设计模式(3)- 状态设计模式State

状态模式用来改变对象的行为，当状态发生变化时，行为也随着发生变化，和switch分支语句有点类似，比如如下的代码：

public class Creature{
private Boolean isFrog = true;//标识
public void greet(){
if(isForg){
System.out.println(“Ribbet!”);
}else{
System.out.println(“Darling!”);
}
}
//转换标识
public void kiss(){
isForg = false;
}
public static void main(String[] args){
Creature creature = new Creature();
creature.greet();
creature.kiss();
creature.greet();
}
}

greet()方法在业务复杂的时候if else会很多，而替换为状态模式就变得优雅了许多：

简单的状态模式

public class Creature{
//状态接口
private interface State{
String response();
}
private class Forg implements State{
public String response(){
return “Ribbet!”;
}
}
private class Prince implements State{
public String response(){
return “Darling!”;
}
}
private State state = new Forg();
public void greet(){
System.out.println(state.response);
}
public void kiss(){
state = new Prince();
}
public static void main(String[] args){
Creature creature = new Creature();
creature.greet();
creature.kiss();
creature.greet();
}
}

在状态模式中可以有多个状态，同时对应多个分支程序;

//状态接口
interface State{
void operation1();
void operation2();
void operation3();
}
//状态实现类1
class implementation1 implements State{
public void operation1(){
System.out.println(“Implementation1.operation1()”);
}
public void operation2(){
System.out.println(“Implementation1.operation2()”);
}
public void operation3(){
System.out.println(“Implementation1.operation3()”);
}
}
//状态实现类2
class implementation2 implements State{
public void operation1(){
System.out.println(“Implementation2.operation1()”);
}
public void operation2(){
System.out.println(“Implementation2.operation2()”);
}
public void operation3(){
System.out.println(“Implementation2.operation3()”);
}
}
//服务提供者
class ServiceProvider{
private State state;
public ServiceProvider(State state){
this.state = state;
}
//状态更改
public void changeState(State newState){
state = newState;
}
public void service1(){
//……
state.operation1();
//……
state.operation3();
}
public void service2(){
//……
state.operation1();
//……
state.operation2();
}
public void service3(){
//……
state.operation3();
//……
state.operation2();
}
}
public class StateDemo{
private ServiceProvider sp = new ServiceProvider(new Implementation1());
private void run(ServiceProvider sp){
sp.service1();
sp.service2();
sp.service3();
}
public static void main(String[] args){
StateDemo demo = new StateDemo();
demo.run(sp);
sp.changeState(new Implementation2());
demo.run(sp);
}
}

状态模式和代理模式有相似之处，都有目标对象和代理的相似概念

但是他们之间是有区别的：

1.代理模式目标对象只有一个，而状态模式可以有多个目标对象，即，多个状态。

2.代理模式是为了控制目标对象的访问，状态模式是为了根据状态或者标示判断使用哪个目标。