Behavioural Design patterns: State

The state pattern deals with altering an object’s behaviour when its state changes.

Imagine the case of a class responsible for generating user interface based on the state. You got anonymous, logged-in and admin users.

We shall create an interface called GreetingState which defines the action of drawing a html text with a welcome message to the user. There is going to be a different implementation according to the states that we have.

package com.gkatzioura.design.behavioural.state;

public interface GreetingState {

    String create();

}

We shall implement the GreetingState for the anonymous user.

package com.gkatzioura.design.behavioural.state;

public class AnonymousGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello anonymous user!</p>";

    @Override
    public String create() {
        return FOOTER_MESSAGE;
    }

}

Then we shall implement the GreetingState for the logged in user. This one would create a personalised message.

package com.gkatzioura.design.behavioural.state;

public class LoggedInGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello %s!</p>";

    private final String username;

    public LoggedInGreetingState(final String username) {
        this.username = username;
    }

    @Override
    public String create() {
        return String.format(FOOTER_MESSAGE,username);
    }

}

And at last the admin Footer.

package com.gkatzioura.design.behavioural.state;

import java.util.Date;

public class AdminGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello %s, last login was at %s</p>";

    private final String username;
    private final Date lastLogin;

    public AdminGreetingState(final String username, Date lastLogin) {
        this.username = username;
        this.lastLogin = lastLogin;
    }


    @Override
    public String create() {
        return String.format(FOOTER_MESSAGE,username,lastLogin);
    }

}

The we shall create the stateui context.

package com.gkatzioura.design.behavioural.state;

import java.io.PrintWriter;

public class StateUIContext {

    private GreetingState greetingState;

    public void setGreetingState(GreetingState greetingState) {
        this.greetingState = greetingState;
    }

    public void create(PrintWriter printWriter) {
        printWriter.write(greetingState.create());
    }
}

Let’s put them all together.

package com.gkatzioura.design.behavioural.state;

import java.io.PrintWriter;
import java.util.Date;

public class StateMain {

    public static void main(String[] args) {

        StateUIContext stateUIContext = new StateUIContext();

        try(PrintWriter printWriter = new PrintWriter(System.out)) {
            stateUIContext.setGreetingState(new AnonymousGreetingState());
            stateUIContext.create(printWriter);
            printWriter.write("\n");
            stateUIContext.setGreetingState(new LoggedInGreetingState("someone"));
            stateUIContext.create(printWriter);
            printWriter.write("\n");
            stateUIContext.setGreetingState(new AdminGreetingState("admin",new Date()));
            stateUIContext.create(printWriter);
            printWriter.write("\n");
        }
    }
}

You can find the sourcecode on github.