仅允许Factory方法实例化对象(防止实例化基类和未初始化的对象)

public class Animal {
    private Animal() {}
    public static Animal createInstance(String type) {
        if (type.equals("dog")) {
            return new Dog();
        }
        return null;
    }
}

public class Dog extends Animal {
    private Dog() {}
}

public abstract class Animal {
    public static Animal createInstance(String type) {
        if (type.equals("dog")) {
            return new Dog();
        }
        return null;
    }
}

public class Dog extends Animal {
    private Dog() {}
}