Generic type instantiation and Lambdas

Question

I am working with anonymous functions and functional interfaces, I have one functional interface that takes two objects of same type and returns true or false.

package elementutils;
@FunctionalInterface
public interface TwoElementPredicate <T> {
    public boolean compare(T a, T b);
    }

I use the functional interface in another class to get the "better element" using anonymous functions, the method betterElement takes two objects and the instance of the functional interface. Then i should be able to create lambdas to compare two objects of the same type in the main.

package elementutils;

public class ElementUtils <T> {
    public  T  betterElement(T a, T b, TwoElementPredicate elements){
    if (elements.compare(a, b) == true) {
        return a;
    }else {
        return b;
    }
    }
    public static void main(String[] args) {
        //String x= ElementUtils.betterElement("java", "python", (a, b) -> a.length() < b.length());
        //int y= ElementUtils.betterElement(2, 3, (a, b) -> a > b);
        //double z= ElementUtils.betterElement(2.5, 3.7, (a, b) -> a > b);
        // all this give errors

    }

}

The functions should take any object as long as they are from the same type. I thought I could achieve that using generic classes but when I instantiate the lambdas it seems that the elements are always of type object so I cannot use length() and cannot assigne them to a String for example.

I hope I explained myself correctly, any help will be highly appreciated.

*I hope i explained myself correctly*: no. What code did you try to compile/execute? What did you expect to happen? What happened instead? Precisely? — JB Nizet, Jul 11 '18 at 17:29
I tried to excute the code in main, the one that's commented. I get only objects when i want different types: string, int, doubles. — Nadia Rodriguez, Jul 11 '18 at 17:33
Don't paraphrase errors you get. Post them. *I expect this code to compile, but instead, I get the following error: * — JB Nizet, Jul 11 '18 at 17:34
See also [*What is a raw type and why shouldn't we use it?*](https://stackoverflow.com/q/2770321/2891664) — Radiodef, Jul 11 '18 at 17:38
As a side note, with the way you're calling the `betterElement` method, it looks like you meant to declare it as `public static T betterElement(...)` instead of putting the type parameter on the class. — Radiodef, Jul 11 '18 at 17:41
1) `betterElement()` is an instance method. You can't call it statically. 2) Java already has a `BiPredicate` interface. 3) If the point is to select one of the inputs, why don't you actually return it? — shmosel, Jul 11 '18 at 17:44
Never use a construct like `if (elements.compare(a, b) == true)` in Java. The left side of the comparison is already a boolean expression, hence you should only write `if(elements.compare(a, b))`. If you understand the ternary operator, you can replace the entire `if` statement with a simple `return elements.compare(a, b)? a: b;`. — Holger, Aug 22 '18 at 13:33

score 4 · Accepted Answer · answered Jul 11 '18 at 17:29

4

You are missing the type parameter T on TwoElementPredicate and hence you are using a raw type

You need to declare the parameter elements as of type TwoElementPredicate<T>

answered Jul 11 '18 at 17:29

Thiyagu

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It is because `betterElement` is not a static method – Thiyagu Jul 11 '18 at 18:44
if I make betterElement static I get: Cannot make a static reference to the non-static type T – Nadia Rodriguez Jul 11 '18 at 18:47
1

Then remove the type parameter `T` on the class and make it a generic method `public static T betterElement(T a, T b, TwoElementPredicate elements)` – Thiyagu Jul 11 '18 at 18:52

score 2 · Answer 2 · answered Jul 11 '18 at 17:30

Your definition of betterElement uses a raw TwoElementPredicate, so its arguments will always be Objects. Instead, you should use a generic parameter, with the same T as the elements:

public T betterElement(T a, T b, TwoElementPredicate<T> elements) {
    // Here ----------------------------------------^

Generic type instantiation and Lambdas

2 Answers2