Question

I am preparing for OCJP. I found a question in dump

import java.util.*;
 public class G1 {
 public void takeList(List<? extends String> list) {
   // insert code here
 }
}

Which three code fragments, inserted independently at line 6, will compile? (Choose three.)

 A. list.add("foo");
 B. Object o = list;
 C. String s = list.get(0);
 D. list = new ArrayList<String>();
 E. list = new ArrayList<Object>();

Answer: B,C,D

I have a query that if List<? extends String that means it matches with String class and all the subclass of String class.

Can anyone justify the answer shown here?

Answer 1

A is incorrect, because your list could be a List<SomeStringSubClass>, and it would thus be invalid to add a String to it. (I know String is final, but the compiler doesn’t care about it here).

E is incorrect because a List<Object> can’t be assigned to a List<? extends String>, obviously, since the objects contained by a List<Object> don’t necessarily are of type String or of a subclass of String.

The other three ones are OK:

• B is OK because everything except primitives is a an Object in Java
• C is OK because a List<? extends String> contains instances of String, or instances of a subclass of String. And an instance of a sublcass is also an instance of the superclass.
• D is OK because an ArrayList<String> is a List<String>, and a List<String> is also a List<? extends String>