State

Intent

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

Class diagram of the original GoF pattern

Improvement in MixJuice

Solved Problem(s)

Extensibility problem

It is impossible to add new operation to State without modifying the source code. (This problem is not mentioned in the GoF book.)

Used programming technique(s)

Abstract method addition to State.

Consequences

• (+) New operations can be added to State without modifying the source code.
• (-) Addition of both a new operation and a new ConcreteState requires a complementary module.

Structure and pseudo code

module original {
  define class Context {
    State state;
    define void request(){
      state.handle();
    }
  }
  define abstract class State {
    define abstract void handle();
  }
  define class ConcreteStateA extends State {
    void handle(){...}
  }
  define class ConcreteStateB extends State {
    void handle(){...}
  }
}
module extension extends original {
  class State {
    define abstract void newOperation();
  }
  class ConcreteStateA {
    void newOperation(){...}
  }
  class ConcreteStateB {
    void newOperation(){...}
  }
}

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