为什么此类不适用于Junit测试？

我没有这个问题的答案，但我认为是这样，因为它只有无效的方法，无论如何也只是在计数。还是有更好的解释？

public class PlayingField implements Security{
    private int kidsCount;

    @Override
    public void addPerson() {
        kidsCount++;
        soundAlarm();

    }

    @Override
    public void removePerson() {
        kidsCount--;

    }

    @Override
    public int getPersonCount() {
        return kidsCount;
    }

    @Override
    public void soundAlarm() {
        if (kidsCount> 50) {
            System.out.println("cant add a kid to the PlayingField");
            kidsCount--;
        }
    }

}

这些方法是从另一个类使用以下方法调用的：

public class KidsShop extends FashionShop {

    private PlayingField playingField;

    public KidsShop(String name, int area, int rent) {
        super(name, area, rent);
        this.playingField = new PlayingField();
    }
    public void addKid() {
        playingField.addPerson();
    }
    public void pickUpKid() {
        playingField.removePerson();
    }
    public int getNumberOfKids() {
        return playingField.getPersonCount();
    }