CS242 Advanced Programming Concepts in Java

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CS242Advanced Programming Concepts in Java

• 9/04/07
• Java Basics

Prof. Searleman jets_at_clarkson.edu
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Outline

• Eclipse do the basic tutorial in the Java
  Development User Guide (HelpWelcome)
• Java Basics
• Primitives vs. Objects
• this vs. this() overloading constructors
• String immutable vs. mutable objects
• OOP concepts
• Composition, Inheritance, Use
• Methods common to all classes
• toString()
• Read OODP, Ch. 2 Effective Java, Ch. 3
• HW1 posted due Thursday, 9/6/07, in class

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Primitive typesnumbers literals

• Primitive types wrapper classes
• Literals
• important point the distinction between a
  primitive and an Object

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float is not Float

• Consider
• float x
• Float y
• What is the difference between x y?

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Primitives Wrapper Classes

• float x
• Float y
• x 2.7f // ok
• y 2.7f // ERROR, why?
• y is a reference (currently NULL)
• y can refer to a Float object
• There are 3 constructors

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Wrapper class

• float x
• Float y
• y new Float(2.7f) // OK
• y new Float(2.7) // OK
• y new Float(2.7) // OK
• A wrapper class provides conversion
  functionality useful in data structures, sorting
  objects, etc.

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float is not Float

• float x 2.7f
• Float y new Float(2.7)
• What is the difference between x y?
• x is a primitive cant send a message to x
• y refers to an object responds to any message
  appropriate for Float

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Circle class Point class

• files Circle.java
• Point.java
• 1 approach for testing include a main program
  in each class
• another approach write a separate test program
• TestCircle.java

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• public class Circle // This class is not
  completely formulated yet
• / private implementation (instance
  variables) /
• private float radius
• private Point center
• / constructor given radius and 2 ints/
• public Circle(float aRadius, int x, int y) /
  implementation /
• / accessor method /
• public float getRadius() return radius
• / mutator method /
• public void setRadius(float newRadius) radius
  newRadius
• / accessor method /
• public Point getCenter() return center
• / method which computes and returns the area
  of the circle /
• public float computeArea() / implementation
  /

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Circle class, revisited

• public class Circle
• private float radius
• private Point center
• / constructs a circle with given radius
  center /
• public Circle(float radius, int x, int y)
• this.radius radius
• this.center new Point(x, y)

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Relationships between classes
public class Circle private float
radius private Point center
A Circle has a Point
UML Class Diagram
FilledCircle
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Relationships between classes
Object
A Circle is an Object
(inheritance)
IS-A
public class Circle / stuff / (everything
is an Object, except for a primitive value)
Circle
UML Class Diagram
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Overloading methods

• multiple versions of a method, distinguished by
  their signatures (name argument list)
• it is very common to overload constructors
• example
• public Circle(int x, int y) // creates unit
  circle
• public Circle(float radius, int x, inty)

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Uses of this

• this reference to current object
• this.radius
• this() way to call another
  constructor in the same class

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Overloading constructors version 1

• public Circle(float radius, int x, int y)
• this.radius radius
• this.center new Point(x,y)
• public Circle(int x, int y)
• this(1.0f, x, y) / reuses code /

advantage can change the implementation without
changing all the other constructors
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Overloading constructors version 2

• public Circle(int x, int y)
• this.radius 1.0f
• this.center new Point(x,y)
• public Circle(float radius, int x, int y)
• this(x, y)
• this.radius radius

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this vs. this()

• this.
• this(

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String

• NOT an array of char
• immutable (i.e. cannot be changed)
• string literal enclosed in double quotes
• methods length(), equals(), toUpperCase(),
  compareTo(), indexOf(), substring(), toString(),
  trim(), replace(),
• use the StringBuffer class if you want to modify
  characters in the buffer

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• String myName Fred
• String yourName Wilma
• String s Hello, yourName
• s myName
• s s.trim()
• Effective Java, Chapter 2, p. 13
• Item 4 avoid creating duplicate objects
• String s new String(silly) // bad idea
• String s silly // much better

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Mutable vs Immutable Objects

• Circle c1 new Circle(1.5f, 0,0)
• The circle is mutable, that is, it can be
  changed (for example, with setRadius)
• String s1 Hello
• The string cannot be changed (can change the
  reference s1, but not the string)
• s1 s1.substring(3,5) // s1 refers to a new
  String
• StringBuffer sBuf1 Hello
• String buffers can be changed
• sBuf1 sBuf1.setCharAt(0, j) // now jello

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String operations

• Parsing strings is an important operation that we
  will look at later (StringTokenizer)
• Converting to a string is another important
  operation

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Converting to a String toString() method

• System.out.println( Answer 42 )
• So the int value 42 is converted to a string
• with the method Integer.toString()

String
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Methods common to all objects

• cf. Effective Java, Chapter 3
• superclass java.lang.Object
• methods toString(), equals(), hashcode(),
  finalize(), clone(), getClass(), wait(),
  notify(), notifyAll()

