CS535 Object-Oriented Programming & Design
Fall Semester, 1996
Java Classes and Objects

1. References
2. Classes
   1. Basic Terms
   2. Fields
   3. Constructors - Initializing Fields
   4. this
   5. Access Levels for Fields and Methods
   6. Object Variables are References
   7. Parameter Passing - By value only
   8. Static Fields and Methods
   9. Constants
   10. Class Names, Packages and Import

                                   Java Classes and Objects Slide # 1

Dr. Who, Meglos

                                   Java Classes and Objects Slide # 2

• Class

• Object

• Inheritance

• Abstraction

• Encapsulation

• Information Hiding

• Hierarchy

                                   Java Classes and Objects Slide # 3

class  BankAccount 
{
   public  float  balance = 0.0F;

   public void deposit( float amount )
   {
      balance += amount ;
   }
}

                                   Java Classes and Objects Slide # 4

class  BankAccount 
{
   public  float  balance = 0.0F;
}

class RunBank 
{
   public static void main( String args[] ) 
   {
      System.out.println( " Start main " );

      BankAccount richStudent  =  new BankAccount( );

      richStudent.balance = (float) 100000;

      BankAccount poorInstructor  =  new BankAccount( );

      poorInstructor.balance =  5.10F;

      System.out.println( "Student Balance: " + richStudent.balance );

      System.out.println( "Prof Balance: " + poorInstructor.balance );

   }
}

                                   Java Classes and Objects Slide # 5

• Each class compiles to its own .class file
• Execute the class with the main program

                                   Java Classes and Objects Slide # 6

                                   Java Classes and Objects Slide # 7

class  BankAccount 
{
   public  float  balance = 0.0F;

   public void deposit( float amount )
   {
      balance += amount ;
   }

   public String toString()
   {
      return "Account Balance: " + balance;
   }
}

class RunBank 
{
   public static void main( String args[] ) 
   {
      BankAccount richStudent  =  new BankAccount( );
      BankAccount poorInstructor;
      poorInstructor  =  new BankAccount( );

      richStudent.deposit( 10000F);
      poorInstructor.deposit( 5.10F );

      System.out.println( "Student Balance: " + richStudent.balance );
      System.out.println( "Prof: " + poorInstructor.toString() );
   }
}

                                   Java Classes and Objects Slide # 8

• When required Java sends toString() message to objects to convert them to strings

This happens in println() and when adding to strings

class RunBank 
{
   public static void main( String args[] ) 
   {
      BankAccount richStudent  =  new BankAccount( );
      BankAccount poorInstructor  =  new BankAccount( );

      richStudent.deposit( 10000F);
      poorInstructor.deposit( 5.10F );

      String profBalance = "Prof: " + poorInstructor;
      System.out.println( profBalance  );
      System.out.println( "Prof: " + poorInstructor );
      System.out.println( richStudent );

   }
}

                                   Java Classes and Objects Slide # 9

class  BankAccount 
{
   public  float  balance;

   public BankAccount( float initialBalance ) //Constructor
   {
      balance = initialBalance;
   }

   public void deposit( float amount )
   {
      balance += amount ;
   }

   public String toString()
   {
      return "Account Balance: " + balance;
   }
}

class RunBank 
{
   public static void main( String args[] ) 
   {
      BankAccount richStudent  =  new BankAccount( 10000F );
      BankAccount poorInstructor  =  new BankAccount( 5.10F );

      System.out.println( "Prof: " + poorInstructor );
      System.out.println( "Student: " + richStudent );
   }
}

                                   Java Classes and Objects Slide # 10

class  ConstructorExample  {

   public  ConstructorExample( )  {
      System.out.println(  "In constructor - no argument"  );
   };

   public  ConstructorExample( int  size)  {
      System.out.println(  "In constructor - one argument" );
   };

   public  void  ConstructorExample( )  {
      System.out.println(  "return type means it is ");
      System.out.println(  "not a constructor "  );
   };
}

class TestConstructor  {

   public static void main( String args[] ) {
      System.out.println( " Start main " );

