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1Java

The Class Explored

Part I

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2Java

Class Constants10

Class members9

Instance members8Destruction7

Instanceof6

This5

Null4

References3

Constructor2

public and private access specifiers1

Contents

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3Java

Command Line Arguments16for-each loop statement15

Initializing arrays14

Arrays13

Immutability12

String11

contents

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4Java

Know

public and private access specifiers

What a constructor is

What references, null, this and instanceofare

How objects are destroyed

The difference between instance and classmember variables

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5Java

The String class

How to work with Arrays

The new for-each loop statement

How to get arguments from the

command line

Know

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Be Able To

Use public and private access specifiers

Write constructors

Write programs using the String class andarrays

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7Java

Public and Private access

Specifiers

Name

Registration number

Name of the degree

Current Semester

Identified by

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8Java

public class Student{

private String name;

private int regNo;

private StringdegreeName;

private intcurrentSemester;

public String getName()

public voidsetName(String nm)

public int getRegNo()public void setRegNo(int r)

public String getDegreeName()public voidsetDegreeName(String dnm)

public int getCurrentSemester()public voidsetCurrentSemester(int i)

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9Java

setRegNo(int r) Student object

regno 12345

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10Java

Constructor

When we create an object using new

keyword a constructor is called.

There can be more than one constructor for a

class.

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11Java

public class Student{

private String name;

private int regNo;

private String degreeName;

private int currentSemester;

/*Constructors 1 for student who have decided the degreethey are going to enroll into */

public Student(String nm, String d){

setName(nm);

regNo=generateRegno();setDegreeName(d);

setCurrentSemester(1);}

A constructor has

the same name as

the class and has

no return value.

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12Java

/*Constructors 2 for student who have notdecided the degree they are going to enrollinto */

public Student(String nm){

setName(nm);

regNo=generateRegno();

setCurrentSemester(1);}

private int generateRegno(){

int nextNo=0;

//logic to generate reg no.

return nextNo;}

// place for setter methodsFolder 1

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13Java

public class StudentTest{

public static void main(String args[]){

//Creating object using constructor 1

Student student1=

new Student(John, M.C.A.);

//Creating object using constructor 2

Student student2=new Student(Mary);

}

}

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14Java

new Student();

class Student{Student(){..}

}

Compiler

looks for

But since we dont have this

constructor the compiler flags an

error.

Can we create student object

similar to college object?

Student student1= newStudent();

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15Java

But what happened in case

of the College class ?

When there are no constructors

explicitly written in a class, the java

compiler inserts a constructor which

does not have any arguments. But

if we explicitly include one or moreconstructors, then the compiler

does not insert any constructor.

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public class College{

private String name;

public void display(String str){

System.out.println(Welcome to !);

System.out.println(name);

}

public static void main(String args[]){

College collegeObject= new College();

collegeObject.name=XYZ College;

collegeObject.display(XYZ College);}

}

publicCollege(){super();

}

Compiler inserts

Folder 2

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17Java

References

Student student2=new Student(Mary);

name:MaryregNo:2

currentSemester:1degreeName:null

student2

HEAPPointingto address

of theactualstudent2object in

memory

STACK

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18Java

Student student1= new Student(Mary);

Student student2=student1;student1.setName(Merry Mary);

System.out.println(student2.getName());

This code snippet prints

Merry Mary. Why ?

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19Java

name:MaryMerry MaryregNo:2

currentSemester:1degreeName:null

student1

student2

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null

Default value of an object reference is null.

class Test{Student student;

void test(){student.display();}

null

An error occurs at runtime

class Test{

void test(){Student student;student.display();

}

An error occurs atcompile time

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21Java

this

public class Student{

String name;

Student(String name){

name=name;

regNo=generateRegno();

currentSemester=1; }}

this.name=name;

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22Java

public class Student{

String name;

public Student(Stringname, String d){

this(name);

setDegreeName(d);

}

public Student(Stringname){

setName(name);

regNo=generateRegno();

setCurrentSemester(1);

}

}

calls

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23Java

instanceof

Usage:

object-refinstanceofclass-name

returns a boolean value.

Example:Student s1= new Student(Mary);

System.out.println(s1 instanceof Student);

System.out.println(s1 instanceof College);

true

Compilation error

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null instanceof Student

returns false.

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Destruction

Objects are automatically Garbage collected. Object is garbage collected

-- if the object reference is set to null and no

other object reference refers to thatobject

OR

-- if the object goes out of scope and itsreference is not assigned to any othervariable outside its scope.

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26Java

heap

Student student1= new Student(Mary);

Student studentref=student1;

student1=null;

student

student1studentref

Will the student object created in the first

line be garbage collected?

null

How many objects are created?

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27Java

Instance members

public class Student{

private String name;

public void setName(String name){...

}

Instance

member

variable

Instance memberfunction

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28Java

Class members

public class Student{

private int gRegNo;

public Student(String nm){

regNo=generateRegno();

setCurrentSemester(1);}

private int generateRegno(){

gRegNo++;

return gRegNo; }}

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29Java

student1

Student

name:MarygRegNo:1regNo:1

Student

name:JohngRegNo:1regNo:1

student2

Again 1! Should be 2

Student student1= new Student(Mary);

Student student2= new Student(John);

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30Java

public class Student{

private static int gRegNo;

Student(String nm){

regNo=generateRegno();

...}

private int generateRegno(){

gRegNo++;

return gRegNo; }

public static int getGRegNo(){

return gRegNo;}

...}Folder 3

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31Java

public class StudentTest(){

public static void main(String args[]){

Student student1= new Student(Mary);

Student student2= new Student(John);

System.out.println( Student.getGRegNo());

System.out.println( student1.getGRegNo());

