How to fix "not a functional interface" problem in my code

Question

I am learning Java predicates and struck here on basic code. Below is my code.

package TestLambda;

public class Animal {
    private String species;
    private boolean canHop;
    private boolean canSwim;

    public Animal(String species,boolean canHop,boolean canSwim){
        this.species = species;
        this.canHop = canHop;
        this.canSwim = canSwim;
    }

    public boolean canHop(){return canHop;}
    public boolean canSwim(){return canSwim;}
    public String toString(){return species;}

Below is my interface.

package TestLambda;

@FunctionalInterface
public interface CheckTrait {
    boolean Test(Animal a);
}

Implementing the interface.

package TestLambda;

public class CheckIfHopper implements CheckTrait {
    public boolean Test(Animal a){
        return a.canHop();
    }
}

Now i am testing a simple Lambda.

package TestLambda;

import java.util.ArrayList;
import java.util.List;

public class LambdaSearch {

    public static void main(String[] args) {
        List<Animal> animals = new ArrayList<>();
        animals.add(new Animal("fish",false,true));
        animals.add(new Animal("dog",true,true));
        animals.add(new Animal("rabbit",true,false));
        animals.add(new Animal("horse",true,false));

        print(animals, a -> a.canHop());
    }

    public static void print(List<Animal> animals,CheckIfHopper checkIfHopper){
        for(Animal animal :animals){
            if(checkIfHopper.Test(animal)){
                System.out.println(animal + " ");
            }
        }
    }

}

Mycode is throwing error:- TestLambda.CheckIfHopper is not a functional interface

What i am doing wrong please?

Answer 1

If your purpose is to test lambda expressions, then CheckIfHopper is redundant here.

Your print method would take a CheckTrait parameter:

public static void print(List<Animal> animals, CheckTrait checkIfHopper){
    for(Animal animal :animals){
        if(checkIfHopper.Test(animal)){
            System.out.println(animal + " ");
        }
    }
}

And the call would be:

print(animals, a -> a.canHop());

Or the following for brevity:

print(animals, Animal::canHop);

You can't make a lambda expression to provide an instance of a class, its target type must be a functional interface (this is essentially what the error message is saying)

You don't need the CheckIfHopper class, because a -> a.canHop() or Animal::canHop provide an implementation of CheckTrait without a class declaration (that's the point of lambda expressions, isn't it...)