src/main/java/io/reactivex/internal/functions/Functions.java
public static <T, U> Function<T, U> justFunction(U value) {
        return new JustValue<T, U>(value);
    }
public static <T, U> Function<T, U> justFunction(U value) {
        return new JustValue<T, U>(value);
    }
Returns a Function that ignores its parameter and returns the given value.
@param <T> the function's input type
@param <U> the value and return type of the function
@param value the value to return
@return the new Function instance