}

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32Java

student1

Student

name:MaryregNo:1

Student

name:JohnregNo:2

student2

Student student1= new Student(Mary);

Student student2= new Student(John);

It is correct now.

gRegNo:2

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34Java

Class Constants

public class Student{public static final intMAX_STUDENTS=3000;..}

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String

Constructors:String()String(String)

Examples of creating String object:String s=abc;

String s= new String();

String s= new String(Hello);String s1= new String(s);

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36Java

Methods of String class:

int length() :

String s= new String(Hello);

System.out.println(s.length());

//prints 5

char charAt(int index)

String s="Have a nice day";

System.out.println(s.charAt(0));

// prints H

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37Java

String s1=abc,s2=def;

String s3=s1+s2; // returns abcdef

String s4=s1+1; // returns abc1

String concat(String str)

String s1=java.concat(c)

//returns javac

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boolean equals(Object object)

boolean equalsIgnoreCase(String anotherString)

Example:

String s1=abc;

String s2=sbc;String s3=ABC;

s1.equals(s2) ;//returns falses1.equalsIgnoreCase(s3) );//returnstrue

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39Java

Wait a minute. Why do we require

equals() method to compare Strings

? Can we not compare using ==.

== is alright with primitive

datatypes. But with

references, == will actually

compare the addresses whilewhat we are want here is to

check equality of value of

strings.

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40Java

public Stringsubstring(int beginIndex)

public Stringsubstring(int beginIndex,int endIndex)

Example:icecream".substring(3)

returns cream

icecream".substring(0,3)

returns ice

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41Java

Comparing two strings:

public intcompareTo(String anotherString)

public intcompareToIgnoreCase(String str)

Example:

String s1="ABC";String s2="acc";

s2.compareTo(s1)

returns 32

s2.compareToIgnoreCase(s1)

returns 1

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42Java

String toLowerCase()

String toUpperCase()

String replace(char oldChar,char newChar)StringreplaceAll(String reg,String replacement)

boolean startsWith(String prefix)public boolean endsWith(String suffix)

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Immutability

String s1="ABC";

String s2=ABC;

ABCs1

s2

String pool

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44Java

Strings are Immutable Objects.

String s1="ABC";s1=DEF

ABC

DEF

s1

String pool

This string object

remains intact. This is

not changed.

New String object is

created

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45Java

String pool

ABC

DEF

s1

s2

Compare this with object references (slide 15)

Assigning string references:

String s1="ABC";String s2=s1;

s2=DEF;

System.out.println(s1);// prints ABC

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Constructor:

String(String newStr)

String s1="ABC";

String s2=new String(ABC);

ABC

ABC

s1

s2

String pool

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47Java

String s1="ABC";

String s2=s1;

System.out.println(s1==s2);

s2="DEF";

System.out.println(s1==s2);String s4="ABC";

System.out.println(s1==s4);

String s3=new String(s1);System.out.println(s1==s3);

What is printed

in each case?

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Arrays

int sum=0,mul=0;int num[]= new int[5];

for(int i=0;i

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int num[]= new int[5];

00000

numnum[0]num[1]

num[2]num[3]num[4]

automatically initialized to 0

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50Java

public class ArrayTest{

static int num[];public static void main(Stringargs[]){System.out.println(num);

System.out.println(num[0]);}}

error at runtime

prints null

Folder 4

E l f f

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51Java

Example for an array of

references

We will implement Stack data structure that can

contain Strings

We need to have an array of String objectsinside the Stack class.

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public class Stack{

private String stackData[];private int top;private final int MAX_CAPACITY;

public Stack(){this(10);}public Stack(int capacity){

stackData=new String[capacity];

top=-1;MAX_CAPACITY=capacity;}

Creates Stack instance

Creates an arrayrepresenting stack

last insertion index

Folder 5

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53Java

public Stringpush(String data)

{

if(!isFull()){stackData[++top]=data;

return stackData[top];}else return "Stack Full ! ";

}If the stack is not full it inserts the data on the top ofthe list.

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54Java

public Objectpop() {if(isEmpty()){

return "Stack Empty ! ";}else {

String obj=stackData[top];

stackData[top]=null;top--;return obj;}

}If the stack is not empty it pops out the data from the

top of the list.

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55Java

public booleanisEmpty(){if(top==-1) return true;else return false; }

public booleanisFull(){if(top==MAX_CAPACITY-1)return true;else return false; }

public static void main(String args[]) {

Stack s=new Stack(3);System.out.println("push Object :

"+s.push("Prema"));

System.out.println("push Object :

"+s.push("Padma"));System.out.println("push Object :

"+s.push("Prasad" ));

System.out.println("remove top "+s.pop());}

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56Java

Change the program to handlethe problem gracefully.

Execute and findout what happens

when you push 4th

element into the

stack?

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57Java

Initializing arrays

int [] a= {1,2,4,8,26};

int [] a=new int[] {1,2,4,8,26};

int a[]= new int[0];

anonymous array

Creates an array of length 0

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58Java

for-each loop statement

This is a new feature in 1.5 .

Convenient way to iterate through arrays ( and

collection)

Syntax:

for(datatype variable: array)statement

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59Java

Example 1:

int a[]= {1,2,3,4,5};

for(int j:a)

System.out.println(j);

Example 2:Student s[]= new Student [2];

s[0]= new Student(Mary);

s[1]= new Student(John);for(Student s1:s)

System.out.println(s1.getName());

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Command Line Arguments

2 command line arguments sent to StudentTest class

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61Java

public class StudentTest{

public static void main(Stringargs[]){

//Creating object using constructor 1

Student student1=

new Student(args[0], args[1]);

}

}Mary M.C.A.

Folder 6

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So what will happen if you

run StudentTest withoutsupplying command line

arguments ?