Guava why toStringFunction not a generic function?

the Guava toStringFunction() has the following declaration:

public static Function<Object, String> toStringFunction() { ... } 

All non-primitive roots at Object, so the function works well.

but when I try to compose it with another function, such as:

Function<Integer, Double> f1 = Functions.compose(Functions.forMap(someMap),
                                                 Functions.toStringFunction());

where someMap variable is a map< String, Double>, So i expect toStringFunction converts Integer to String and then forMap converts String to Double. But I get a compiler error:

    Function<Integer, Double> f1 = Functions.compose(Functions.forMap(someMap),
                                                    ^
  required: Function<Integer,Double>
  found:    Function<Object,Double>
2 errors

My two questions are:

1.how to specifically tell the compiler that toStringFunction should be Function< Integer, String>? A simple cast will not work, and I am looking for a real composition of the two functions.

Function< Integer, String> g1 = (Function< Integer, String>)Functions.toStringFunction(); 

// cause error 

2.Had the toStringFunction written as the following:

 @SuppressWarnings("unchecked") 
  public static <E> Function<E, String> toStringFunction(){
    return (Function<E, String>) ToStringFunction.INSTANCE;
  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String>{
    INSTANCE;


    // @Override public String toString(){
    //   return "Functions.toStringFunction()";
    // }

    @Override public String apply(Object o){
      return o.toString();
    }
  }

I can specify the type:

Function<Integer, Double> f1 = Functions.compose(Functions.forMap(someMap),
                                                 Functions.<Integer>toStringFunction());

and this works fine. But what is the motivation for the version Guava team have right now?