Counter.java

package math;

class Counter {

    private int counter;

    Counter() {
        counter = 0;
    }

    Counter increment() {
        counter = 1;
        return this;
    }

    Counter decrement() {
        counter = -1;
        return this;
    }

    int getCounter() {
        return counter;
    }

}

CounterTest.java

package math;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

import static org.junit.Assert.assertEquals;

@RunWith(JUnit4.class)
public class CounterTest {

    private Counter sut;

    @Before
    public void setup() {
        sut = new Counter();
    }
        
    @Test
    public void whenInitialised_thenCounterIsZero() {
        final int expected = 0;
        final int actual = sut.getCounter();
        assertEquals(expected, actual);
    }

    @Test
    public void whenIncrement_thenCounterIsPlusOne() {
        final int expected = 1;
        final int actual = sut.increment().getCounter();
        assertEquals(expected, actual);
    }

    @Test
    public void whenDecrement_thenCounterIsMinusOne() {
        final int expected = -1;
        final int actual = sut.decrement().getCounter();
        assertEquals(expected, actual);
    }

}