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//Functional Interface: an interface with only one abstract method. Must be a non-default, non-static, non-private method.
//That's called being implicitly abstract. You can add the abstract modifier if it makes you feel better.
interface FunctionalInterface{
    int giveNumber();
}
public class Main {

    public static void main(String[] args) {

        //The lambda expressions will form the implementation of the abstract method.
        //Lets work with one way, in which we assign it to a variable reference
        FunctionalInterface interface1;
        interface1 = () -> 123; //We have now defined the behavior of the giveNumber method in terms of the interface1 object.
        //Technically, we have turned this small piece of code here into its own object and we can call upon it whenever we want by using the object and its method.

        System.out.println(interface1.giveNumber()); //Calling upon our new lambda expression
        
        //We can redefine the lambda expression
        interface1 = () -> 123 * 5;

        System.out.println(interface1.giveNumber());
        
        //Keep in mind the expression must match our abstract method in terms of return type and parameter types
        interface1 = () -> "booty"; //wont work because we are trying to return a string
    }
}

interface FunctionalInterface{
    //We use parameters this time in our abstract method
    double giveResult(double a, double b);
}
public class Main {

    public static void main(String[] args) {

        //Think of some code that can match our abstract method
        FunctionalInterface adding = (double a, double b) -> a + b; //You can keep the parameters unspecified too, then it will be inferred
        System.out.println(adding.giveResult(5.0, 6.0));

        //Let's define another implementation in which we subtract
        FunctionalInterface subtracting = (a, b) -> a - b;
        System.out.println(subtracting.giveResult(7.0, 2.0)); //Param types must match abstract method(ints can be used because they will basically be autoboxed to a double)

    }
}