Java Intro

INTRODUCTION TO JAVA PROGRAMMING

Java: Write Once, Run Anywhere (3)

• Java has been used by large and reputable companies to create serious stand-alone applications.

• Example:– Eclipse1: started as a programming environment created by IBM for

developing Java programs. The program Eclipse was itself written in Java.

1 For more information: http://www.eclipse.org/downloads/

http://www.eclipse.org/downloads/

Java Development Kit (JDK)

It is a collection of development tools.

These are used for developing and running java

programs.

Components of JDK Javac (Java compiler)

Java (Java Interpreter)

Javap (Java disassembler- disassembles one or more class files)

Javah (for cretaing C header files or source or output files)

Javadoc (for creating HTML documents)

Appletviewer (for viewing Java applets)

Jdb (Java Debugger)

Text Editor

Java Source Code

javac

Java Class File

java

Output

javadoc

javah

jdb

HTML Files

Header Files

Process of Building and Running Java Programs

Java Runtime Environment vs. Java Development Kit

A Java distribution comes typically in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK).

The Java runtime environment (JRE) consists of the JVM and the Java class libraries and contains the necessary functionality to start Java programs.

The JDK contains in addition the development tools necessary to create Java programs. The JDK consists therefore of a Java compiler etc.

Java Virtual Machine(JVM)

• The JVM is the environment in which Java programs execute.• It is a software that is implemented on top of real hardware and

operating system. • When the source code (.java files) is compiled, it is translated into

byte codes and then placed into (.class) files. • The JVM executes these bytecodes. So Java byte codes can be

thought of as the machine language of the JVM. • A JVM can either interpret the bytecode one instruction at a time or

the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler (e.g Oracle JRockit JVM )

• The JVM must be implemented on a particular platform before compiled programs can run on that platform.

The Java SDK comes in three versions: J2ME - Micro Edition (for handheld and portable devices) J2SE - Standard Edition (PC development) J2EE - Enterprise Edition (Distributed and Enterprise Computing)

The SDK is a set of command line tools for developing Java applications: javac - Java Compiler java - Java Interpreter (Java VM) appletviewer - Run applets without a browser javadoc - automated documentation generator jdb - Java debugger

The SDK is NOT an IDE (Integrated Development Environment) Command line only. No GUI.

The Java Software Development Kit (SDK)

While it should be your goal to learn as many packages as you can, there are some packages you will use more than others:

Commonly Used Packages

Language(general)

GUI

Misc. Utilities and Collections

Input/Output

Networking

java.lang

java.awtjava.awt.eventjavax.swing

java.util

java.io

java.net

Common classes used for all application development

Graphical User Interface, Windowing,Event processing

Helper classes, collections

File and Stream I/O

Sockets, Datagrams

FIRST JAVA PROGRAMCOMPILING AND EXECUTING

Getting Started(1) Create the source file:

– open a text editor, type in the code which defines a class (HelloWorldApp) and then save it in a file (HelloWorldApp.java)

– file and class name are case sensitive and must be matched exactly (except the .java part)

Example Code: HelloWorldApp.java

public class HelloWorldApp {

public static void main(String[] args) {

// Display "Hello World!" System.out.println("Hello World!");

} }

Java is CASE SENSITIVE!

Getting Started(2) Compile the program:

– compile HelloWorldApp.java by using the following command:

javac HelloWorldApp.java

it generates a file named HelloWorldApp.class

‘javac’ is not recognized as an internal or external command, operable program or batch file. javac: Command not found

if you see one of these errors, you have two choices:1) specify the full path in which the javac program locates every time. For example:

C:\j2sdk1.4.2_09\bin\javac HelloWorldApp.java

2) set the PATH environment variable

Getting Started(3) Run the program:

– run the code through:java HelloWorldApp

– Note that the command is java, not javac, and you refer toHelloWorldApp, not HelloWorldApp.java or HelloWorldApp.class

Exception in thread "main" java.lang.NoClassDefFoundError:HelloWorldApp

if you see this error, you may need to set the environment variable CLASSPATH.

