Restricting inheritance…
Sometimes in your design you would like to stop some classes and
methods being inherited. The final keyword in Java achieves this
functionality. When you declare a class to be final, no other
sub class of it can be created. Similarly, when you declare a
method to be final, no sub class can override that method.
Overriding member variables…
So far we have talked about how can you override the methods
declared in the super class. It is also possible to hide the
member variables declared in the super class, by declaring a
member variable in the sub class with the same name as in super
class. The type of the field does not matter, only the name
needs to match. Notice that when you hide a member variable in
the sub class, you can still access the super class variable by
using the super keyword. This means that the member variables
cannot be overridden but can only be hidden. It is important to
notice this subtle distinction between overriding and hiding.
The logic behind that Java allows the hiding of member
variables, is that you can then create a sub class of any super
class without detailed knowledge of its internal implementation.
This is more in the line of encapsulation.
How it works…
We have just seen how powerful inheritance mechanism is and how
relatively it is to implement in real world. We will later in
this article examine some of the pitfalls with this inheritance
mechanism but for the time being, let us see how the inheritance
works in Java.
Object Initialization and Inheritance
When an object is initialized, all the instance variables
defined within the class of that object and also all the
instance variables defined in its super class get initialized.
Consider the following example in Listing 3.
public class Vehicle
{
private String registration;
public void accelarate()
{
}
public void stop()
{
//stop the engine
}
}
public class Car extends Vehicle
{
private String model;
public Car()
{
}
public void stop()
{
//stop the engine
//lock the steering wheel
}
public void switchOnWiper()
{
}
}
Listing 3 A Simple inheritance model
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