Chapter 7

Section 7.1

a class establishes structure and purpose of an object, but doesn’t reserve memory space (unless member variables are static)
classes are the plan and objects carry out that plan
inheritance is idea of deriving a new class fro an existing one
new class automatically contains some or all variables and methods in original class, then you can add more variables and methods to the inherited class to define it better
idea of software reuse
original class is parent class or superclass or base class
derived class is child class or subclass
listing 7.1, 7.2, 7.3, p. 324-326
don’t have to instantiate parent class to be able to instantiate child class
inheritance is only one-way
child class is more specific version of parent class
not all variables and methods will be inherited – private ones will not, public ones will
protected is a third visibility modifier that makes variables and methods public to inherited classes, but private to rest of world
inherited variables and methods have same visibility modifier as parent
constructors are not inherited – inherited class will have its own
can call parent constructor by using super – a general reference to parent class
listing 7.4, 7.5, 7.6, p. 328-330
child constructor is responsible for calling parent – must be first thing done in child constructor – if you don’t do this, Java does it automatically
this is all single inheritance – Java doesn’t support multiple inheritance, but can implement numerous interfaces

child class can override a parent method by using the same name and signature of parent’s method
listing 7.7, 7.8, 7.9, p. 332-334
can also redefine variables from parent class – called shadowing – this isn’t done often since inherited variables already exist for child

a class can have as many children as needed, and the hierarchy levels can keep growing as necessary
children of same parent are siblings, but they aren’t related by inheritance
inheritance is transitive through the tree, though
in Java all classes are derived from Object class
abstract class is generic class that can’t be instantiated, and has methods with no definition – similar to an interface
abstract classes can have methods that aren’t abstract, though
use keyword abstract
serve as placeholders in class hierarchy
listing 7.10,7.11, 7.12, p. 339-341
usually located high in a tree
if a child of an abstract class doesn’t define all abstract methods, it too is then abstract
can’t use final or static with abstract classes – why?

a reference declared to refer to some class object can be used to refer to any related class objects – makes that reference polymorphic
listing 7.13, 7.14, 7.15, 7.16, 7.17, 7.18, 7.19, p.345-354

one interface can be derived from another – forms interface hierarchies
child interface inherits all abstract methods and constants or parent, so any class implementing child has to implement all these methods

we have used inheritance all along every time we define our classes to extend Applet
we also extended listener classes
we could also extend an event adapter class
listing 7.23, p. 360

GUI component is object that represents a visual entity in an applet or application
applet itself is a special component called a container, that can hold other components
components generate certain events
each event has corresponding listener interface and adapter class
listing 7.24, p. 364 – uses labels and text fields
text field generates ActionEvent when enter is hit in text field
paint method is defined in Component class
listing 7.25, 7.26, p. 367-369 – uses canvas