Calculator.java

package demo;

public class Calculator {

    enum Operation {
        Addition, Subtraction
    }

    public int calculate(Operation operation, int a, int b) {
        switch (operation) {
            case Addition:
                return addition(a, b);
            case Subtraction:
                return subtraction(a, b);
        }

        throw new RuntimeException("Unsupported exception");
    }

    private int addition(int a, int b) {
        return a + b;
    }

    private int subtraction(int a, int b) {
        return a - b;
    }

}

CalculatorTest.java

package demo;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

public class CalculatorTest {

    private Calculator calculator;

    @Before
    public void setUp() throws Exception {
        calculator = new Calculator();
    }

    @Test
    public void shouldPerformAddition() throws Exception {
        int actual = calculator.calculate(Calculator.Operation.Addition, 1, 3);
        int expected = 4;

        Assert.assertEquals(expected, actual);
    }

    @Test
    public void shouldPerformSubtraction() throws Exception {
        int actual = calculator.calculate(Calculator.Operation.Subtraction, 3, 1);
        int expected = 2;

        Assert.assertEquals(expected, actual);
    }

}