Delving Further into Object-Oriented Concepts - Beginning Java Programming: The Object-Oriented Approach - page 246

ways to check for equality, so the purposes of your program will help you determine an appropriate
implementation. Two cars might be equal if they have the same vehicle identification number (the
same exact car), or you might consider them equal if the make, model, and year are the same (the
same kind of car).
The finalize() method is used as part of garbage collection. There is a brief section on this topic
later in this chapter. It is worth noting that Object is default finalize() method doesn't do any-
thing, but it can be overridden to clean up loose ends when an object becomes garbage. Oracle notes
that when and even if the finalize() method is called is uncertain, so it should not be relied upon
to close resources.
The getClass() method cannot be overridden. It will always return a Class object that you can use
to get information about the class, such as the class name, the superclass, or any interfaces it imple-
ments. The Class class has dozens of methods you can use to learn more about a class.
The hashCode() method returns a value representing the object's memory address by default. By defi-
nition, if two objects are equal, their hashcode must also be equal. Therefore, when you override the
equals() method, you must also override the hashCode() method to maintain this relationship.
The toString() method returns a String representation of an object. The String representation
depends on the object, so you should consider overriding this method in your own classes. The
toString() method is the method called when you print an object, so if you properly override the
toString() method, you can easily print objects in a format useful to you and your program.
abstract classes and methods
Abstract classes cannot be instantiated. They are used as superclasses when you want all instances
to belong to a subclass, but the subclasses share attributes or methods that are better defined in a
superclass. For example, you might have an abstract class called Shape with several subclasses, like
Rectangle , Circle , and Triangle . It might not make sense to be able to create a Shape without
a more specific type, so you can make Shape abstract and create all shape objects as members of
a subclass. The keyword abstract is used in the class declaration, like public abstract class
Shape { } .
Abstract classes can also have implemented methods and abstract methods. Abstract methods are
declared without an implementation. In the Shape example, you might declare a calculateArea()
method in the Shape class, but allow each subclass to implement it in its own way, since each type
of shape has a different area formula. All abstract methods must eventually be implemented in one
of the subclasses. All classes that contain an abstract method must be abstract themselves. Abstract
methods are declared using the abstract keyword, but instead of opening curly brackets, the dec-
laration is completed with a semicolon, like public abstract double calculateArea(); . The
classes for Shape and Circle are shown as examples for the abstract class and method syntax.
public abstract class Shape {
private String color;
public Shape(String color) {
this.setColor(color);
}