What Is It?

JUnit is a framework for facilitating unit tests in Java.

JUnit provides the following:

• A way to mark certain methods as tests
• A way to execute and manage the results of these tests (test runner)
• Assert functions that allow to declare when a test should fail and when succeed

The entire framework is contained within a single JAR file:

• This JAR file is usually called junit-*.jar (where * is the version number)
• It can be downloaded from here
• Tip: to locate the JUnit JAR file on your computer, you can use:

    find / -name "junit*.jar" 2>/dev/null
  

JUnit 4 vs. JUnit 3

JUnit 4 (current version):

• Located in package org.junit
• To mark a method as a test, add annotation @Test to the method

JUnit 3 (deprecated but still widely used):

• Located in package junit.framework
• To mark a method as a test, the containing class must extend junit.framework.TestCase, and the name of the method must start with test

How To Run Tests

• From command line (requires the junit.jar on the classpath):

    java org.junit.runner.JUnitCore <ClassContainingTests>
  

• Create a custom class with a main method (test runner) and call e.g. JUnitCore.runClasses

Notes

• The assert functions of JUnit 4 (e.g. assertEquals) are in the class org.junit.Assert
• The JUnit 4 API documentation can be found here