      ConstructorExample  test  =  new ConstructorExample( );
      ConstructorExample  x  =  new ConstructorExample(5);

      System.out.println( " Done with Constructors " );
      test.ConstructorExample ();
   }
}

                                   Java Classes and Objects Slide # 11

class  ImplicitConstructorOnly  {
   int  size  =  5;
}

class  OneConstructor  {

   OneConstructor(  String  message  )  {
      System.out.println(  message  );
   }
}

class  TwoConstructors  {
   TwoConstructors  (  String  message  )  {
      System.out.println(  message  );
   }

   TwoConstructors  (  )  {
      System.out.println(  "No argument Constructor"  );
   }
}

class  Constructors  {

   public static void main( String args[] ) {
   ImplicitConstructorOnly  ok  =  new  ImplicitConstructorOnly();

   TwoConstructors  alsoOk  =  new  TwoConstructors();

   OneConstructor  compileError  =  new  OneConstructor();
   }
}

                                   Java Classes and Objects Slide # 12

class OverLoad
{
   public void same() {
      System.out.println( "No arguments" );
   }

   public void same( int firstArgument ) {
      System.out.println( "One int arguments" );
   }

   public void same( char firstArgument ) {
      System.out.println( "One char arguments" );
   }

   public int same( int firstArgument ) {      // Compile Error
      System.out.println( "One char arguments" );
      return 5;
   }

   public void same( char firstArgument, int secondArgument) {
      System.out.println( "char + int arguments" );
   }

   public void same( int firstArgument, char secondArgument ) {
      System.out.println( "int + char arguments" );
   }
}

                                   Java Classes and Objects Slide # 13

Refers to the object on which the method operates

Used to return self from method and pass self as a parameter

Java's this differs from C++'s this

The difference occurs with inheritance

class  BankAccount 
{
   public  float  balance;

   public BankAccount( float initialBalance )
   {
      this.balance = initialBalance;
   }

   public void deposit( float amount )
   {
      balance += amount ;
   }

   public String toString()
   {
      return "Account Balance: " + balance;
   }
}

                                   Java Classes and Objects Slide # 14

class  BankAccount 
{
   public  float  balance;

   public BankAccount( float initialBalance )
   {
      this.balance = initialBalance;
   }

   public BankAccount deposit( float amount )
   {
      balance += amount ;
      return this;
   }
}

class RunBank 
{
   public static void main( String args[] ) 
   {
      BankAccount richStudent  =  new BankAccount( 10000F );

      richStudent.deposit( 100F ).deposit( 200F ).deposit( 300F );

      System.out.println( "Student: " + richStudent.balance );
   }
}

                                   Java Classes and Objects Slide # 15

class CustomerList
{
   public BankAccount[] list = new BankAccount[ 100 ];
   public int nextFreeSlot = 0;

   public void add( BankAccount newItem )
   {
      list[ nextFreeSlot++ ] =  newItem;
   }
}

class  BankAccount 
{
   public  float  balance;

   public BankAccount( float initialBalance ) 
   {
      this.balance = initialBalance;
   }

   public void badBalanceCheck( CustomerList badAccounts )
   {
      if ( balance <= 0F ) badAccounts.add( this );
   }
}

class RunBank 
{
   public static void main( String args[] ) 
   {
      BankAccount richStudent  =  new BankAccount( 10000F );
      CustomerList customersToDrop = new CustomerList();
      richStudent.badBalanceCheck( customersToDrop );
   }
}

