Simple example of JUnit

JUnitTest.java

import org.junit.Test;
import static org.junit.Assert.assertEquals;

import java.util.Arrays;

public class JUnitTest {
    @Test
    public void CalculatorTest() {
        Calculator calculator = new Calculator();
        assertEquals(10, calculator.sum("1234"));
    }

}

class Calculator {
    public int sum(String digits) {
        return Arrays.stream(digits.split("")).map(Integer::valueOf)
                .reduce(Integer::sum).get();
    }
}