Core Java Design Pattern : Behavioral Pattern : State Design Pattern

• State Design Pattern
• Examples

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Also Known As : Objects for States

Use the State pattern in either of the following cases:

• An object’s behavior depends on its state, and it must change its behavior at run-time depending on that state.
• Operations have large, multipart conditional statements that depend on the object’s state. This state is usually represented by one or more enumerated constants. Often, several operations will contain this same conditional structure. The State pattern puts each branch of the conditional in a separate class. This lets you treat the object’s state as an object in its own right that can vary independently from other objects.

Consider a class TCPConnection that represents a network connection. A TCPConnection object can be in one of several different states: Established, Listening, Closed. When a TCPConnection object receives requests from other objects, it responds differently depending on its current state. For example, the effect of an Open request depends on whether the connection is in its Closed state or its Established state. The State pattern describes how TCPConnection can exhibit different behavior in each state.

The State pattern allows an object to change its behavior when its internal state changes. This pattern can be observed in a vending machine. Vending machines have states based on the inventory, amount of currency deposited, the ability to make change, the item selected, etc. When currency is deposited and a selection is made, a vending machine will either deliver a product and no change, deliver a product and change, deliver no product due to insufficient currency on deposit, or deliver no product due to inventory depletion.

State.java

package com.javaskool.state;

public interface State {
void grantPermission(StateContext stc);
void requestPermission(StateContext stc);
String getStatus();
}

AcceptedState.java

package com.javaskool.state;

public class AcceptedState implements State {
	
	@Override
	public String getStatus() {
		// TODO Auto-generated method stub
		return "Request received";
	}

	@Override
	public void grantPermission(StateContext stc) {
		// TODO Auto-generated method stub
		System.out.println("Invalid state");
	}

	@Override
	public void requestPermission(StateContext stc) {
		// TODO Auto-generated method stub
		System.out.println("Requesting Permission");
		stc.setState(stc.getRequestedState());
	}
}

GrantedState.java

package com.javaskool.state;

public class GrantedState implements State{
	@Override
	public String getStatus() {
		// TODO Auto-generated method stub
		
		return "Request received";
	}

	@Override
	public void grantPermission(StateContext stc) {
		// TODO Auto-generated method stub
		
		System.out.println("Invalid State");
	}

	@Override
	public void requestPermission(StateContext stc) {
		// TODO Auto-generated method stub
		System.out.println("Invalid State");
	}
}

RequestedState.java

package com.javaskool.state;

public class RequestedState implements State {
	
	
	@Override
	public String getStatus() {
		// TODO Auto-generated method stub
		System.out.println("Requested Permission");
		return null;
	}

	@Override
	public void grantPermission(StateContext stc) {
		// TODO Auto-generated method stub
		System.out.println("Granting Permission");
		stc.setState(stc.getGrantedState());
	}

	@Override
	public void requestPermission(StateContext stc) {
		// TODO Auto-generated method stub
		System.out.println("Permission already requested");
	}
}

StateContext.java

package com.javaskool.state;

public class StateContext {
	private State accepted;
	private State requested;
	private State granted;
	private State state;
	public StateContext() {
		super();
		// TODO Auto-generated constructor stub
		accepted=new AcceptedState();
		requested=new RequestedState();
		granted=new GrantedState();
	}
	public void acceptApplication(){
		state=accepted;
	}
	public void requestPermission(){
		state.requestPermission(this);
	}
	public void grantPermission(){
		state.grantPermission(this);
	}
	public String getStatus(){
		return state.getStatus();
	}
	public void setState(State s){
		state=s;
	}
	public State getAcceptedState(){
		return accepted;
	}
	public State getGrantedState(){
		return granted;
	}
	public State getRequestedState(){
		return requested;
	}
}

TestDrive.java

package com.javaskool;

import com.javaskool.state.StateContext;

public class TestDrive {
	public static void main(String arg[]){
		StateContext ctx=new StateContext();
		ctx.acceptApplication();
		ctx.requestPermission();
		ctx.grantPermission();
		System.out.println(ctx.getStatus());		
	}
}

Output

c:\>javac -d . *.java
c:\>java com.javaskool.TestDrive
Requesting Permission
Granting Permission
Request received

UML Diagram for Request-Grant-Permission Application having Implementation of State Design Pattern

Download:

State Design Pattern Code (10 KB)