Nullable (java5 21.0.1 API)

```
@Documented
@Retention(CLASS)
@Target({METHOD,FIELD,PARAMETER,LOCAL_VARIABLE})
public @interface Nullable
```
An element annotated with Nullable claims null value is perfectly valid to return (for methods), pass to (parameters) or hold in (local variables and fields). Apart from documentation purposes this annotation is intended to be used by static analysis tools to validate against probable runtime errors or element contract violations.
By convention, this annotation applied only when the value should always be checked against null because the developer could do nothing to prevent null from happening. Otherwise, too eager Nullable usage could lead to too many false positives from static analysis tools.
For example, Map.get(Object key) should not be annotated Nullable because someone may have put not-null value in the map by this key and is expecting to find this value there ever since.
On the other hand, the Reference.get() should be annotated Nullable because it returns null if object got collected which can happen at any time completely unexpectedly.

- Optional Element Summary
  
  Optional Elements
  Modifier and Type Optional Element Description
  
  java.lang.String value

- Element Detail
  - value
```
@NonNls
java.lang.String value
```
    Returns:
    
    textual reason when the annotated value could be null, for documentation purposes.
    
    Default:
    
    ""