OnJava8-Examples/patterns/StateDemo.java

// patterns/StateDemo.java
// (c)2021 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Basic demonstration of the State pattern.

class State {
  private State implementation;
  protected State() {}
  public State(State imp) {
    implementation = imp;
  }
  public void change(State newImp) {
    implementation = newImp;
  }
  // Forward method calls to the implementation:
  public void f() { implementation.f(); }
  public void g() { implementation.g(); }
  public void h() { implementation.h(); }
}

class Implementation1 extends State {
  @Override public void f() {
    System.out.println("Implementation1.f()");
  }
  @Override public void g() {
    System.out.println("Implementation1.g()");
  }
  @Override public void h() {
    System.out.println("Implementation1.h()");
  }
}

class Implementation2 extends State {
  @Override public void f() {
    System.out.println("Implementation2.f()");
  }
  @Override public void g() {
    System.out.println("Implementation2.g()");
  }
  @Override public void h() {
    System.out.println("Implementation2.h()");
  }
}

public class StateDemo {
  static void test(State s) {
    s.f();
    s.g();
    s.h();
  }
  public static void main(String[] args) {
    State s = new State(new Implementation1());
    test(s);
    System.out.println("Changing implementation");
    s.change(new Implementation2());
    test(s);
  }
}
/* Output:
Implementation1.f()
Implementation1.g()
Implementation1.h()
Changing implementation
Implementation2.f()
Implementation2.g()
Implementation2.h()
*/