Q12 K&B-Generic

[nitude gupta]

 import java.util.*;
public class Group extends HashSet<Person> {
public static void main(String[] args) {
Group g = new Group();
g.add(new Person("Hans"));
g.add(new Person("Lotte"));
g.add(new Person("Jane"));
g.add(new Person("Hans"));
g.add(new Person("Jane"));
System.out.println("Total: " + g.size());
}
public boolean add(Object o) {
System.out.println("Adding: " + o);
return super.add(o);
}
}
class Person {
private final String name;
public Person(String name) { this.name = name; }
public String toString() { return name; }
}   

it has options

  Which of the following occur at least once when the code is compiled and run?
(Choose all that apply.)
A. Adding Hans
B. Adding Lotte
C. Adding Jane
D. Total: 3
E. Total: 5
F. The code does not compile.
G. An exception is thrown at runtime.  

didn't get the explaination given that

F is correct. The problem here is in Group's add() method—it should have been
add(Person), since the class extends HashSet<Person>.

can you also explain why it works when we change add(object ) to add(Person)

i have question that isn't the return statement of add() method wrong is it possible we can use super anywhere else rather than first statement in a menthod

[Sunny Mattas]

Hi,

public class Group extends HashSet<Person> { 

I think above line means Group class extends a HashSet class. But using <Person> we have made sure only Person is added to the HashSet.

 public boolean add(Object o) {  
 System.out.println("Adding: " + o);  
 return super.add(o);  
 }

But in above code we are calling add method of base class of Group class which in this case is HashSet and trying to add the Object (Remember compiler only knows about reference type not real object) using super.add(o) .This will not work as HashSet is only allowed to take Person types in this case.And we are getting compiler error.

i have question that isn't the return statement of add() method wrong is it possible we can use super anywhere else rather than first statement in a method

We can use super() super as first statement, which calls the construtor of parent class but super.add() makes sure
add method of parent class is called even if there is a same add method present in current class.super can be used anywhere except static methods or blocks to call the members of base class

Regards
Sunny Mattas
SCJP5

[Omar Al Kababji]

in the class of the HashSet class you will be having something like this line of code

public class HashSet<T>{
   public boolean add(T item) {    
      //code to add the item object to the HashSet
   }  
}

so when you extend the HashSet class by specifying the generic type, all the T objects gets replaced with Person, which was specified in the extends statement

public group extends HashSet<Person>

so at this point you will be able to add only with Person objects on the group class.


if you take a look on how you use comparable and comparators you would get a better understand too ;)