DoFn.StateId (core 2.44.0 API)

```
@Documented
@Retention(RUNTIME)
@Target({FIELD,PARAMETER})
@Experimental(STATE)
public static @interface DoFn.StateId
```
Annotation for declaring and dereferencing state cells.
To declare a state cell, create a field of type StateSpec annotated with a DoFn.StateId. To use the cell during processing, add a parameter of the appropriate State subclass to your @ProcessElement or @OnTimer method, and annotate it with DoFn.StateId. See the following code for an example:
```
new DoFn<KV<Key, Foo>, Baz>() {

  @StateId("my-state-id")
  private final StateSpec<ValueState<MyState>> myStateSpec =
       StateSpecs.value(new MyStateCoder());

  @ProcessElement
   public void processElement(
       @Element InputT element,
      @StateId("my-state-id") ValueState<MyState> myState) {
     myState.read();
     myState.write(...);
   }
 }
 
```
State is subject to the following validity conditions:
- Each state ID must be declared at most once.
- Any state referenced in a parameter must be declared with the same state type.
- State declarations must be final.

- Required Element Summary
  
  Required Elements
  Modifier and Type Required Element Description
  
  java.lang.String value
  The state ID.

- Element Detail
  - value
```
java.lang.String value
```
    The state ID.