## State pattern

### Introduction

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

### Participants:

#### Context

- defines the interface of interest to clients.
- maintains an instance of a ConcreteState subclass that defines the current state.
- this participant can not be read-only.
- this participant can not be an interface.

In the "Attribute" field you can specify the name of an attribute where Context keeps a reference to the current ConcreateState object.

#### State

- defines an interface for encapsulating the behavior associated with a particular state of the Context.
- this participant can not be read-only.

#### Concrete state

- each Concrete State implements a behavior associated with a state of the Context.
- this participant can not be read-only.

The "Copy documentation" option controls whether to copy JavaDoc comments from methods in interfaces participating in the pattern to the stubs of these methods made by the pattern in classes implementing such interfaces.

The "Create pattern links" option controls whether to generate additional links which can be used by this pattern later to determine classes and interfaces participating in the pattern. That means that if you selected this option and used the pattern to create a set of classes and interfaces, the pattern invoked for some participant later (via the popup menu item "Choose Pattern...") will automatically find (if it is possible) all other participants and fill in participant fields with their names.

Also, if you applied the pattern with this option on and invoked the pattern via the popup menu item "Choose Pattern..." for some participant later, the pattern will have additional field called "Use selected class as", containing possible roles for the selected element.

This option is very useful when you are planning to change something in the classes/interfaces participating in the pattern. For example, if this option is on and after creation of the classes and interfaces you added several methods to certain interface-participant (and this change must be reflected somehow in other participants), all you have to do is to select this changed interface, invoke the "Choose Pattern..." dialog for this element and select the original pattern. After that the pattern will determine other participants and you will only have to press "Finish". The pattern will modify all other classes and interfaces accordingly to your changes.

### Applicability

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.