TypeSafety

[Harikrishna Gorrepati]

Hi, This is from Whizlabs..Code, question and my assumptions are given below. Please advice.

import java.util.ArrayList;
import java.util.List;

class A {
}

class B extends A {
}

class C extends A {
}

class D extends B {
}

/**
 * Which of the following are true..
 * 1. The type List<A> is assignable to List --Given answer is FALSE. My answer is True
 * 2. List<Object> is assignable to any List reference --Given answer is FALSE. My answer is True
 * 3. List<? extends B> is assignable to List<? extends A>--Given answer is TRUE. My answer is False
 */

//I gave following code to demonstrate my answers. Please advice where I am going wrong.

public class Question39_Generics_Ask {
    static void callMethod(List list) {

    }

    public static void main(String[] args) {

	//Option 1..My assumption #1
	List<A> callMethod = new ArrayList<A>();
	callMethod(callMethod);

	//Option 1..My assumption #2
	List<A> a = null;
	List l = a;
	
	//Option 2..My assumption #1
	List<Object> option2 = new ArrayList<Object>();
	callMethod(option2);

	//Option 2..My assumption #2
	List ll;
	ll = List<Object>; //compile Error
	
	//Option 3..My assumption #1
	List<? extends B> option3 = new ArrayList<? extends B>();//Compile Error.
	callMethod(option2);
	
	//Option 3..My assumption #2
	List<? extends B> aa;
	List<? extends A> bb = aa;
    }
}   

[Ankit Garg]

ll = List<Object>;

How will this line compile? List is an interface, there is no new keyword and its missing brackets at the end.

For answer 1 and 2 I agree with you.

List<? extends B> option3 = new ArrayList<? extends B>();

You can never instantiate a wildcard type, this is why this line doesn't compile.

List<? extends B> aa;  
List<? extends A> bb = aa;

The 2nd line doesn't compile as aa might not have been initialized. Otherwise it compiles fine which makes answer 3 correct...

[Harikrishna Gorrepati]

Hi Ankit, assignment means, option2 at Line 42 is assigning at Line 26 "list" reference variable (or) reference variable "aa" at line 54 is assigning to bb (or) both ?

[Ankit Garg]

Technically when you pass some variables in a method call, its not assignment. But most of the rules that apply to assignment will apply to it. So in this case, rules of generics apply to both line 54 and 42...