Inheritance - Java Concepts - page 645

A DVANCED T OPIC 10.5: Inheritance and the equals
Method

You just saw how to write an equals method: Cast the otherObject
parameter to the type of your class, and then compare the fields of the implicit
parameter and the other parameter.

But what if someone called coin1.equals(x) where x wasn't a Coin
object? Then the bad cast would generate an exception, and the program would
die. Therefore, you first want to test whether otherObject really is an
instance of the Coin class. The easiest test would be with the instanceof
operator. However, that test is not specific enough. It would be possible for
otherObject to belong to some subclass of Coin . To rule out that
possibility, you should test whether the two objects belong to the same class. If
not, return false .

if (getClass() != otherObject.getClass()) return
false;

Moreover, the Java language specification [ 1 ] demands that the equals method
return false when otherObject is null .

Here is an improved version of the equals method that takes these two points
into account:

public boolean equals(Object otherObject)
{
if (otherObject == null) return false;
if (getClass() != otherObject.getClass())
return false;
Coin other = (Coin) otherObject;
return name.equals(other.name) && value ==
other.value;
}

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471

When you define equals in a subclass, you should first call equals in the
superclass, like this:

public CollectibleCoin extends Coin
{