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>