开闭原则说的通俗一点就是对扩展开放，对修改关闭；这样原则设计出来的系统，遇到增加功能的需求时，几乎不用修改源代码，只是增加几个类，然后调用就好。这样既增加了新功能满足了需求，又维护了原本系统的稳定性。下面我写一个demo,具体分为5步，作用是比较两个事物的大小，比如可以比较2个人的大小、2个房子的大小、2头猪的大小等，看了demo就会明白。

一、类图

二、需要比较的对象，暂时只有房子、人

public class House {
    int width;
    int length;
    String addr;

    public House (int width, int length, String addr){
        this.width = width;
        this.length = length;
        this.addr = addr;
    }

    public int getWidth() {
        return width;
    }
    public void setWidth(int width) {
        this.width = width;
    }
    public int getLength() {
        return length;
    }
    public void setLength(int length) {
        this.length = length;
    }
    public String getAddr() {
        return addr;
    }
    public void setAddr(String addr) {
        this.addr = addr;
    }

    public int getSquare() {
        return width * length;
    }


    @Override
    public String toString() {
        return "House [width=" + width + ", length=" + length + ", addr="
                + addr + "]";
    }
}

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

三、抽象类，把随着比较对象不同而改变的方法抽象出来

public abstract class CompareObject<T> {
    protected T object1;
    protected T object2;

    public void setData(T arg1, T arg2) {
        object1 = arg1;
        object2 = arg2;
    }


    public abstract T getMax();

    public abstract T getMin();
}

四、比较类，主要实现具体的比较方法，人呢是比较年龄，房子是比较面积。

public class CompareHouse extends CompareObject<House> {


    @Override
    public House getMax() {
        if (object1.getSquare() > object2.getSquare()) {
            return object1;
        }
        return object2;
    }

    @Override
    public House getMin() {
        if (object1.getSquare() > object2.getSquare()) {
            return object2;
        }
        return object1;
    }

    public Object getCheaper() {

        return null;
    }
}

public class ComparePerson extends CompareObject<Person> {


    @Override
    public Person getMax() {
        if (object1.getAge() > object2.getAge()) {
            return object1;
        }
        return object2;
    }

    @Override
    public Person getMin() {
        if ((object1).getAge() > (object2).getAge()) {
            return object2;
        }
        return object1;


    }
}

五、调用

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Person person = new Person("petter", 20);
        Person person1 = new Person("mery", 21);
        ComparePerson comparePerson = new ComparePerson();
        comparePerson.setData(person, person1);
        Log.d(TAG,"Max_Person"+ comparePerson.getMax().toString());
//        house
        House house = new House(8, 12, "beijing");
        House house1 = new House(10, 11, "shenzhen");
        CompareHouse compareHouse = new CompareHouse();
        compareHouse.setData(house, house1);
        Log.d(TAG,"Max_House"+ compareHouse.getMax().toString());
    }
}

运行结果：

2019-03-14 18:09:26.008 15027-15027/com.example.openingdemo D/MainActivity: Max_PersonPerson{name='mery', age=21}
2019-03-14 18:09:26.009 15027-15027/com.example.openingdemo D/MainActivity: Max_HouseHouse [width=10, length=11, addr=shenzhen]

这样做的好处就是，如果下次要添加比较2头猪的需求时，我们无需改动原本的代码，只要添加一个pig类、一个ComparePig类就好了。