Java summary: Classes

1. Package (optional)

   package package-name; // defines package to which this file belongs

2. Imports

Import all external classes you want to use.


import java.awt.Color;      // Import one class (Color) of the java.awt package
import java.awt.event.*     // Import all classes

3. Class Definition

[visibility][abstract] class class-name [extends parent-class] [implements interface-name...] { class-body }

visibility	The default visibility is package visibility -- class is accessible to everyone in package `public` -- can be used by everyone. `private` -- visible only in this class. `protected` -- visible in this class and all subclasses.??
abstract	abstract class
parent-class	This is the name of the parent class of this class (only one). The default parent class in java is `Object`.
interface-name	For each interface that is implemented: this class must define all methods of the interface
class-body	consists of attributes and methods

Class Types

concrete class	All methods are concrete, have an implementation. Only these can be instantiated to create an object.
abstract class	Got some abstract (non-implemented) methods. Should be overridden by child class to create a concrete class.
interface class	No attributes or implemented methods (pure abstract class). Use keyword interface in definition.

An inner-class or nested class is defined in another class, as opposed to top-level classes. Can only be used inside that class. Can access all members of the outer class! As for example by an event listener.

4. Objects

Variable	Node myNode;	A reference to an object, null if empty.
Creation	myNode = new Node();	A class in instantiated and the object is created by calling the constructor method.
Deletion	-	Objects are automatically deleted when it has no more references (= garbage collection).
Passing objects as arguments	Do(myNode);	Always by reference.
Returning objects	return myNode;	Returns a reference.

Special Variables

final	Constant, eg final int MAX_PAGES = 32;
static	Class-level variable, only one value per class. Eg object counter, Class.sCounter++;
super	Reference to the parent class, eg. super.Draw();
this	Self-reference, eg. when returning a reference to yourself.

Object Operators

object.f	Member (attribute or method) of object or class.
x == y	Comparison, true if x and y refer to the same object, otherwise false (even if values of the objects are the same).
x != y	As above for inequality.
comparison	Compare object values with .equals() or .compareTo()
x = y	Assignment copies the reference, not the object. Both variables refer to the same object. Clone the object to make a copy.
x instanceof co	true if the object x is an instance of class co, or is an instance of the class of co.