Difference between the two method references to instance methods

Question

import java.util.List;
import java.util.function.*;

interface SomeTest <T>
{
    boolean test(T n, T m);
}

class MyClass <T>
{
    boolean myGenMeth(T x, T y)
    {
        boolean result = false;
        // ...
        return result;
    }
}

class Timepass
{
    public static void main(String args[])
    {
        SomeTest <Integer> mRef = MyClass <Integer> :: myGenMeth;   //Statement 1

        Predicate <List<String>> p = List<String> :: isEmpty;       //Statement 2
    }
}

My query

In the above code, Statement 1 produces two compile time errors

1- Can not find method myGenMeth(Integer, Integer)

2- Non static method myGenMeth(T, T) can not be referenced from static context

Where as, Statement 2 shows no error.

1- What is the difference between Statement 1 and Statement 2??

2- How the `Statement 2` is working fine.

(I am not asking why the `Statement 1` is producing error).

Both questions have already been answered in the [previous](http://stackoverflow.com/questions/31245127/syntax-for-specifying-a-method-reference-to-a-generic-method) question that you asked. Quit posting duplicates. — Chetan Kinger, Jul 07 '15 at 12:03
Both questions are different. And the difference is easily noticable. — kevin gomes, Jul 07 '15 at 12:06
The accepted answer to your previous question already explains the reason for statement 1 resulting in a compilation error (See the comments section if you have a short term memory ;) ) — Chetan Kinger, Jul 07 '15 at 12:07
@ChetanKinger : You are right, but my question is why the second statement is not producing error — kevin gomes, Jul 07 '15 at 12:09
@ChetanKinger : Stop arguing about the question, If you find the answer, then please post the answer — kevin gomes, Jul 07 '15 at 12:12
I am not arguing. Just keeping the site clean. Edit your question and remove parts that are not required. — Chetan Kinger, Jul 07 '15 at 12:15
@ChetanKinger : Time and again you are just asking me to edit my question, without answering it. Just read the two last points in my question, and if you have their answer, please post. — kevin gomes, Jul 07 '15 at 12:18
Thanks for editing your question and removing the part that has already been answered. — Chetan Kinger, Jul 07 '15 at 12:26
@ChetanKinger : add `import java.util.function.*;` Then it will compile — kevin gomes, Jul 07 '15 at 12:45

score 1 · Answer 1 · answered Jul 07 '15 at 17:04

Because you have method references to instance methods, but don't specify any specific instance, the instance needs to be passed as a parameter to the interface method.

For statement 2, you can pass this instance to the test method of Predicate:

p.test(new ArrayList<>());

But for statement 1, test doesn't take an instance as a parameter:

mRef.test(new MyClass<>(), 1, 2);

To make this compile, SomeTest needs to be changed to:

interface SomeTest<T> {
    boolean test(MyClass<T> instance, T n, T m);
}

Alternatively, you can make the method references refer to specific instances, then the interface method doesn't need to contain that parameter:

SomeTest <Integer> mRef = new MyClass<Integer>()::myGenMeth;
mRef.test(1, 2);

Supplier<Boolean> p = new ArrayList<>()::isEmpty;
p.get();

Difference between the two method references to instance methods

My query

2- How the Statement 2 is working fine.

(I am not asking why the Statement 1 is producing error).

1 Answers1

2- How the `Statement 2` is working fine.

(I am not asking why the `Statement 1` is producing error).