Explaining first program

public class HelloWorldApp {


// Display "Hello World!" System.out.println("Hello

World!"); } }

public: so that JVM can access the main method

static: so that JVM can call main method w/o creating object.

void : no return type

main: starting point

String args[] : command line arguments in form of string array.

Explaining first programpublic class HelloWorldApp {


// Display "Hello World!" System.out.println("Hello World!");

} }

Inbuilt class in Java.lang package

Object of printStream class

Is a method of printstream class

Characteristics of Java• Java Is Simple • Java Is Object-Oriented • Java Is Distributed • Java Is Interpreted • Java Is Robust • Java Is Secure • Java Is Architecture-Neutral • Java Is Portable • Java's Performance • Java Is Multithreaded • Java Is Dynamic


Java is partially modeled on C++, but greatly simplified and improved. Some people refer to Java as "C++--" because it is like C++ but with more functionality and fewer negative aspects.


Java is inherently object-oriented. Although many object-oriented languages began strictly as procedural languages, Java was designed from the start to be object-oriented. Object-oriented programming (OOP) is a popular programming approach that is replacing traditional procedural programming techniques.

One of the central issues in software development is how to reuse code. Object-oriented programming provides great flexibility, modularity, clarity, and reusability through encapsulation, inheritance, and polymorphism.


Distributed computing involves several computers working together on a network. Java is designed to make distributed computing easy. Since networking capability is inherently integrated into Java, writing network programs is like sending and receiving data to and from a file.


You need an interpreter to run Java programs. The programs are compiled into the Java Virtual Machine code called bytecode. The bytecode is machine-independent and can run on any machine that has a Java interpreter, which is part of the Java Virtual Machine (JVM).


Java compilers can detect many problems that would first show up at execution time in other languages.

Java has eliminated certain types of error-prone programming constructs found in other languages.

Java has a runtime exception-handling feature to provide programming support for robustness.


Java implements several security mechanisms to protect your system against harm caused by stray programs.


Write once, run anywhere

With a Java Virtual Machine (JVM), you can write one program that will run on any platform.


Because Java is architecture neutral, Java programs are portable. They can be run on any platform without being recompiled.


Java’s performance Because Java is architecture neutral, Java programs are portable. They can be run on any platform without being recompiled.


Multithread programming is smoothly integrated in Java, whereas in other languages you have to call procedures specific to the operating system to enable multithreading.


Java was designed to adapt to an evolving environment. New code can be loaded on the fly without recompilation. There is no need for developers to create, and for users to install, major new software versions. New features can be incorporated transparently as needed.

JAVA AND C++Main Differences

Java vs C++• Java is (an interpreted) write once, run anywhere language.

- Write once, run everywhere does (nearly) NEVER work with C++, but does (nearly) ALWAYS work with JAVA.

- Acheived using bytecode concept.

• You don't have separated HEADER-files defining class-properties– You can define elements only within a class.

– All method definitions are also defined in the body of the class.

– File- and classname must be identical in JAVA

– Instead of C++ “#include“ you use the “import keyword“.• For example: import java.awt.*;.

• #include does not directly map to import, but it has a similar feel to it

JAVA vs C++

• Instead of controlling blocks of declarations like C++ does, the access specifiers (public, private, and protected) are placed on each definition for each member of a class– Without an explicit access specifier, an element defaults to "friendly,"

which means that it is accessible to other elements in the same package (which is a collection of classes being ‘friends‘)

– The class, and each method within the class, has an access specifier to determine whether it’s visible outside the file.

JAVA vs C++

• Everything must be in a class.– There are no global functions or global data. If you want the equivalent

of globals, make static methods and static data within a class.

– There are no structs or enumerations or unions, only classes.

– Class definitions are roughly the same form in Java as in C++, but there’s no closing semicolon.

• Java has no preprocessor. – If you want to use classes in another library, you say import and the

name of the library.

– There are no preprocessor-like macros.

– There is no conditional compiling (#ifdef)

JAVA vs C++

• All the primitive types in Java have specified sizes that are machine independent for portability. - The char type uses the international 16-bit Unicode character set, so it

can automatically represent most national characters.

• Type-checking and type requirements are much tighter in Java. – For example:

• 1. Conditional expressions can be only boolean, not integral.• 2. The result of an expression like X + Y must be used; you can’t just say "X

+ Y" for the side effect.

