collections framework doubt

[Radha Kamesh]

Hi friends ,

will such questions ( implementing comparator... using compare() etc come for the exam?)

import java.util.*;
2.   public class Car implements Comparator {
3.     public int wheels = 0;
4.     public Car(){}
5.     public Car(int wheels) {
6.       this.wheels = wheels; 
7.     }
8.     public int compare(Object o1, Object o2) {
9.       Car c1 = (Car)o1;
10.      Car c2 = (Car)o2;
11.      return new Integer(c1.wheels).compareTo(new Integer(c2.wheels));
12.    }
13.    public String toString() {
14.      return new Integer(wheels).toString();
15.    }
16.    public static void main(String[] args) {
17.      Car c1 = new Car(1);
18.      Car c2 = new Car(2);
19.      Car c3 = new Car(3);
20.      TreeSet list = new TreeSet(new Car());
21.      list.add(c3);
22.      list.add(c1);
23.      list.add(c2);
24.      Iterator it = list.iterator();
25.      while (it.hasNext())
26.        System.out.print (it.next() + " ");
27.    }
28.  }

choose:
Compilation succeeds, compare() throws ClassCastException
Compilation succeeds, the output is 0 1 2 3
Compilation succeeds, the output is 1 2 3
Compile time error, the compare() method does not have the correct signature
Compile Time error, compareTo method from the Comparator interface has not been implemented.


Solution provided:
Answer 3 is correct.
The compilation succeeds and the output will be in ascending order (e.g.: 1 2 3).
The class Car is a Comparator but at the same time also a business object that contains the total of wheels for the car.

At line 20 the TreeSet is created, the TreeSet of the constructor takes in the Car() Comparator, this constructor creates a new empty set sorted according to the Car criteria. All elements must be mutually comparable by the car.compare(Object o1, Object o2) method. The Car object used to create the TreeSet does not belong to the set.

The cars are sorted by the total amount of wheels. 3 new objects are created and added to a list. When the TreeSet start sorting it will call the compare method and it compares the total of wheels. It uses the Integer compareTo method.

[Vikrant Sahdev]

Depends whether you are taking SCJP 1.4 or 1.5

[marc weber]

Originally posted by Vikrant Sahdev:
Depends whether you are taking SCJP 1.4 or 1.5

Aren't these questions on both?

[Radha Kamesh]

Sorry i forgot to mention that...

I meant the SCJP 1.4 exam...

Thanks,
Radha Kamesh
[ August 31, 2007: Message edited by: Radha Kamesh ]