question from nikos blog(generics)

[Arjun Srivastava]

Will this code compile successfully?
1.

PriorityQueue<?> queue1 = null;
         PriorityQueue queue2 = null;
         queue1 = queue2; 

answer Yes, without warnings.
here why generic code + non-generic code is not generating warning?

2.

  PriorityQueue queue1 = null;
           PriorityQueue<Integer> queue2 = null;
           queue2 = queue1;

code 2 generates warning
o.k. because generic code + non-generic code is used.

[Abimaran Kugathasan]

For the first one, both are implicitly same! So no warnings!

[Arjun Srivastava]

Sir,one more question from same source

   
import java.util.*;
class Parent {
	void say(List<String> list) {
		System.out.println("parent");
	}
}
class Child extends Parent {
	void say(List list) {
		System.out.println("child");
	}
}
public class Main{
public static void main(String[] java) {
Parent c = new Child();
	c.say(new LinkedList<Long>());
}
}

here as method say() is overrided,so object type determines which method will invoked,so as child method is non generic it can hold anything(i.e. not primitive).
this code gives compiler error, why so?

Also if we put

 c.say(new LinkedList<String>()); 

at line 16 it invokes child class say() method.
output:child

[Abimaran Kugathasan]

If I ask, Can you assign like this?

List<String> list = new LinkedList<Long>();

[Abimaran Kugathasan]

And for the output as child, generics only for compiler! Do you got the point, why do these methods override even though the differ the method signature?

[Arjun Srivastava]

Abimaran Kugathasan wrote:If I ask, Can you assign like this?

List<String> list = new LinkedList<Long>();

no , parameter type should have to be same on the both sides until and unless we are using wildcards(?).

[Abimaran Kugathasan]

Arjun Srivastava wrote:
no , parameter type should have to be same on the both sides until and unless we are using wildcards(?).

So, that's why the compiler gives an Error!

[Arjun Srivastava]

but as overriding is happening, so it should consider the non-generic method which can take anything
but why it is considering generic method here of parent class just because of parent reference type?
but finally it invokes child method if we use line below

c.say(new LinkedList<String>());

[Abimaran Kugathasan]

When compile time, it check the reference type, and at the time, generics also there. But at runtime, the generic and non-generic versions are same, VMI is there! {Generics only for compiler}

[Arjun Srivastava]

Abimaran Kugathasan wrote:When compile time, it check the reference type, and at the time, generics also there. But at runtime, the generic and non-generic versions are same, VMI is there! {Generics only for compiler}

thanks, it is more clear now.
wonderful explanation

[Abimaran Kugathasan]

You are Welcome, If you want to check more, decompile the class file and check the methods! You'll get it!