                                   Java Classes and Objects Slide # 16

                                   Java Classes and Objects Slide # 17

class  Death  {
   int  myId;

   public  Death  ( int  sequenceNumber)  {
      myId  =  sequenceNumber;  }

   public  void  finalize( )  {
      System.out.println(  myId  );    }
}

class  Finalization  {
   public static void main( String args[] ) {

      Death  sampleObject;
      for (  int  k = 0;  k < 5;  k++ )
         sampleObject  =  new Death(  k  );
   }
}

class  FinalizationForced  {
   public static void main( String args[] ) {

      Death  sampleObject;

      for (  int  k = 0;  k < 5;  k++ )
         sampleObject  =  new Death(  k  );

      System.gc();
      System.runFinalization();  }
}

                                   Java Classes and Objects Slide # 18

Members declared public are accessible anywhere the class is accessible

Inherited by all subclasses

Members declared private are accessible to an inherited by subclasses, and accessible by code in the same package

Members declared private are accessible only in the class itself

Members declared with on access modifier are accessible only to code in the same package

                                   Java Classes and Objects Slide # 19

class AccessLevels 
{
   public  int  publicObjectVariable ;
   protected  float  protectedObjectVariable  =  10;
   private  int[]  privateObjectVariable;

   int  noExplicitAccessLevel  =  publicObjectVariable;

   public  AccessLevels (  int  startValue  )  
   {
      System.out.println(  " Start Constructor"  );
      privateObjectVariable  =  new  int[ startValue ];
   }

   public  void  sampleMethod(  int  value  )  
   {
      System.out.println( " In method" );
      privateObjectVariable[ 1 ]  =  value;
   }
}
class TestAccessLevels 
{
   public static void main( String args[] ) 
   {
      AccessLevels   test  =  new AccessLevels ( 11 );

      test.publicObjectVariable  =  100;   // Ok
      test.protectedObjectVariable=  100;   // Ok
      test.privateObjectVariable[ 1 ]  =  100;   // Compile Error
      test.noExplicitAccessLevel  =  100;   // Ok
   }
}

                                   Java Classes and Objects Slide # 20

class  Hide
{
   public void publicAccess()
   {
      System.out.println( "Start public access " );
      internalWorker();
      realPrivateWorker();
   }

   protected void internalWorker()
   {
      System.out.println( "In internal worker " );
   }

   private void realPrivateWorker()
   {
      System.out.println( "In Private worker " );
   }

   public static void main( String[] args )
   {
      Hide me = new Hide();
      me.publicAccess();
   }
}

                                   Java Classes and Objects Slide # 21

class  BankAccount 
{
   private  float  balance;

   public BankAccount( float initialBalance ) 
   {
      balance = initialBalance;
   }

   public void deposit( float amount )
   {
      balance += amount ;
   }

   public String toString()
   {
      return "Account Balance: " + balance;
   }
}

                                   Java Classes and Objects Slide # 22

Decomposability Composability Protection      Continuity Understandability                  

       Decomposability Composability          Continuity Understandability                  

                                   Java Classes and Objects Slide # 23

• All fields should be protected or private to outside access
• All fields should be hidden from methods in same class

class  BankAccount 
{
   private  float  balance;

   protected float getBalance()
   {
      return balance;
   }

   protected void setbalance( float newBalance)
   {
      balance = newBalance;
   }

   public BankAccount( float initialBalance ) 
   {
      setbalance( initialBalance );
   }

   public void deposit( float amount )
   {
      setbalance( getBalance() + amount );
   }

   public String toString()
   {
      return "Account Balance: " + getBalance();
   }
}

                                   Java Classes and Objects Slide # 24

class  Student  
{
   public  char  grade;
}

class PointerTest 
{

   public static void main( String args[] ) 
   {
      Student  sam  =  new  Student();

      sam.grade  =  'A';

      Student  samTwin  =  sam;

      samTwin.grade  =  'C';

      System.out.println( sam.grade );
   }
}

                                   Java Classes and Objects Slide # 25

class  Parameter  
{
   public  void  noChangeArgument(  char  grade  )  
   {
      grade  =  'A';
   };
};

class TestParameter   
{

   public static void main( String args[] ) 
   {

      char  samGrade  =  'C';

