Dog.java

package pet;

class Dog {

    private static final String DEFAULT_NAME = "Spot";

    private String name;

    Dog(String name) {
        if (null == name) {
            name = DEFAULT_NAME;
        }

        this.name = name;
    }

    String getName() {
        return name;
    }

}

DogTest.java

package pet;

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 DogTest {

    @Test
    public void whenNullNameProvided_thenNameIsSpot() {
        final String expected = "Spot";
        final String actual = (new Dog(null)).getName();
        assertEquals(expected, actual);
    }

}