JAVA vs C++

• There are Strings in JAVA– Strings are represented by the String-class, they aren‘t only some

renamed pointers– Static quoted strings are automatically converted into String objects. – There is no independent static character array string like there is in C

and C++.

• There are no Java pointers in the sense of C and C++

JAVA vs C++

• Although they look similar, arrays have a very different structure and behavior in Java than they do in C++.– There’s a read-only length member that tells you how big the array is, and

run-time checking throws an exception if you go out of bounds.

– All arrays are created on the heap, and you can assign one array to another (the array handle is simply copied).

• There is a garbage collection in JAVA– Garbage collection means memory leaks are much harder to cause in Java,

but not impossible.

• There are no destructors in Java. – There's no need because of garbage collection.

JAVA vs C++

• Java has method overloading, but no operator overloading.

• Java has both kinds of comments like C++ does.

• There’s no goto in Java. – The one unconditional jump mechanism is the break label or continue

label, which is used to jump out of the middle of multiply-nested loops.

• Java contains standard libraries for GUIs– Simple, robust and effective way of creating user-interfaces– Graphical output as part of the language

Features of C++ omitted in JAVA1. No Typedef, pre-processor(#define,#include), Structure &

Unions.2. No standalone functions, global functions, global data

everything in a class.3. Member functions to be defined only inside a class (No

inline functions)4. No goto and delete keyword used.5. No default arguments to functions.6. Objects are passed by reference only not by value but

references are themselves values.7. No Pointers so no use of :: & -> operators8. No Multiple Inheritance interfaces instead.9. No forward declaration, no semicolon at end of class.

Features of C++ omitted in JAVA10. No automatic type conversion that results in loss of

precision.11. Primitive data type size is same for all platforms12. No copy constructors as all objects are passed by references

only.13. No destructors as work done automatically by Garbage

collector..14. No declaration of static member outside class.15. Interpretation of private, public and protected keyword is

different.16. No Templates but support collections/generics17. No virtual functions all dynamic binding (final keyword)18. No Operator Overloading.

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Lexical Issues

• There are many atomic elements of Java. Java programs are a collection of whitespace, identifiers, comments, literals, operators, separators and keywords.

• Whitespace– Java is a free-form language. It means we do not need to follow any special

indentation rules. In java whitespace is a space, tab, or newline.• Identifiers

– Identifiers are used for class names, method names and variable names.– An identifier may be any descriptive sequence of uppercase and lowercase

letters, numbers, or the underscore and dollor-sign characters.• Eg: AvgTemp, count, $calculate, s5, value_display

– An identifier must not begin with a number because it leads to invalid indentifier.• Eg: 5s, yes/no

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Lexical Issues

• Literals– A constant value in Java is created by using a literal representation. A literal is

allowed to use anywhere in the program.

Eg: Integer literal : 100 Floating-point literal : 98.6 Character literal : ‘s’ String literal : “sample”

• Comments// This comment extends to the end of the line./*..............the multi-line comments are given with in this comment............................................................................................................*//**....... Documentation comments..........*/(readable to both human and computer)

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Lexical Issues• Separators

Separators are used to terminate statements. In java there are few characters are used as separators . They are, parentheses(), braces{}, brackets[], semicolon;, period., and comma,.

• Java Keywords ( 49 )

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Data Types

• Data type specify the size and type of values• Java defines eight simple types of data. These can be put into

four groups:1. Integers2. Floating-point numbers3. Characters4. Boolean

Range: -2x-1 to 2x-1-1 where x is number of bits.

Leftmost bit is reserved for the sign.

