学步园

1.假如有一个Busisness类要用到一个Printer类来输出内容，可以这样写

package com.test;

public class TestInterfaceOne {
	public static void main(String args[]) {
		Business business = new Business();
		business.UsePrinterToPrint();
	}
}

class Business {
	
	public void UsePrinterToPrint() {
		PrinterA printerA = new PrinterA();
		printerA.print();
	}
}

class PrinterA {
	public void print(){
		System.out.println("this is printed by PrinterA");
	}
}

2.但是注意到Printer类现在是PrinterA，以后还可能被替换为PrinterB，PrinterC，PrinterX...，要替换的话会频繁改动Business里面的UsePrinterToPrint()里面的代码，

所以抽象出一个接口类IPrinter，像这样:

package com.test;

public class TestInterfaceTwo {
	public static void main(String args[]) {
		Business business = new Business();
		business.UsePrinterToPrint();
	}
}

class Business {
	
	public void UsePrinterToPrint() {
		IPrinter printer = new PrinterB();
		printer.print();
	}
}

interface IPrinter {
	public void print();
}

class PrinterA implements IPrinter {
	public void print(){
		System.out.println("this is printed by PrinterA");
	}
}

class PrinterB implements IPrinter {
	public void print(){
		System.out.println("this is printed by PrinterB");
	}
}

3.改成接口后，以后每次更换Printer，只需要改动UsePrinterToPrint()里面new出来的Printer就行了，

但是这样子仍然要改动Business里面的代码，如果改动一个类后需要改动另一个类的代码，那么就说这两个类之间有依赖关系，

为了不让Business与不同的Printer之间有依赖关系，使得以后可以不改动Business的代码，

我们引入了一个工厂类PrinterFactory作为他们之间的"中介"，由PrinterFactory来决定要生产出哪一个Printer来给Business用，像这样:

package com.test;

public class TestInterfaceThree {
	public static void main(String args[]) {
		Business business = new Business();
		business.UsePrinterToPrint();
	}
}


class Business {
	
	public void UsePrinterToPrint() {
		
		new PrinterFactory().getPrinter().print();		
	}
}


class PrinterFactory {
	public IPrinter getPrinter() {
		return new PrinterB();
		//return new PrinterA;
	}
}

interface IPrinter {
	public void print();
}

class PrinterA implements IPrinter {
	public void print(){
		System.out.println("this is printed by PrinterA");
	}
}

class PrinterB implements IPrinter {
	public void print(){
		System.out.println("this is printed by PrinterB");
	}
}

4. 这样，以后我们要更换Printer，只需要改动工厂PrinterFactory里面的getPrinter()方法就行了，不用改动Business里面的代码