Anytime you override equals() you should also override
hashCode(). The hashCode() method should
ideally return the same int for any two objects that
compare equal and a different int for any two objects that don't
compare equal, where equality is defined by the
equals() method. This is used as an index by the
java.util.Hashtable class.
In the Car example equality is determined
exclusively by comparing license plates; therefore only the
licensePlate field is used to determine the hash code. Since
licensePlate is a String, and since the
String class has its own hashCode()
method, we can sponge off of that.
public int hashCode() {
return this.licensePlate.hashCode();
}
Other times you may need to use the bitwise operators to merge hash
codes for multiple fields. There are also a variety of useful
methods in the type wrapper classes (java.lang.Double,
java.lang.Float, etc.) that convert primitive data
types to integers that share the same bit string. These can be used
to hash primitive data types.