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Data Types Integers

Floating-point

Name Width(Size) Rangebyte 8 bits -128 to 127short 16 bits -32768 to 32767int 32 bits -2147483648 to 2147483647long 64 bits -264-1 to 264-1 -1

Name Width(Size) Rangefloat 32 bits 1.4e-045 to 3.4e+038double 64 bits 4.9e-324 to 1.8e+038

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Data Types

Characters• A char variable stores a single character from the Unicode

character set• A character set is an ordered list of characters, and each

character corresponds to a unique number• Unicode is an international character set, containing symbols

and characters from many world languages• The Unicode character set uses 16 bits per character, allowing

for 65,536 unique characters• Character literals are enclosed in single quotes:• ‘A' ‘c' ‘3' '$' ‘.' '\n‘

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Data Types

• You might have heard about the ASCII character set • ASCII is older and smaller than Unicode, but is still quite popular• The ASCII characters are a subset of the Unicode character set, including:

• uppercase letters A, B, C, …• lowercase letters a, b, c, …• digits 0, 1, 2, …• special symbols *, &, +, ?, …• control characters backspace ‘\b’• escape sequences) new line ‘\n’, …

Characters• Data type used to store character is char. • It requires 16 bits.• Range of a char is 0 to 65536 (no negative char)

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Data Types

Boolean• Used to test a particular condition• It can take only two values: true and false• Uses only 1 bit of storage• All comparison operators return boolean value

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Constants

• Refer to fixed values that do not change during the execution of a program.

• Integer Constants: sequence of digits• 123 037 0X2

• Real Constants: numbers containing fractional parts• 0.0083 -0.75 435.36

• Single character Constants: single character• ‘5’ ‘A’ ‘;’

• String Constants: sequence of characters• “Hello World”“?+;;;;….#”

• Backslash character (Symbolic) Constants: used in output methods• ‘\n’ ‘\b’ ‘\t’ ‘\’’ ‘\\”’ ‘\\’

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Constants

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Symbolic Constants

• Some constants may appear repeatedly in a number of places in

the program.

• These constants can be defined as a symbolic name

• Make a program:

• easily modifiable

• More understandable– final type symbolic name = valueex: final float PI = 3.14159

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Variables

• An identifier that denotes a storage location used to store a data value.• May take different values at different times during the execution of the

program• Naming Conventions: may consists of alphabets, digits, underscore and dollar

with following conditions• Must not begin with a digit• Uppercase and Lowercase are distinct.• Should not be a keyword• Space is not allowed

• Variable declaration and assignment– <type> variable_name;– Variable_name =value; // initializtaion– Variable_name=Math.sqrt(Variable_name* Variable_name);//

dynamic initialization

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Scope of Variables

Classified into three kinds:• Instance Variables

• Created when the objects are instantiated• Take different values for each object

• Class Variables• Global to a class• Belong to the entire set of objects that class creates• Only one memory location is created

• Local Variables• Used inside methods or inside program blocks.• Blocks are defined between opening brace { and a closing brace }• Visible to program only from the beginning to the end of the block.

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Type Casting

• The process of converting one data type to another is called casting

type variable1 = (type) variable2

int m = 50;

byte n = (byte) m• Casting into a smaller type may result in a loss of data• Automatic Conversion:(smaller to larger-promotion)

Automatic type conversion is possible only if the destination type has enough space to store the source value.

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Type Casting

Type Casting

Widening (implicit)

Narrowing (Explicit)

Assigning a smaller type to a larger oneNo need to write anything (automatic conversion)

byte b = 75int a =b

Assigning a larger type to a smaller oneWrite the target data type within parenthesis

int a = 75byte b = (byte) a

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Type Casting

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Type Casting

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Type Casting

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Array Array is a list of finite number n of homogeneous data elements

such that:

• The elements of the array are referenced respectively

by an index set of n consecutive numbers.

• The element of the array are stored respectively in

successive memory location.

• The number n of elements is called the length or size.

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Creation of Arrays• After declaring arrays, we need to allocate memory for

storage array items.• In Java, this is carried out by using “new” operator, as

follows:Arrayname = new type[size];

• Examples: students = new int[7];

Declaring and defining in the same statement:int[] students=new students[10];double myList[] = new double[10];

0123456

students

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Initialization of Arrays1. Once arrays are created, they need to be initialised with

some values before access their content. A general form of initialisation is:

Arrayname [index/subscript] = value;2. Example:

students[0] = 50;students[1] = 40;

3. Array index starts with 0 and ends with n-14. Trying to access an array beyond its

boundaries will generate an error message.students[7] = 100;

50students

0123456

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Explicit initialization1. Arrays can also be initialised like standard variables at the time of their

declaration.

