@GwtCompatible public interface Function<F,T>
The Functions class provides common functions and related utilites.
 
See the Guava User Guide article on the use of Function.
@Nullable T apply(@Nullable F input)
input. This method is generally
 expected, but not absolutely required, to have the following properties:
 Objects.equal(a, b) implies that Objects.equal(function.apply(a),
     function.apply(b)).
 NullPointerException - if input is null and this function does not accept null
     argumentsboolean equals(@Nullable Object object)
Most implementations will have no reason to override the behavior of Object.equals(java.lang.Object).
 However, an implementation may also choose to return true whenever object is a
 Function that it considers interchangeable with this one. "Interchangeable"
 typically means that Objects.equal(this.apply(f), that.apply(f)) is true for all
 f of type F. Note that a false result from this method does not imply
 that the functions are known not to be interchangeable.
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