1. Classes in Java
Specifics for Java
Copyright Curt Hill

2. The Class Declaration public class x{ … }
A public class x must be in a file x.java
Every application must be a public class
Every applet must be a public class
The class defines properties and methods
Copyright Curt Hill

3. Properties Also known as fields The variables of a class
May be primitives or objects
Belong to class and are instantiated with the class
Copyright Curt Hill

4. Properties and Visibility
Variables may be declared using any visibility
Protected, which gives access to derived classes but not clients
Friendly visibility is also very commonly used
Private is usually only used on classes that will be superclasses
Public gives client access and is seldom used
Copyright Curt Hill

5. Methods and Visibility
In functions the preferred visibility is public
Friendly is also common
Protected and private are not as frequent but have their uses
Copyright Curt Hill

6. Java Files
A file with a .java extension must contain one and only one public class
It must have same name as file
It may also contain other classes of friendly (but not private) visibility outside the public class
These are put in file in any order
The friendly classes function as helper classes for the public class
Copyright Curt Hill

7. Order of declaration In C++ items could be declared almost anywhere
The declaration had to precede the use
In Java a class can be used before its declaration
Provided declaration is in the same file
However an include provides declaration as well
Copyright Curt Hill

8. Class derivation
A class that is only slightly different from another class would be easier to derive that start over or copy
The keyword is extends and is used in every applet public class newclass extends java.applet.Applet
Copyright Curt Hill

9. Extends
This keyword states that the new class starts with every property and method of the old class
Applet is the super class or base class
newclass is the sub class or derived class
Copyright Curt Hill

10. The Derived Class This new class changes the functionality in some way
It may add new properties or methods
It may overrides any or all old methods
Those methods left alone continue to function
Copyright Curt Hill

11. Accessing the Super class
If method X of the super class has been overridden it is still accessible
Call function super.x(…)
Using the super class’s constructor is just super(…)
Copyright Curt Hill