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toString() method

• c1 new Circle(1.5f, 0, 0)
• System.out.println( c1 )

Prints Circle_at_1afa3 !!
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Item 9 Always override toString()

• Effective Java, Chapter 3, p. 42
• When practical, the toString method should
  return all of the interesting info contained in
  the object.
• Note that toString should never print anything

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• public class Circle
• // other stuff
• public String toString() // 1st version
• return \n \t center
• center.getX() ,
• center.getY()
• \n \t radius radius

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• public class Circle
• public String toString()
• return \t center center
• \n \t radius radius
• // alternative implementation
• // other methods

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• Circle c1 new Circle(1.5f, 10, 20)
• System.out.println( c1 )

prints center lt10,20gt radius 1.5
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toString() called automatically

• System.out.println( Answer 42 )
• System.out.println( d1 )
• System.out.println( d1.topFace() )
• System.out.println( d1.toString() )

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Overriding toString()

• It is recommended that you specify the format of
  the return value for classes associated with a
  value, and to document your intentions.
• examples
• phone number format (XXX) YYY-ZZZZ
• social-security number XXX-XX-XXXX

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Methods common to all objects

• cf. Effective Java, Chapter 3
• superclass java.lang.Object
• methods toString(), equals(), hashcode(),
  finalize(), clone(), getClass(), wait(),
  notify(), notifyAll()
• Item 9 Always override toString()

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is not equals()

• tests for equivalence (exactly same object)
• compare primitives using and !
• equals() tests whether 2 objects are equal in
  value
• compare objects using equals(), unless
• you intend to compare for equivalence

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• Integer n1 new Integer(42)
• Integer n2 new Integer(42)
• System.out.println( n1 n2 )
• prints false
• System.out.println( n1.equals(n2) )
• prints true
• n2 n1
• System.out.println( n1 n2 )
• now prints true

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Comparing things

• ! means not equivalent
• compare primitives using and !
• compare objects using equals()
• This is especially true when comparing two
  strings!
• unless, of course, you want to test for
  equivalence

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• cat cat true
• String s1 cat
• String s2 c
• String s3 s2 at
• s1 s3 false
• s1.equals(s3) true

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OOP

• composition has-a relationship
• inheritance is-a relationship
• overloading methods
• overriding methods
• polymorphism many-shaped

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Polymorphism

• x1.draw()
• x2.draw()
• x3.draw()
• what happens depends on what type of object x is

Circle Square FilledCircle
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Relationships between classes
public class Circle / stuff / public class
FilledCircle extends Circle
Circle
(inheritance)
IS-A
A FilledCircle is a Circle
FilledCircle
UML Class Diagram
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• //// file Circle.java
• public class Circle
• private float radius
• private Point center
• / constructors, getRadius(), setRadius(),
• getCenter(), computeArea() /
• / want to create a subclass of Circle, namely
  FilledCircle /

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Recall Uses of this

• this reference to current object
• this.radius
• this() way to call another
  constructor in the same class

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Uses of super

• super reference to the superclass
• super.radius // caution inaccessible
• super() way to call a constructor for the
  superclass
• public FilledCircle(float radius, int x, int y)
• super(radius, x, y)

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• //// file FilledCircle.java
• import java.awt.Color
• public class FilledCircle extends Circle
• private Color fillColor Color.black
• public FilledColor(float radius, int x, int y)
  
• super(radius, x, y)
• public FilledColor(
• float radius, int x, int y, Color
  newFillColor)
• super(radius, x, y)
• fillColor newFillColor

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UML class diagram
IS-A
IS-A
1
HAS-A
(composition)
IS-A
(inheritance)
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Inheritance

• A subclass inherits all instance variables and
  all methods from its superclasses
• e.g. FilledCircle inherits radius and center from
  Circle along with the methods getRadius(),
  setRadius(), computeArea(), etc.

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Packages

• default package
• access modifiers
• - public
• - private
• - protected
• - none (defaults to package access)

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• So FilledCircle cant directly manipulate radius
  or center (only Circle can)
• public everyone can use it
• private only within the class
• protected subclasses have access
• package everyone in same package

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• Overloading
• multiple versions of a method, distinguished by
  their signatures
• (name and argument list)
• Overriding
• when a subclass completely replaces a method that
  it inherits from its superclass (same signature)

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IS-A
IS-A
HAS-A
1
IS-A
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methods toString(), equals(), getClass(),
methods constructors, get/setCenter(),
get/setRadius(), computeArea() toString()
methods constructors, get/setColor() toString()
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• public class FilledCircle
• extends Circle
• private Color fillColor Color.black
• public String toString()
• return super.toString()
• \n \t color fillColor
• // other methods for the class

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• Circle c1 new Circle(1.5f, 10, 20)
• FilledCircle fc1
• new FilledCircle(2.0f, 1, 1, Color.cyan)
• System.out.println( c1 )
• System.out.println( fc1 )

center lt10,20gt radius 1.5
center lt1,1gt radius 2.0 color
java.awt.Colorr0,g255,b255