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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).

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  • Both questions have already been answered in the previous question that you asked. Quit posting duplicates. Commented Jul 7, 2015 at 12:03
  • Both questions are different. And the difference is easily noticable. Commented Jul 7, 2015 at 12:06
  • 3
    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 ;) ) Commented Jul 7, 2015 at 12:07
  • @ChetanKinger : You are right, but my question is why the second statement is not producing error Commented Jul 7, 2015 at 12:09
  • That has also been answered in your previous question. Commented Jul 7, 2015 at 12:10

1 Answer 1

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1

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();
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