A computer system generally consists of input/output devices, primary and secondary memory, and a central processing unit. A computer can only run programs in its own machine language, which is based on the binary code. Special programs known as compilers and interpreters translate source code programs written in a high-level language, such as Java, into machine language object code programs.
Application software refers to programs designed to provide a particular task or service; systems software assists the user in using application software.
The client/server model is a form of distributed computing in which part of the software for a task is stored on a server and part on client computers.
HyperText Markup Language (HTML) is the language used to encode WWW documents.
A Java program is a set of interacting objects. This is the basic metaphor of object-oriented programming.
An object in a Java program encapsulates the program's attributes (or variables) and actions (or methods). A variable is a named memory location where data of an appropriate type can be stored. A method is a named section of code that can be called (or invoked) when needed.
An object's methods are used to pass messages to it.
A class is an abstract template that defines the characteristics and behaviors of all objects of a certain type.
An object is an instance of a class. An object has instance methods and instance variables. A class method (or class variable) is a method (or variable) that is associated with the class itself, not with its instances.
A constructor is a special method that is used to construct objects.
Java classes are organized into a class hierarchy, with the Object class at the top of the hierarchy. For a given class, classes that occur below it in the hierarchy are called its subclasses, while classes that occur above it are called its superclasses.
Classes inherit attributes and methods from their superclasses. This is known as class inheritance.
The main principles of the object-oriented programming approach are as follows:
- Divide and Conquer: Successful problem solving involves breaking a complex problem into objects.
- Encapsulation and Modularity: Each object should be assigned a clear role.
- Public Interface: Each object should present a clear public interface that determines how other objects will use it.
- Information Hiding: Each object should shield its users from unnecessary details of how it performs its role.
- Generality: Objects should be designed to be as general as possible.
- Extensibility: Objects should be designed so that their functionality can be extended to carry out more specialized tasks.
- Abstraction is the ability to focus on certain important features of an object while ignoring other features.
