K & B: Collections Self-Test Question

[Venkat Sidh]

Hi,

Here is the question on Page 630 & Question #12 ,

 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; }
}
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.

The code doesn't compile and it is fine, but if I change the code to,

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

And changing Person's class to

 class Person {
private String name;
public Person(String name) { this.name = name; }
public String toString() { return name; }
public boolean equals(Object o) {
if(o instanceof Person && this.name == ((Person)o).name)
return true;
return false;
}
public int hashcode() {
return 5;
}
} 

then the answer is A,B,C,E but I thought it should be A,B,C,D since Person overrides equals() and hashcode() and mutually comparable then how come duplicates are added to Set. Am I missing something here? Thanks.

[Keith Lynn]

I admit I puzzled over this for awhile until I spotted the error.

Note that in your definition of Person, you have the method hashcode().

However, the method you need to override from Object is hashCode().

[Venkat Sidh]

Oops...good caught. I never realized it's a typo error.

[B]

hashcode() is not same as hashCode()...

[/B]ofcourse

Anyway its a good lesson to prepare for test.

[Matthew Gerald]

This had me a bit boggled as well, nice catch on that Keith.

[Burkhard Hassel]

Hi ranchers,


annotations are not on the exam. But use it!

@Override

above your methods, and the compiler would shout out loud.

eg

class A {
void m2l() {
System.out.println("class A");
}
}
class B extends A {
[B]@Override[/B]
void m21() {
System.out.println("class B");
}
}

will not compile, because one method is called em-two-el and the other m-twentyone.


Yours,
Bu.
[ October 28, 2006: Message edited by: Burkhard Hassel ]