State pattern
The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern's superclass.
The state pattern can be interpreted as a strategy pattern which is able to switch the current strategy through invocations of methods defined in the pattern's interface.
This pattern is used in computer programming to encapsulate varying behavior for the same object based on its internal state. This can be a cleaner way for an object to change its behavior at runtime without resorting to large monolithic conditional statements[1]:395 and thus improve maintainability.[2]
Structure
Example
Java
The state interface and two implementations. The state’s method has a reference to the context object and is able to change its state.
interface Statelike {
void writeName(StateContext context, String name);
}
class StateLowerCase implements Statelike {
@Override
public void writeName(final StateContext context, final String name) {
System.out.println(name.toLowerCase());
context.setState(new StateMultipleUpperCase());
}
}
class StateMultipleUpperCase implements Statelike {
/** Counter local to this state */
private int count = 0;
@Override
public void writeName(final StateContext context, final String name) {
System.out.println(name.toUpperCase());
/* Change state after StateMultipleUpperCase's writeName() gets invoked twice */
if(++count > 1) {
context.setState(new StateLowerCase());
}
}
}
The context class has a state variable that it instantiates in an initial state, in this case StateLowerCase. In its method, it uses the corresponding methods of the state object.
class StateContext {
private Statelike myState;
StateContext() {
setState(new StateLowerCase());
}
/**
* Setter method for the state.
* Normally only called by classes implementing the State interface.
* @param newState the new state of this context
*/
void setState(final Statelike newState) {
myState = newState;
}
public void writeName(final String name) {
myState.writeName(this, name);
}
}
The demonstration below shows the usage:
public class DemoOfClientState {
public static void main(String[] args) {
final StateContext sc = new StateContext();
sc.writeName("Monday");
sc.writeName("Tuesday");
sc.writeName("Wednesday");
sc.writeName("Thursday");
sc.writeName("Friday");
sc.writeName("Saturday");
sc.writeName("Sunday");
}
}
With the above code, the output of main() from DemoOfClientState should be:
monday TUESDAY WEDNESDAY thursday FRIDAY SATURDAY sunday
See also
References
- 1 2 Erich Gamma, Richard Helm, Ralph Johnson, John M. Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
- ↑ Jaeger, Thomas. "The State Design Pattern vs. State Machine". www.codeproject.com. CodeProject. Retrieved 5 May 2015.
- 1 2 State pattern in UML and in LePUS3
- ↑ legend
External links
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The Wikibook Computer Science Design Patterns has a page on the topic of: State implementations in various languages |