This small example shows you how to write a unit test. You need to have a JDK installed and a text editor. (In general it is recommended to use a build tool for building your software and running the tests.)

Create a new folder junit-example and download the current junit-4.XX.jar from JUnit's release page and Hamcrest to this folder. Change to the folder junit-example. All files are created within this folder and all commands are executed there, too.

Create a new file Calculator.java and copy the following code to this file.

public class Calculator { public int evaluate(String expression) { int sum = 0; for (String summand: expression.split("\\+")) sum += Integer.valueOf(summand); return sum; } }

Now compile this class:

javac Calculator.java 

The Java compiler creates a file Calculator.class.

Create the file CalculatorTest.java and copy the following code to this file.

import static org.junit.Assert.assertEquals; import org.junit.Test; public class CalculatorTest { @Test public void evaluatesExpression() { Calculator calculator = new Calculator(); int sum = calculator.evaluate("1+2+3"); assertEquals(6, sum); } }

Compile the test. On Linux or MacOS

javac -cp .:junit-4.XX.jar:hamcrest-core-1.3.jar CalculatorTest.java 

and on Windows

javac -cp .;junit-4.XX.jar;hamcrest-core-1.3.jar CalculatorTest.java 

The Java compiler creates a file CalculatorTest.class.

Run the test from the command line. On Linux or MacOS

java -cp .:junit-4.XX.jar:hamcrest-core-1.3.jar org.junit.runner.JUnitCore CalculatorTest 

and on Windows

java -cp .;junit-4.XX.jar;hamcrest-core-1.3.jar org.junit.runner.JUnitCore CalculatorTest 

The output is

JUnit version 4.12 . Time: 0,006 OK (1 test) 

The single . means that one test has been run and the OK in the last line tells you that your test is successful.

Modify Calculator.java in order to get a failing test. Replace the line

sum += Integer.valueOf(summand); 

with

sum -= Integer.valueOf(summand); 

and recompile the class.

javac Calculator.java 

Run the test again. On Linux or MacOS

java -cp .:junit-4.XX.jar:hamcrest-core-1.3.jar org.junit.runner.JUnitCore CalculatorTest 

and on Windows

java -cp .;junit-4.XX.jar;hamcrest-core-1.3.jar org.junit.runner.JUnitCore CalculatorTest 

Now the test fails and the output is

JUnit version 4.12 .E Time: 0,007 There was 1 failure: 1) evaluatesExpression(CalculatorTest) java.lang.AssertionError: expected:<6> but was:<-6> at org.junit.Assert.fail(Assert.java:88) ... FAILURES!!! Tests run: 1, Failures: 1 

JUnit tells you which test failed (evaluatesExpression(CalculatorTest)) and what went wrong:

java.lang.AssertionError: expected:<6> but was:<-6>