How to bind a Java Supplier to an instance of an object?

Question

How can I bind a Java Supplier to an existing instance of an Object? For example, if I want to write my own compareTo() method with this header:

public static int myCompareTo(Object o1, Object o2, Supplier<Comparable> supplier) {...}

I want be able to call it like:

myCompareTo("Hello", "Hello2", String::length);

where String (with the capital letter) is a class and no object. So how can I bind the instance o1 to the supplier?

Answer 1

Thanks for your answers. Actually I figured it out now. I wanted to have the supplied object instances (o1 and o2) to execute the given method. I found out that Supplier was the wrong interface instead I had to use Function. Here you can see my working simplified example:

public static <T> int myCompareTo(T o1, T o2, Function<T, Comparable> getter) {
        return getter.apply(o1).compareTo(getter.apply(o2));
}

The reason, the interface has to be Function and not Supplier is, that only Function is equivalent to a lambda expression taking an object and calls the referenced method on the object.

For example, if you define the method reference as:

Function<TypeOfInstance, ReturnTypeOfReferencedMethod> methodReference = TypeOfInstance::referencedMethod();

then the equivalent lambda expression being executed is:

(instance) -> instance.referencedMethod()

Additional Information:

Edit: I know I could have done the same by using Comparator, but this example is very simplified. In my application a Function of this kind is neccessary. I had to create a compareTo function that sorts an ArrayList by more than one attribute because the main sorting attribute may not be unique in the list. I want to share my code with you, because I think it can be a interesting insight for you.

public static <T> int ultimateCompare(T o1, T o2, Function<T, Comparable>... getters) {
    for (Function<T, Comparable> getter : getters) {
        int result = getter.apply(o1).compareTo(getter.apply(o2));
        if (result != 0) return result;
    }
    return 0;
}

With this for example, you can sort a list of persons by last name and if two of them are identical, you can use the first name to sort. With this solution you can change sorting at runtime.

Answer 2

Here's what you were searching for (I believe):

public static <T, U extends Comparable<U>> int compare(T o1, T o2, Function<T, U> mapper) {
    return mapper.apply(o1).compareTo(mapper.apply(o2));
}

You can call that like so:

compare("str1", "str2", String::length); // 0