Basic 2 Classes and Objects

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Basic -2Classes and Objects
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Classes and Objects

• A class is a complex data TYPE
• An object is an instance of a class.
• Example
• Class Person
• Objects several Person instances are all around
  you

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Trivial Example Class Person

• A Person has some properties
• Attributes or fields
• In addition it may have some behavior
• methods
• A java class defines properties and behavior
  common to all people (all Person objects)
• Each person gets their own field copies

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Class Person - example

• class Person
• String name
• int height // in inches
• int weight // in pounds
• // well worry about methods later
• void setWeight(int newWeight)
• void setHeight(int newHeight)

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Objects

• An object is an instance of a class type
• An object variable is a reference to the objects
  data structure
• Object variable can be seen as a pointer
• Declaring an object variabile ! creating an
  object
• Declaration provides a null pointer
• Must create an object with the newkeyword
• Calls a constructor method of the class
  (optionally with arguments)
• Dynamic allocation

Variable declaration
Person p1 p1 new Person() p1.name your
name
Object instantiation
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Object Variables

• Multiple variables can point to same object
• A variable can point to NO object
• its value is null
• NOTE passing an object as parameter in a method
  actually passes a copy of the reference
• Method manipulates the same referenced object

Person p1, p2 p1new Person()
p1.name John p2 p1
System.out.println(p1.name) System.out.println(p2
.name) Person p3 p3new
Person() p3.name Dan p2p3
System.out.println(p2.name)
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Illustration
name John height 0 weight 0
p1
p2
name Dan height 0 weight 0
p3
Java graciously initializes primitive fields for
you
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Object creation caveats

• What if we dont use new?
• Person p
• System.out.println(p.name)
• Hgtjavac Person.java
• Person.java15 Variable p may not have been
  initialized.
• System.out.println(p.name)
• 1 error

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Second Try

• Person p new Person()
• p null
• System.out.println(p.name)
• Compilation works fine but
• Hgtjava Person
• Exception in thread "main" java.lang.NullPointerEx
  ception
• at Person.main(Person.java16)

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Object equality

• RECALL variables are references

Person p1 new Person() Person p2 new
Person() p1.name Jane p2.name
Jane If(p1 p2) System.out.println(Same!")
else System.out.println(Different!")
Person Jane
p1
Person Jane
p2
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More on equality equals()

• All classes are equipped with a method equals()
• For comparing objects of same class
• Can be (re-)defined wrt class logic

boolean equals(Person otherPerson) if
name.equals(otherPerson.name) System.out.printl
n(Same!") return true
System.out.println(Different!") return
false
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Object Destruction

• Programmer cannot explicitly destroy an
  instantiated object
• JVM takes care of garbage collection
• Implicitly frees up memory from useless objects
• Object is useless when program keeps no more
  references to it
• e.g when program exits the context where object
  was created (block, method variables)
• Or when variables point to other objects or are
  assigned the null value

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Lets play around with Person

• p1.weight 200 // bad but dealable
• p1.weight 700 // unfortunate, but possible
• p1.weight -20 // unreal
• Solution Have class modify attribute. Allows
  sanity checks.
• Provide that behavior through a method
• p1.setWeight(700) // OK. Weight is now 700.
• p1.setWeight(-20)
• Error, weight must be positive number
• // weight still 700

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Solution?

• class Person
• void setWeight(int newWeight)
• if (newWeight lt 0)
• System.err.println(
• Error, weight must be positive number)
• else
• weight newWeight

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New problem

• p1.setWeight(-20)
• Error, weight must be positive number
• p1.weight -20 // Yo, Im the boss
• Assigning weight directly bypasses sanity check
  by the setWeight() method. We still have a
  problem.
• Solution change attribute visibility to make the
  bare weight attribute inaccessible from outside
  Person

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New solution

• Class Person
• private String name
• private int height // in inches
• private int weight // in pounds
• public void setWeight(int newWeight)
• if (newWeight lt 0)
• System.err.println(
• Error, weight must be positive number)
• else
• weight newWeight

Within same class no dot notation and no
visibility issues
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New solution in action

• class Test
• public static void main(String args)
• Person p1 new Person()
• p1.weight 20
• gtjavac Test.java
• gtTest.java4 Variable weight in class Person not
  accessible from class Test.
• p1.weight 20
• 1 error
• Denied!

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Finish up

• Need to add a getWeight(), since we can no longer
  access weight directly
• public int getWeight(void)
• return weight
• Also, need get() and set() functions for name and
  height.

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Accessor Functions for Encapsulation

• Generally make fields private and provide public
  getField() and setField() accessor functions.
• For this course ALWAYS
• unless theres a specific reason not to
• Usually there isnt
• Need to justify explicitly otherwise!
• Encapsulation keeps programs tidy
• Enables clear definition of interactions between
  classes
• Public parts of a class represents its
  Application Programming Interfaces (API)
• Private parts hide details of the class
  implementation
• If interface remains stable, even if
  implementation changes, the rest of application
  is unaffected
• Example height in cm. and weight in Kg.

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More on Visibility
/ Represents a 2d geometric point / package
2d class Point private float x,y //
coordinates public Point(float coordX, float
coordY) xcoordX ycoordY public
void move(float dX, float dY) xdX
ydY public String desc() return
Point (" x ", " y ")"

• Methods / Attributes
• public
• private
• package (default)
• protected

Constructor with parameters
At-a-glance look at the class API
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Constructors
public class Point private float x,y
public Point() xy0 // same as default
public Point(float coordX,
float coordY) xcoordX ycoordY
public Point(float xy) this(xy,xy)

• Multiple constructors with different sets of
  parameters
• No need to specify result type
• new operator invoked with parameters matching
  those of some constructor
• If programmer provides no constructor, Java
  provides default constructor (no parameters)

Constructor chaining
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Default Constructor

• Takes no parameters, initializes everything to
  defaults
• 0, null etc.
• Graciously provided by Java for absent-minded
  programmers -)
• Only exists if there are no explicitly defined
  constructors
• Any constructor, even one taking parameters,
  means no default

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Overloading

• Multiple constructors is a prominent example of
  overloading
• Multiple methods can have same name but different
  signatures

public class Point private float x,y
public void move(float dx, float dy) xdx
ydy public void move(float dxy)
move(dxy, dxy)
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Strings

• Strings in Java are objects of class
  java.lang.String
• Java.lang is a fundamental library
• String has a lot of nice features check it out
  in the javadocs!

char characters H', e', l', 'l',
'o' String salutationnew String(characters) Sy
stem.out.println(salutation) System.out.println(s
alutation.length()) String s
World salutation s System.out.println
(salutation)
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Comparing Strings

• if (s1 s2) .
• NO!
• if (s1.equals(s2))
• Prominent case of equals() vs.

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Turning objects into Strings

• All classes have a method toString() to provide a
  textual description of objects
• Can be (re-)defined wrt class logic

public class Person ... public String
toString() String ret ret name is tall
ret height ret inches and weighs
ret weight ret pounds. return
ret ...
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Turning objects into Strings

• toString()can be invoked implicitlly
• When object is used within String manipulation
  operations

Person assistantnew Person(Mike) System.out.p
rintln(the assistant is " assistant)
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Summary

• Introduction to the Java language
• What is Java, what is a Java program
• Style, Javadocs
• Language basics
• Types, variables
• Classes, objects, arrays
• Visibility and encapsulation
• Some utilities
• String, toString()
• System.out