Please Explain Java 8 Method Reference to instance Method using class name

Question

public interface MyFunc {

    boolean func(T v1, T v2);

}

public class HighTemp {

    private int hTemp;

    HighTemp(){

           


        

Answer 1

For starters I'm not a professional programmer. I too had a great difficulty in understanding the so called "Reference to an Instance Method of an Arbitrary Object of a Particular Type" I think this might be helpful for somebody who comes here from a google search.
I understood it a little bit with the help of lambda expressions.

In your code HighTemp::lessThanTemp as a Lambda expression would look like (x,y)->{x.lessThanTemp(y);} Replacing the method reference with this lambda expression would produce the same result. The above Lambda expression or the method reference both tell the interface method what to do.
When you use the method reference it tells the interface method to use the referred method from the given class, to carryout its function. Therefore if you convert HighTemp::lessThanTemp to English words it would sound something like "implement the lessThanTemp method form the class HighTemp as the implementation of the interface function". As you might've noticed in that case the return types and the argument types should be compatible. Otherwise you cannot implement an interface.

I would provide you another simple example code. More examples helps to understand this concept.

interface myint{
    int returnit(Test t ,int y);
}
class Test{
    int x=0;
    public Test(int x){
        this.x=x;
    }

    public int addNumbers(int y){
        return x+y;
    }
    public int subtractNumbers(int y){
        return x-y;
    }

}

public class myclass{
    private static void myMethod(Test t,myint inf,int y){
        int x=inf.returnit(t, y);
        System.out.println(x+"");
    }
    public static void main(String[] args){
        myMethod(new Test(4),Test::addNumbers,7);
        myMethod(new Test(4),Test::subtractNumbers,7);
    }
}

Output would be:

11
-3

This is the simplest way I could imagine it. See how return types and argument types gets matched using the above sentence pattern. Spend some time on it.