Type arrayname[] = {list of values};

2. Example:

int[] students = {55, 69, 70, 30, 80, 90, 45};

3. Creates and initializes the array of integers of length 7.

4. In this case it is not necessary to use the new operator.

55697030809045

students

0123456

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Array: Default ValuesWhen an array is created, its elements are assigned the default value of

0 for the numeric primitive data types, '\u0000' for char types, and false for boolean types

Once an array is created, its size is fixed. It cannot be changed. You can find its size using

arrayRefVar.length

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Processing Array Elements using loop• Often a for( ) loop is used to process each of the elements of the array in

turn.

• The loop control variable, i, is used as the index to access array components

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Array as Parameters• Methods can accept arrays via parameters• Use square brackets [ ] in the parameter declaration:

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Returning Array

• A method may return an array as its result

double [] readArray(int n){

double result[] = new double[n]; for (i = 0; i < n; i++){

result[i] = i;}return result;

}

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Two Dimensional Arrays• Two dimensional arrays allows us to store data that are recorded in

table.int[][] student=new int[4][4];4 arrays each having 4 elementsFirst index: specifies array (row)Second Index: specifies element in that array (column)

10 20 85 45

56 58 25 58

45 85 65 78

65 14 28 56

0 1 2 3

0

1

2

3

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Accessing 2D Array Elements Sum the Elements

• Here is code that sums all the numbers in table. • The outer loop iterates four times and moves down the

rows. • Each time through the outer loop, the inner loop iterates

five times and moves across a different row.

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Declaration and Instantiation of 2D Array

Declaring and instantiating two-dimensional arrays are accomplished by extending the processes used for one-dimensional arrays:

• Declaration:int myArray [][];

• Creation or Instantiation:myArray = new int[4][3]; // ORint myArray [][] = new int[4][3];

• Initialisation:- Single Value;

myArray[0][0] = 10;- Multiple values:

int table[2][3] = {{10, 15, 30}, {14, 30, 33}};int table[][] = {{10, 15, 30}, {14, 30, 33}};

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2D Array as “Array of Arrays”

int table [][] = new int[4][5];• The variable table references an array of four elements. • Each of these elements in turn references an array of five integers.

Variable Size Arrays

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Java treats multidimensional arrays as “arrays of arrays”. It is possible to declare a 2D arrays as follows:

[0]

[1]

[2]

int a[][] = new int [3][];

a[0]= new int [2];

a[1]= new int [3];

a[2]= new int [4];

[0] [1]

[0] [1] [2]

[0] [1] [2] [3]

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Multidimensional ArraysExample: A farmer has 10 farms of beans each in 5

countries, and each farm has 30 fields!Three-dimensional array:

long[][][] beans=new long[5][10][30];//beans[country][farm][fields]

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Varying length in Multidimensional Arrays

Same features apply to multi-dimensional arrays as those of 2

dimensional arrays

long beans=new long[3][][]; //3 countries

beans[0]=new long[4][]; //First country has 4 farms

beans[0][4]=new long[10];//Each farm in first country has 10

fields

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Exercise-11. WAP in Java that create an array and simple print the element of the array.

(Array Traversing).2. WAP in Java that implements following three methods:

1. boolean Search( int [] A, int Key): it takes an array A and a Key as input and returns true if Key is in the array else return false.

2. int Position(int [] A, int Key): it takes an array A and a Key as input and returns the position of Key in the array.

3. int [] Sort(int [] A): it takes an array A and returns another array. The output array has same element as in input array but in ascending order.

3. WAP in Java that takes an two dimensional array as input parameter to a method and returns one dimensional array. Each element of the output array is the sum of the element of corresponding row in input array.

4. WAP in Java to add, multiply, and subtract two Matrices. The decision of operation should be made based on user’s choice.

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Classes

•A class is a user-defined data type. Once defined, this new type can be used to create variables of that type.

•These variables are termed as instances of classes, which are the actual objects.

•A class is a template for an object, and an object is an instance of a class.

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Classes and Objects

• An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.