      Parameter  Test  =  new  Parameter();

      Test.noChangeArgument(  samGrade  );

      System.out.println(  samGrade  );
   };
};

C

                                   Java Classes and Objects Slide # 26

• Use return
• Use references (arrays or objects)

class TestParameter   
{

   public static void main( String args[] ) 
   {
      char  samGrade  =  'C';

      samGrade  =  inflateGrade(  samGrade  );

      System.out.println(  samGrade  );
   };

   public  static  char  inflateGrade(  char  grade  )  
   {
      switch ( grade ) 
      {
         case  'A': 
         case  'B':
            return 'A';
         case  'C':
            return 'B';
         case  'D':
         case  'F':
            return 'C';
      };
      return 'A';
   }; 
};

                                   Java Classes and Objects Slide # 27

class TestParameter   
{

   public static void main( String args[] ) 
   {
      char[]  classGrades  =  new char[ 30 ];

      classGrades[ 1 ]  =  'B';

      TestParameter.changeArgument(  classGrades  );

      System.out.println(  classGrades[ 1 ]  );
   };

   static  public  void  changeArgument(  char[]  grade  )  
   {
      grade[1]  =  'A';
   };
};

                                   Java Classes and Objects Slide # 28

class  Student  
{
   public  char  grade;
};

class  Parameter  
{

   public  void  changeArgument(  Student  lucky  )  
   {
      lucky.grade  =  'A';
   };
};

class TestParameter   
{

   public static void main( String args[] ) 
   {
      Student    sam  =  new Student();

      sam.grade   =  'B';

      Parameter  Test  =  new  Parameter();

      Test.changeArgument(  sam  );

      System.out.println(  sam.grade );
   };
};

                                   Java Classes and Objects Slide # 29

The same static field is shared between all object of the class

Static fields exist before creating an object of the class

Same as C++ static data members

class  StaticFields
{
   public  static  int  size  =  10;

   public void increaseSize( int increment )
   {
      size =  size + increment;
   }
}

class  DemoStaticFields
{
   public  static  void  main( String[]  arguments )
   {
      System.out.println( "Size " + StaticFields.size );
      StaticFields top  =  new  StaticFields();
      StaticFields bottom  =  new  StaticFields();
      top.size = 20;
      System.out.println( "Size " + bottom.size );
   }
}

                                   Java Classes and Objects Slide # 30

class  StaticMethods
{
   public  static  int  size  =  10;

   public static void increaseSize( int increment )
   {
      size =  size + increment;
   }
}

class  DemoStaticMethods
{
   public  static  void  main( String[]  arguments )
   {
      StaticMethods.increaseSize( 30 );
      System.out.println( "Size " + StaticMethods.size );

      StaticMethods top  =  new  StaticMethods();

      top.increaseSize( 20 );
      System.out.println( "Size " + StaticMethods.size );
   }
}

                                   Java Classes and Objects Slide # 31

class  StaticFields  
{
   public  static  int  size  =  10;
   protected  static int[]  classArray  =  new int [ size ];

   public  int  localInt;

   static                  // Run when class is first loaded 
   {
      size  =  20;
   };

   public  static  void  classMethod(  int  value  )  
   {
      System.out.println( " In class method" );
      size  =  value;
      localInt  =  value;   // Compile Error
   }
}

class TestStaticFields  
{

   public static void main( String args[] ) 
   {
      StaticFields     test  =  new  StaticFields  (  );
      StaticFields     secondObject  =  new  StaticFields  (  );

      StaticFields.size  =  100;
      System.out.println(  test.size  );   // Print 100
      StaticFields.localInt  =  99;   // Compile Error
      test.localInt  =  99;
   }
}

