java开闭原则实例

题记：

以后想多写几篇blog，不为别的，为了加深自己学习印象吧。也许有人会问，你都做java这么久了还在学这些啊，没错，正是因为我做久了java，才发现自己的基础知识有些欠缺，概念懂一些，但未必深刻，所以还要再学一下下。。。

java开闭原则，即对扩展开放，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。

下面我会写一个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瞬间明白。

一、类图

二、需要比较的对象，暂时只有房子、人

//房子
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 + "]";
	}
	
	
}

//人
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 {

	protected Object object1;
	protected Object object2;
	
	public void setData(Object arg1, Object arg2){
		
		if (isAccept(arg1,arg2)) {
			object1 = arg1;
			object2 = arg2;
		}else {
			throw new RuntimeException( arg1 + " and " + arg2 + " isn't the them type ");
		}
		
	}
	
	
	protected abstract boolean isAccept(Object arg1, Object arg2);

	
	public abstract Object getMax();
	
	
	public abstract Object 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。

/**
 * @author lei
 *
 */
public class ComparePerson extends CompareObject{

	@Override
	public Object getMax() {
		Person person1 = (Person) object1;
		Person person2 = (Person) object2;
		if (person1.getAge() > person2.getAge()) {
			return person1;
		}
		return person2;
	}

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

	@Override
	protected boolean isAccept(Object arg1, Object arg2) {
		if ((arg1 instanceof Person) && (arg2 instanceof Person)) {
			return true;
		}
		return false;
	}

	
	
}


public class CompareHouse extends CompareObject {

	@Override
	protected boolean isAccept(Object arg1, Object arg2) {
		if ((arg1 instanceof House) && (arg2 instanceof House)) {
			return true;
		}
		return false;
	}

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

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

}

五、最后的调用

public class MainClass {

	
	public static void main(String[] args) {
		
		Person person1 = new Person("petter", 20);
		Person person2 = new Person("mery", 21);
		ComparePerson comparePerson = new ComparePerson();
		comparePerson.setData(person1, person2);
		System.out.println("max_person: " + comparePerson.getMax().toString());
		
		//house
		House house1 = new House(8,12," ShangHai ");
		House house2 = new House(10,11," ShenZheng ");
		CompareHouse compareHouse = new CompareHouse();
		compareHouse.setData(house1, house2);
		System.out.println("max_House: " + compareHouse.getMax().toString());
	}

}

打印结果：

max_person: Person [name=mery, age=21]
max_House: House [width=10, length=11, addr= ShenZheng ]

好了，demo已经结束，差不多已经明白了吧。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。