Object comparision with equals()

Question

hi all ..
i have stuck in a problem I'm testing a small system where there are 4 classes mainly an abstract parent class and two other sub classes and a client Class ,respectively ,Vehicle, Car , Van and Manager. In my "Manager" class there is a method [setCarier()] that checks whether the object that is passed to the method is a "Car "or "Van" and do something accordingly. code is given bellow
note: there is another method[returnRegNo()] in the Vehicle class that returns the registration number.
1 public void setCarier(Vehicle v) throws NullPointerException
2 {
3 if(v.equals(null))throw new NullPointerException("Reffering to a null value");
4 if(v.equals(new Car(v.returnRegNo())))
5 {  // do something}
6 else{ // do something else}
7 }
problem : is it wrong if i try to compare my unpredicted Vehicle object i.e "v" with the expected object i.e Car using equals() method, as i have done in the 4th line. Because when i try to test this method in junit by giving an instance of Car and using assertEquals() method it reports bellow description
expected:<Availability: AVAILABLE Registration NO xc45ghy Max Weight: 500 Type of the Vehical: Car> but was:<null>
but if i use v instanceof Car the testing works properly. please kindly explain me whats wrong with this equals() method.

Answer 1

Hello Chandima,
I'm not sure what you are trying to accomplish, but checking whether a is of type B is generally discouraged. Maybe you should introduce an enum:
public abstract class Vehicle {
     public enum Type {
          CAR, VAN;
     public abstract Type getType();
     /* further Vehicle code */
public class Car extends Vehicle {
     public Type getType() {
          return Vehicle.Type.CAR;
}As a side note: Line 3 of your code is confusing. When you encounter a NullPointerException, you don't expect it to be thrown explicitly. If you feel you have to compare to null then throw an IllegalArgumentException or use an assert statement:
if(v == null) {
     throw new IllegalArgumentException("v = " + v);
// or
assert v != null : "v = "+ v;EDIT: changed getType() to abstract, makes way more sense.. ;)
With kind regards
Ben
Edited by: BenSchulz on Jan 29, 2008 11:51 AM