Java 1.7 Override of hashCode() not behaving as I would expect

Question

I have a class where I have overridden both the hashCode method as well as the equals method. The equals method behaves as I would expect it to, however the hashCode method does not seem to behave as I would expect. I'm assuming therefor my expectation is incorrect, but not sure why. Below are the overridden methods:

public class Car extends SomeBaseClass implements Cloneable, Serializable {
private static final long serialVersionUID = 1L;
private String id;
private String name;
private String carName;
private String carModel;
private String displayTextCar;


public boolean equals(Car car)
{
    return (getCarName().equals(car.getCarName())  && getCarModel().equals(car.getCarModel()));
}

public int hashCode()
{
    return (this.getCarName() + this.getCarModel()).hashCode();

} 

Now I have a test class where create two car objects, and call the equals method and then put each instance of car into a HashMap. I set each instance to have the same car Name and Model and calling equals method in fact returns true. However even though each instance returns the same hashCode, when I add them to the HashMap, it keeps two objects int the Map, whereas I would expect the second put to replace the first object in the map ??? Below is the guts of the test class:

HashMap<Car,String> testMap;

Car testCar1 = new Car();
testCar1.setCarName("DaveCar");
testCar1.setCarModel("DaveModelTest");
System.out.println("Car Hash 1: " + testCar1.hashCode());

Car testCar2 = new Car();
testCar2.setCarName("DaveCar");
testCar2.setCarModel("DaveModelTest");
System.out.println("Car Hash 2: " + testCar2.hashCode());

//hashCodes prints identical numbers

System.out.println("Car 1 equal Car 2 ?? " + testCar1.equals(testCar2));
//returns true

testMap.put(testCar1, "3");     
testMap.put(testCar2, "16");

System.out.println("Map size is " + testMap.size());
//I would expect the size to be 1 here, but it's in fact 2.

So this doesn't seem correct to me, I have naturally left some of the code out here, but this is the basic principal. Hoping someone can point out where I have gone wrong here. Note that I did use Eclipse to generate hashCode and equals methods and that worked correctly, however it's bugging me that my implementation of hashCode did not work as I expected, even though both objects seemingly returned the same value for the hashCode. Appreciate anyone's input.

Answer 1

The problem is that you have provided a wrong equals: it should be equals(Object), not equals(Car).

Essentially, you have provided an overload instead of an override, so HashMap keeps calling the equals from the base class.

Fixing this problem is simple: add an override that does the cast, and calls the equals method that you wrote, like this:

@Override
public boolean equals(Object other) {
    return (other instanceof Car) && equals((Car)other);
}

Note the use of @Override annotation. It helps Java help you spot issues like this automatically.

Note: with this problem out of the way, consider implementing your hashCode method in a more "frugal" way. Rather than creating a throw-away (this.getCarName() + this.getCarModel()) string simply for the purpose of obtaining its hash code, consider rewriting the method as follows:

public int hashCode() {
    return 31*getCarName().hashCode() + getCarModel().hashCode();
}

or in Java 1.7+ you could write

public int hashCode() { // Thanks, fge, for a nice improvement!
    return Objects.hash(carName, carModel);
}

Answer 2

The problem is not with .hashCode(); the problem is that you don't override .equals()!

Look at your prototype:

public boolean equals(Car car)

Now have a look at the documentation for Object...

What you should override is:

public boolean equals(Object obj)

Hence the error. You did implement hashCode correctly, however you use Object's .equals() implementation.