IT. Expert System.
Design patterns
State Pattern
In State pattern a class behavior changes based on its state. This type of design pattern comes under behavior pattern.
In State pattern, we create objects which represent various states and a context object whose behavior varies as its state object changes.
Implementation
We're going to create a State interface defining a action and concrete state classes implementing the State interface. Context is a class which carries a State.
StaePatternDemo, our demo class will use Context and state objects to demonstrate change in Context behavior based on type of state it is in.
Step 1
Create an interface.
Image.java
public interface State {
public void doAction(Context context);
}
Step 2
Create concrete classes implementing the same interface.
StartState.java
public class StartState implements State {
public void doAction(Context context) {
System.out.println("Player is in start state");
context.setState(this);
}
public String toString(){
return "Start State";
}
}
StopState.java
public class StopState implements State {
public void doAction(Context context) {
System.out.println("Player is in stop state");
context.setState(this);
}
public String toString(){
return "Stop State";
}
}
Step 3
Create Context Class.
Context.java
public class Context {
private State state;
public Context(){
state = null;
}
public void setState(State state){
this.state = state;
}
public State getState(){
return state;
}
}
Step 4
Use the Context to see change in behaviour when State changes.
StatePatternDemo.java
public class StatePatternDemo {
public static void main(String[] args) {
Context context = new Context();
StartState startState = new StartState();
startState.doAction(context);
System.out.println(context.getState().toString());
StopState stopState = new StopState();
stopState.doAction(context);
System.out.println(context.getState().toString());
}
}
Step 5
Verify the output.
Player is in start state
Start State
Player is in stop state
Stop State
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