Class Name: Circle Data Fields:

radius is _______ Methods:

getArea

Circle Object 1 Data Fields:

radius is 10


radius is 25


radius is 125

A class template

Three objects of the Circle class

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Class cont..//Basic form of a class

class classname{

type variable1type variable2-------------------------------type

methodname1(parameter_list){

//body of method}type

methodname2(parameter_list){

//body of method}---------------

}

//A Simple Classclass Box{

double width

double height

double depth

}

Data is encapsulated in a class by placing data fields inside the body of the class definition.

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Declaring (creating) ObjectsDeclaring Object Reference Variables

ClassName objectReference;

Example: Box myBox;

Creating Objects

objectReference = new ClassName();

Example: myBox = new Box();

The object reference is assigned to the object reference variable.

Declaring/Creating Objects in a Single Step

ClassName objectReference = new ClassName();

Example: Box myBox = new Box();

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Accessing Objects

• Referencing the object’s data:

objectRefVar.data

e.g., myBox.width

myBox.height

myBox.depth

• Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myBox.getArea()

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Classes and ObjectsTrace Code

Box myBox = new Box();

Box yourBox = new Box();

yourBox.width = 100;

yourBox.height = 10;

yourBox.depth= 50;

Declare myBox

nullmyBox

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Classes and ObjectsTrace Code, cont.

nullmyBox

Box

Width HeightDepth





yourBox.depth= 50;

Create a Box

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myBox

Box

Width HeightDepth





yourBox.depth= 50;

reference value

Assign object reference to myBox

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Classes and Objects

yourBox

reference valuemyBox

Box

Width HeightDepth





yourBox.depth= 50;

null

Declare yourBox

Trace Code, cont.

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Classes and Objects

Create a new Box object

yourBox


Box

Width HeightDepth





yourBox.depth= 50;

null

Box

Width HeightDepth

Trace Code, cont.

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Classes and Objects

yourBox


Box

Width HeightDepth





yourBox.depth= 50;

Box

Width HeightDepth

Trace Code, cont.

Assign object reference to yourBox

reference value

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Classes and Objects

yourBox


Box

Width HeightDepth





yourBox.depth= 50;

Trace Code, cont.

reference value

Box

Width =100Height=10Depth =50

Change variables in yourBox

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yourBox


Box

Width HeightDepth





yourBox.depth= 50;

myBox = yourBox; reference value

Box

Width =100Height=10Depth =50

Assigning Object Reference Variables

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Garbage Collection• When no references to an object exist, that object is assumed too

be no longer needed, and the memory occupied by the object can be reclaimed.

• As shown in the previous figure, after the assignment statement myBox = yourBox, myBox points to the same object referenced by yourBox. The object previously referenced by myBox is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

• If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable.

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The finalize() Method• Sometimes an object will need to perform some action when it is

destroyed.

• The garbage collector calls a special method named finalize in your object if that method exists.

• If an object hold some non-Java resources (file handle or window character font) or any reference to other objects, these resources can be freed using finalize method.

• Avoiding circular reference.

protected void finalize(){//finalization code here

}

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Differences between Variables of Primitive Data Types and Object Types

1

c: Circle

radius = 1

Primitive type int i = 1 i

Object type Circle c c reference

Created using new Circle()

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Copying Variables of Primitive Data Types and Object Types

1

c1: Circle

radius = 5

Primitive type assignmenti = j

Before:

i

2j

2

After:

i

2j

Object type assignmentc1 = c2

Before:

c1

c2

After:

c1

c2

c2: Circle

radius = 9

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Introducing Methodstype name(parameter-list){

//body of methodreturn value; (if type is not void)

}

• Define the interface to most classes.• Hide specific layout of internal data structures(Abstraction)• Add method to Box class

void volume(){System.out.print(“Volume is: ”);;System.out.println(width*height*depth);

}

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Returning a Value

double volume(){return width*height*depth;}

• The type of data returned by a method must be compatible with the return type specified by the method.

• The variable receiving the value returned by a method must also be compatible with the return type specified for the method.

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Parameterized Methodsvoid setDim(double w, double h, double d){width = w;height = h;depth = d;

}

Parameters: variable defined by a method that receives a value when the method is called

Arguments: value that is passed to a method when it is invoked.

