Objects and Object References

1
Objects and Object References

• Lecture 9

2
Objectives

• Java was designed to do object-oriented
  programming, abbreviated OOP. OOP is a
  programming technique that uses Objects.
• Objectives
• What is an object?
• What makes an object?
• Software Objects
• Using a Class to make Objects
• Constructors
• Primitive data types versus Objects
• Object References

3
What is an Object?

• Think about the things in the world that are
  objects?
• A pen is an object.
• A computer keyboard is an object.
• Bank account is an object
• The class room is an object
• So, what all objects have in common?
• An object has identity (it acts as a single
  whole).
• An object has state (it has various properties,
  which might change).
• An object has behavior (it can do things and can
  have things done to it).
• Example
• you can think of your bank account as an object.
  Your account has properties (the balance,
  interest rate, owner) and you can do things to it
  (deposit money, cancel it) and it can do things
  (charge for transactions, appreciate interest.)

4
Software objects

• It is convenient to have "software objects" that
  are similar to "real world objects." This makes
  the program and its computation easier to think
  about.
• Software objects will have identity, state, and
  behavior just as do real world objects. Of course
  software objects exist entirely within a computer
  system and don't directly affect real world
  objects.

• Software objects have identity because each is a
  separate chunk of memory.
• Software objects have state. Some of the memory
  that makes a software object is used for
  variables which contain values.
• Software objects have behavior. Some of the
  memory that makes a software object is used to
  contain programs (called methods) that enable the
  object to "do things." The object does something
  when one of its method runs.
• Both variables and methods are called members
  of the object.

5
Using a Class to make Objects

• To create an object, there needs to be a
  description of it. A Class is a description of a
  kind of object.

• A class is like a cookie cutter that can be used
  many times to make many cookies. There is only
  one cookie cutter, but can be used to make many
  cookies.
• Different cookies may have different
  characteristics, even though they follow the same
  basic pattern.
• Cookies can be created. And cookies can be
  destroyed (just ask Cookie Monster). But
  destroying cookies does not affect the cookie
  cutter.

6
Example Program
class StringTester public static void main
( String args ) String str1 // str1 is
a variable that refers to an object, //
but the object does not exist yet. int len
// len is a primitive variable of type int
str1 new String("Salam Shabaab")//create an
object of type String len str1.length() //
invoke the object's method length()
System.out.println("The string is " len "
characters long")

• When a programmer wants to create an object the
  new operator is used with the name of the class.
  Creating an object is called instantiation.
• Once the object has been created (with the new
  operator). The variable str1 is used to refer to
  this object.
• Java uses "dot notation" to access any of its
  members as follows
• referenceToAnObject.memberOfObject
• Method names have "()" at their end. Often there
  is additional information inside the "()", but
  they are required even if they contain nothing.

7
Constructors

• A constructor has the same name as the class. The
  line from the above program
• str1 new String("Salam Shabaab")
• creates a new object of type String. The new
  operator says to create a new object. It is
  followed by the name of a constructor. The
  constructor String() is part of the definition
  for the class String.
• Constructors often are used with values (called
  parameters) that are to be stored in the data
  part of the object that is created. In the above
  program, the characters "Salam Shabaab" (not
  including the quote marks) are stored in the new
  object.
• There are usually several different constructors
  in a class, each with different parameters.
  Sometimes one is more convenient to use than
  another, depending on how the new object's data
  is to be initialized.

8
Primitive data types Objects

• In Java, a piece of data either is of a primitive
  data type or is an object data type.
• The only type of data a programmer can define is
  an object data type (a class). Every object in
  Java is an instance of a class. The class
  definition has to exist first before an object
  can be constructed.
• A programmer may define a class using Java, or
  may use predefined classes that come in class
  libraries (like String).

9
Object References

• An object reference is information on how to find
  a particular object. The object is a chunk of
  main memory a reference to the object is a way
  to get to that chunk of memory.
• Objects are created while a program is running.
  Each object has a unique object reference, which
  is used to find it. When an object reference is
  assigned to a variable, then that variable says
  how to find that object.
• In Java, there are only primitive variables and
  object reference variables, and each contains a
  specific kind of information

10
Object References - Examples

• Example of two objects and two reference
  variables
• Example of one object and two reference variables

String strA new String (Java) String strB
new String (C)
String strA new String (Java) String strB
strA
strB
11
Equality of Reference Variable Contents

• The operator does NOT look at objects!   It
  only looks at references (information about where
  an object is located.)