                                   Java Classes and Objects Slide # 32

class Top
{
   public  static  void  main( String[]  arguments )
   {
      System.out.println( "In top main" );
      Bottom.main( arguments );
   }
   public static void print()
   {
      System.out.println( "In top" );
   }
}

class Bottom
{
   public  static  void  main( String[]  arguments )
   {
      System.out.println( "In bottom main" );
      Top.print( );
      OddMain( arguments );
   }
}

class OddMain
{
   public  static  int  main( String[]  arguments )
   {
      System.out.println( "In odd main" );
      Top  hat  =  new Top();

      String[]  message = { "Hi", "Mom" };
      hat.main( message );
      return 5;
   }
}

                                   Java Classes and Objects Slide # 33

class  Constants  
{

   public  static  final  int  SIZE  =  10;

   protected  static  final  int[] CLASS_ARRAY  =  new int [ size ];

   public  final  int  MUST_INITIALIZE_HERE;  // Compile Error

   public  final  char  grade  =  'A';

   public  void  trouble()
   {
      final int willNotWork = 123;      // Compile Error
   }
}

class TestConstants  
{
   public static void main( String args[] ) 
   {
      System.out.println(  Constants.SIZE  );   // Prints 10

      Constants  test  =  new  Constants();

      System.out.println(  test.grade   );   // Prints A

      System.out.println(  Constants.grade  );   // Compile Error
   }
}

                                   Java Classes and Objects Slide # 34

class Output {

   public static void main( String args[] ) {
      java.io.PrintStream  myOut  =  System.out;
      myOut  .print(  "Look Mom, No System.out" );
   }
}

                                   Java Classes and Objects Slide # 35

import  java.io.PrintStream;

class Output {

   public static void main( String args[] ) {
      PrintStream  myOut  =  System.out;
      myOut.print(  "Look Mom, No System" );
   }
}

import  java.io.*;

class Output {

   public static void main( String args[] ) {
      PrintStream  myOut  =  System.out;
      myOut.print(  "Look Mom, No System" );
   }
}

                                   Java Classes and Objects Slide # 36

package  sdsu.roger;

public  class  Sample  {
   public  void  hello()  {
      System.out.println( "Hello for package sdsu.roger" );
   }
}

setenv CLASSPATH

'.:/opt/java/classes:/home/ma/whitney/java/classes'

import  sdsu.roger.Sample;

class TestPackage {

   public static void main( String args[] ) {
      Sample  me  =  new  Sample();
      me.hello();
   }
}

                                   Java Classes and Objects Slide # 37

package SearchTree;

public class Leaf
{
   public Leaf()
   {
      System.out.println( "Leaf in a binary search tree" );
   }
}

package Botany;

public class Leaf
{
   public Leaf()
   {
      System.out.println( "Leaf in a real tree" );
   }
}

class Test 
{
   public static void main( String args[] )
   {
      Botany.Leaf green = new Botany.Leaf();
      SearchTree.Leaf node = new SearchTree.Leaf();
   }
}

                                   Java Classes and Objects Slide # 38

import SearchTree.Leaf;

class Test 
{
   public static void main( String args[] )
   {
      Botany.Leaf green = new Botany.Leaf();
      Leaf node = new Leaf();
   }
}

import SearchTree.Leaf;
import Botany.Leaf;         // Compile error

class Test 
{
   public static void main( String args[] )
   {
      Botany.Leaf green = new Botany.Leaf();
      Leaf node = new Leaf();
   }
}

import SearchTree.Leaf;
import Botany.*;

class Test 
{
   public static void main( String args[] )
   {
      Botany.Leaf green = new Botany.Leaf();
      Leaf node = new Leaf();
   }
}

                                   Java Classes and Objects Slide # 39

Accessible to code in and outside a package

Accessible to code in package only

No subclasses outside package

package Botany;

public class Leaf
{
   
   public Leaf()
   {
      System.out.println( "Leaf in a real tree" );
   }
}

package Botany;

class BotanyHelper
{
   // Only code in package Botany can use this class
}

                                   Java Classes and Objects Slide # 40