What does function interface do in the lambda expression? [duplicate]

Question

@FunctionalInterface
public interface myFunction<T> {
  abstract double apply(T t);
}

The above code is my functional interface.

And the following code is my lambda expression. What does functional interface do in my lambda expression as there is only a simple 'double'? If there is no functional interface, what will there be like?

import java.util.HashSet;

public class TestFindArea {
public static double findArea(myFunction<Double> f,double a,double b){
    return (f.apply(a)+f.apply(b))*(b-a)/2;
}
    public static void main(String[] args) {
        System.out.println(findArea((x) -> x+2 , 4, 8) );

Answer 1

What does functional interface do in my lambda expression as there is only a simple 'double'?

your functional interface represents a function that takes a generic type T and returns a double, meaning wherever we utilize the functional interface as the target type, eventually, we will need to assign or pass it a function which satisfies the function descriptor of its SAM. Thus, in your case, the first argument to findArea((x) -> x+2 , 4, 8) is the function being passed to the findArea method and can be read as "given a number of type Double, add the number 2 to it and return the result".

If there is no functional interface, what will there be like?

Then you cannot pass functions as arguments to other functions nor can you use functions as return types for other functions and nor can you assign a lambda expression to an interface.