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ConstructorsBox(){ //default constructor}Box(){width = 10;height = 10;depth = 10;

}Box(double w, double h, double d){width = w;height = h;depth = d;

}Box myBox = Box();Box yourBox = Box(20,20,20);

• Constructors are a special kind of methods that are invoked to construct objects.

• A Constructor initializes an object immediately upon creation.

• Automatic initialization.

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Constructors cont…• A constructor with no parameters is referred to as a default

constructor.

• Constructors must have the same name as the class itself.

• Constructors do not have a return type—not even void.

• Implicit return type of a class’ constructor is the class type itself.

• Constructors are invoked using the new operator when an object is created.

• Constructors play the role of initializing objects.

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The this Keyword• this can be used inside any method to refer to the current

object.

• When you want to pass the current object to a method.

• To resolve any name space collisions that might occur between instance variables and local variables.Box(double width, double height, double depth){this.width = width;this.height = height;this.depth = depth;

}

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The this Keyword Cont…class Data{

private String data_string;Data(String S){data_string = s;}public String getData(){return data_string;}public void printData(){

Printer p = new Printer();p.print(this);

}}class Printer{

void print(Data d){System.out.println(d.getData);}}public class app{

public static void main(String args[]){

(new Data(“Hello from Java”)).printData();}

}

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Overloading Methods• Method overloading defines several different versions of a method

all with the same name, but each with a different parameter list.

• At the time of method call, java compiler will know which one you mean by the number and/or types of the parameters.

• Overloaded methods must differ in the type and/or number of their parameters.

• Implements polymorphism

• May have different return types.

• To overload a method, you just define it more than once, specifying a new parameter list different from every other

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Overloading Methods exampleClass calculator{int add(int op1, int op2){return op1+op2;}int add(int op1, int op2, op3){return op1+op2+op3;}

} In C you have three functions to get the absolute value of different data types–

abs() for integer, fabs() for floating point, and lfabs() for long integer.

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Overloading Constructors• Works like overloading other methods.

• Define the constructor a number of times, each time with a different parameter list.

//when all dimensions specifiedBox(double w, double h, double d){Width = w;Height = h;Depth = d;

}//when cube is createdBox(double l){Width = Height = Depth = l;

}

Box myBox = new Box(10,20,15);Box yourBox = new Box(25);

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Using Objects as Parameters• An object of a class can be passed as parameter to both methods and

constructors of a class./*construct a new object so that it is initially the same as some existing object.

Define a new constructor Box that takes an object of its class as a parameter.

*/Box(Box ob){Width = ob.Width;Height = ob.Height;Depth = ob.Depth;

}Box myBox = new Box(10,20,15);Box yourBox = new Box(myBox);

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Using Objects as Parameters cont…//objects may be passed to methodsclass Test{int a,b;Test(int i, int j){a = i;b = j;}//return true if obj is equal to the invoking obbjectboolean equals(Test obj){if(obj.a == a && obj.b == b) return true;else return false;}

}Test ob1 = new Test(100, 22);Test ob2 = new Test(100, 22);Test ob3 = new Test(-1,-1);ob1.equals(ob2); trueOb1.equals(ob3); false

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Type of Argument Passing Call-by-value:

• When you pass an item of a simple data type to a method.• Method only gets a copy of the data item.• The code in the method cannot affect the original data item at all.

class Test{void meth(int i, int j){i=i*2;j=j/2;}

}

Test ob = new Test();int a = 15, b = 20;Systeem.out.println(“ a and b before call: “ +a+” “+b); 15 20ob.meth(a,b);Systeem.out.println(“ a and b after call: “ +a+” “+b); 15 20

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Type of Argument Passing cont… Call-by-reference:

• When you pass an object to a method.• Java actually passes a reference to the object.• Code in the method can reach the original object.• Any change made to the passed object affect the original object.

class Test{int a,b;Test(int i, int j){a = i;b = j;

}void meth(Test o){o.a = o.a*2;o.b = o.b/2;}

}Test ob = new Test(15,20);Systeem.out.println(“ a and b before call: “ +a+” “+b); 15 20ob.meth(ob);Systeem.out.println(“ a and b after call: “ +a+” “+b); 30 10

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Returning Objects from Methods

• A method can return objects just like other data types.

