Inheritance is extending the capabilities and functionality of a class
Inheritance allows you to build a hierarchy of classes, with parents (called superclasses) and children (called subclasses)
A superclass has one or more extensions - subclasses that are built upon the superclass
All classes in Java are subclasses of java.lang.Object
Once a superclass is defined, use the extends keyword to define a subclass.
Example - the Box superclass --
public class Box
{
private boolean open; // member variables
private String content;
public Box (String content) // constructor
{
open = true;
this.content = content;
}
public void setOpen(boolean open)
{
this.open = open;
}
// more ... see p. 133
}
The LockableBox subclass --
public class LockableBox extends Box
{
private String password;
private boolean locked;
}
LockableBox objects have the member variables and methodsfrom Box as well as it's own member variables and methods
Use the super keyword to reference methods and members from the superclass --
public LockableBox (String password)
{
super("");
//call to Box's constructor
this.password = password;
//initialize LockableBox's password member var
super.setOpen(true);
//initialize LockableBox's open member var
//(which was inherited from Box ) to true
//using Box's setOpen method
}
Overriding Methods -
When the functionality of the superclass' methods isn't quite right for the subclass
Example - consider the setOpen method -
 | for a Box object, merely pass the open state: |
| public void setOpen(boolean open) |
| for a LockableBox object, we should test if the box is locked
if we're trying to open it: |
