection() {
assertTrue(collection.isEmpty());
}


@Test
public void testOneItemCollection() {
collection.add("itemA");
assertEquals(1, collection.size());
}
}
Given this test, the methods might execute in the following order:

setUp()
testEmptyCollection()
setUp()
testOneItemCollection()

The ordering of test-method invocations is not guaranteed, so testOneItemCollection() might be executed before testEmptyCollection(). But it doesn't matter, because each method gets its own instance of the collection.

Although JUnit provides a new instance of the fixture objects for each test method, if you allocate any external resources in a @Before method, you should release them after the test runs by annotating a method with @After. The JUnit framework automatically invokes any @After methods after each test is run. For example:

package junitfaq;

import org.junit.*;
import static org.junit.Assert.*;
import java.io.*;

public class OutputTest {

private File output;

@Before
public void createOutputFile() {
output = new File(...);
}

@After
public void deleteOutputFile() {
output.delete();
}

@Test
public void testSomethingWithFile() {
...
}
}

With this test, the methods will execute in the following order:

createOutputFile()
testSomethingWithFile()
deleteOutputFile()

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