CSC 313 Java Objects in Java

1
CSC 313 JavaObjects in Java
2
Java Data Types

• Java has two kinds of types
• fundamental, primitive or simple
• derived or composite
• Derived or composite types are represented as
  Object References.
• An object is a class instance or an array. The
  reference values (often just references) are
  pointers to these objects, and a special null
  reference, which refers to no object.

3
Java Program Structure

• A Java program always consists of a number of
  classes. There is at least one class in every
  program, and there can be many.
• Classes specify the objects you use in
  object-oriented programming.
• You must give each file the same name as that of
  the class that is defined within it.

4
Classes Encapsulation

• The class as a data type can encapsulate
• variables or fields, which represent features or
  properties that class instances share
• procedures
• Procedures used to create or construct instances
  of a class are known as constructors.
• Procedures that represent class operations or
  behaviors are know as methods.

5
Object-oriented Programming

• In Java almost everything is an object. So What
  are Objects?
• Anything can be thought of as an object. Objects
  are all around you. You can consider Tree to be a
  particular class of objects, and then the tree in
  my yard which I call myOak, the tree in your yard
  which I call thatDarnedTree, and a
  generalSherman, the well-known redwood, could be
  instances of Tree. In other words, they are
  objects of the class Tree.

6
What are Objects?
7
Define a Class of Objects - Hat
8
Operating on Objects
9
Complete Class Definition
10
Encapsulation (1)

• Encapsulation refers to hiding items of data and
  methods within an object by specifying them as
  private in the definition of the class. In the
  Hat class we defined earlier, the instance
  variables, name, size and hatOn were
  encapsulated.
• They were only accessible through the methods
  defined for the class.

11
Encapsulation (2)

• Being able to encapsulate members of a class in
  this way is important for the security and
  integrity of class objects.
• Another major advantage of encapsulation-the
  ability to hide the implementation of a class. A
  particular object, for example a cowboyHat, will
  incorporate, or encapsulate, the name, the size
  of the object, and the status of the hat in the
  instance variable hatOn. Only the constructor,
  and the putHatOn(), takeHatOff() and getSize()
  methods can be accessed externally.

12
Wrapper Classes for Primitive Types

• Objects are an integral part of the Java
  language.
• Many of Javas build-in collection classes
  store only data of type object.
• To allow these collection classes (vector, stack,
  linked list etc.) to store primitive data, Java
  provides wrapper classes for converting primitive
  data to Object type.
• These wrapper classes for the primitive data are
  displayed in the following slide.

13
Wrapper Classes for Primitive Types
14
Wrapper Class Methods

• All of these wrapper classes directly or
  indirectly inherit from Object.
• Therefore standard methods are available such
  as
• toString
• Clone
• equals

15
Constants Utility Methods

• The Java platform includes constants and utility
  methods related to the primitive types in wrapper
  classes. For example
• Integer.parseInt (String)
• Double.parseDouble(String)
• Character.isDigit
• Character.isSpace

16
Object-oriented programming

• Inheritance
• Software reusability
• Classes are created from existing ones
• Absorbing attributes and behaviors
• Adding new capabilities
• Convertible inherits from Automobile
• Polymorphism
• Enables developers to write programs in general
  fashion
• Handle variety of existing and yet-to-be-specified
  classes
• Helps add new capabilities to system

17
Object-oriented programming

• Inheritance
• Subclass inherits from superclass
• Subclass usually adds instance variables and
  methods
• Single vs. multiple inheritance
• Java does not support multiple inheritance
• Interfaces (discussed later) achieve the same
  effect
• Is a relationship
• Composition
• Has a relationship

18
Composition vs. Inheritance

• Inheritance
• Is a relationship
• Teacher is an Employee
• Composition
• Has a relationship
• Employee has a TelephoneNumber

19
Superclasses and Subclasses

• Is a Relationship
• Object is an object of another class
• Rectangle is a quadrilateral
• Class Rectangle inherits from class Quadrilateral
• Form tree-like hierarchical structures

20
Summary

• Java programs are implicitly object-oriented
• A java program always consists of a number of
  classes. There is at least one class in every
  program, and there may be many.
• The class as a data type can encapsulate
• variables or fields, which represent features or
  properties that class instances share
• Procedures
• Wrapper classes are provided by Java for
  converting primitive data type Object.

21
Thats All For Today Folks!