State

The patterns in this chapter describe how to communicate your use of state. Objects are convenient packages of behavior which is presented to the outside world and state which is used to support that behavior. One of the advantages of objects is that they mince all of the state of a program into tiny pieces, each effectively its own little computer. Large libraries of state, promiscuously referenced, make further changes to code difficult because the effect of a code change on the state is hard to predict. With objects, it is easier to analyze what state will be affected by a change, because the namespace of referenceable state is so much smaller.

The chapter contains the following patterns:

• State
  Compute with values that change over time.
• Access
  Maintain flexibility by limiting access to state.
• Direct Access
  Directly access state inside an object.
• Indirect Access
  Access state through a method to provide greater flexibility.
• Common State
  Store the state common to all objects of a class as fields.
• Variable State
  Store state whose presence differs from instance to instance as a map.
• Extrinsic State
  Store special-purpose state associated with an object in a map held by the user of that state.
• Variable
  Variables provide a namespace for accessing state.
• Local Variable
  Local variables hold state for a single scope.
• Field
  Fields store state for the life of an object.
• Parameter
  Parameters communicate state during the activation of a single method.
• Collecting Parameter
  Pass a parameter to collect complicated results from multiple methods.
• Optional Parameter
  Pass a parameter or use default parameter’s value.
• Var Args
  Pass various count of parameters.
• Parameter Object
  Consolidate frequently used long parameter lists into an object.
• Constant
  Store state that doesn’t vary as a constant.
• Role-Suggesting Name
  Name variables after the role they play in a computation.
• Declared Type
  Declare a general type for variables.
• Initialization
  Initialize variables declaratively as much as possible.