src/main/java/io/reactivex/internal/functions/Functions.java
@SuppressWarnings("unchecked")
    public static <T> Function<T, T> identity() {
        return (Function<T, T>)IDENTITY;
    }
@SuppressWarnings("unchecked")
    public static <T> Function<T, T> identity() {
        return (Function<T, T>)IDENTITY;
    }
Returns an identity function that simply returns its argument.
@param <T> the input and output value type
@return the identity function