Java: Class and Interface Concepts

Class and Interface Concepts

Here are some of the basic building blocks of Objected-Oriented Programming that you will become familiar with:

class

A collection of fields (instance and class variables) and methods.

instance variable (aka field variable or member variable

An instance variable is a variable that is defined in a class, but outside of a method. There is one copy of the variable for every instance (object) created from that class.

A common problem is trying to reference an instance variable from a static method. A static method (eg, main) can only reference static variables in its own class (or its own local variables).

class variable (aka static variable)

A class variable or static variable is defined in a class, but there is only one copy regardless of how many objects are created from that class. It's common to define static final variables (constants) that can be used by all methods, or by other classes. Color.blue is an example of a static final variable.

constructor

When an object is created, a constructor for that class is called. If no constructor is defined, a default constructor is called. It's common to have multiple constructors taking different numbers of parameters. Before a constructor is executed, the constructor for the parent class is called implicitly if there is no parent constructor called explicitly on the first line.

inner class

If a class is defined within another class, it is an inner class. There are two common reasons to do this: to attach an anonymous listener to a control at the point where the control is being built, and to have access to the fields and methods of the enclosing class.

override (applies to methods)

If a subclass redefines a method defined in a superclass, the method in the superclass is overridden. A common use of this is in defining a subclass of JPanel that you use for graphics. When you define the paintComponent method, you are overrid