Inheritance - Java Concepts - page 632

BankAccount anAccount; // OK
anAccount = new BankAccount(); // ErrorȌ BankAccount is
abstract
anAccount = new SavingsAccount(); // OK
anAccount = null; // OK
The reason for using abstract classes is to force programmers to create subclasses.
By specifying certain methods as abstract, you avoid the trouble of coming up with
useless default methods that others might inherit by accident.
Abstract classes differ from interfaces in an important wayȌthey can have
instance fields, and they can have concrete methods and constructors.
A DVANCED T OPIC 10.2: Final Methods and Classes
In Advanced Topic 10.1 you saw how you can force other programmers to create
subclasses of abstract classes and override abstract methods. Occasionally, you
may want to do the opposite and prevent other programmers from creating
subclasses or from overriding certain methods. In these situations, you use the
final keyword. For example, the String class in the standard Java library has
been declared as
public final class String { . . . }
That means that nobody can extend the String class.
The String class is meant to be immutableȌstring objects can't be modified by
any of their methods. Since the Java language does not enforce this, the class
designers did. Nobody can create subclasses of String ; therefore, you know that
all String references can be copied without the risk of mutation.
You can also declare individual methods as final:
public class SecureAccount extends BankAccount
{
. . .
public final boolean checkPassword(String
password)
{
. . .