JUnit 4

Download junit and hamcrest jar files.

Create the class under test and the test.

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

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 both classes then run.

javac Calculator.java

javac -cp .:junit-4.12.jar:hamcrest-core-1.3.jar CalculatorTest.java

java -cp .:junit-4.12.jar:hamcrest-core-1.3.jar org.junit.runner.JUnitCore CalculatorTest
JUnit version 4.12
.
Time: 0.025

OK (1 test)

Alternate version with Maven in github