• The object created by a method will continue to exist as long as

there is a reference to it.

• No need to worry about an object going out-of-scope because

the method in which it was created terminates.

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Returning Objects Exampleclass Test{int a;Test (int i){a = i;}Test incrByTen(){Test temp = new Test(a+10);return temp;}

}Class RetOb{public static void main(String args[]){Test ob1 = new Test(2);Test ob2;ob2 = ob1.incrByTen();System.out.println(“ob1.a: “+ob1.a);System.out.println(“ob2.a: “+ob2.a);}

}ob1.a = 2;ob2.a = 12;

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Visibility Modifiers Accessor Methods

• Visibility modifiers specify which parts of the program may see and use any particular class/method/field.

• How a member can be accessed is determined by the access specifier that modifies its declaration.

• Java has three visibility modifiers: public, private, and protected.

When no access specifier is used , then by default the member of a class is public within its own package but can not be accessed outside its package.(default visibility)

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Visibility Modifiers - Classes• A class can be defined either with the public modifier or

without a visibility modifier (default visibility).

• If a class is declared as public it can be used by any other class

• If a class is declared without a visibility modifier it has a default visibility.

• A member is a field, a method or a constructor of the class.

• Members of a class can be declared as private, protected, public or without a visibility modifier (default):

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Public Visibility• Members that are declared as public can be accessed from any

class that can access the class of the member

• We expose methods that are part of the interface of the class by declaring them as public

• We do not want to reveal the internal representation of the object’s data. So we usually do not declare its state variables as public (encapsulation)

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Private Visibility• A class member that is declared as private, can be accessed only

by code that is within the class of this member.

• We hide the internal implementation of the class by declaring its state variables and auxiliary methods as private.

• Data hiding is essential for encapsulation.

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class Data{public int [] a={0,0,0,0,0,0,0};}class app{public static void main(String args[]){

Data d= new Data();for(int i=0;i<d.a.length;i++)System.out.println(d.a[i]);

}}

class Data{public int [] a={1,0,0,0,0,0,0};}

class app{void print(Data obj){System.out.println(obj.a[0]); }

public static void main(String args[]){

Data d= new Data();app b=new app();b.print(d);for(int i=0;i<d.a.length;i++)System.out.println(d.a[i]);

}}

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class Test {int a; // default accesspublic int b; // public accessprivate int c; // private access// methods to access cvoid setc(int i) { // set c's valuec = i;}

int getc() { // get c's valuereturn c;}}

class AccessTest {public static void main(String args[]) {Test ob = new Test();// These are OK, a and b may be accessed directlyob.a = 10;ob.b = 20;// This is not OK and will cause an error// ob.c = 100; // Error!// You must access c through its methodsob.setc(100); // OKSystem.out.println("a, b, and c: " + ob.a + " " +ob.b + " " + ob.getc());}}

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The static Keyword• Create a member that can be used by itself, without reference to

a specific instance or object.

• A static member can be accessed before any objects of its class are created.

• You can declare both methods and variables to be static.

• These variables are, essentially, global variables.

• No copy of a static variable is made when objects of its class are declared.

• All instances of the class share the same static variables.

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static MethodsMethods declared as static have several restrictions:

• They can only call other static methods.

• They must only access static data.

• They cannot refer to this or super in any way.

final Keyword

• A variable can be declared as final.

• contents of the variable cannot can not be modified.

• Must initialize a final variable when it is declared.

– Final int a=10;

– Final flaot = 10.45f

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// Demonstrate static variables, methods, and blocks.class UseStatic{static int a = 3;static int b;static void meth(int x) {System.out.println("x = " + x);System.out.println("a = " + a);System.out.println("b = " + b);}static {System.out.println("Static block initialized.");b = a * 4;}public static void main(String args[]) {meth(42);}}

Output:Static block initialized.x = 42a = 3b = 12

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class StaticDemo {static int a = 42;static int b = 99;static void callme() {System.out.println("a = " + a);}

class StaticByName {public static void main(String args[]) {StaticDemo.callme();System.out.println("b = " + StaticDemo.b);}}

Output:a = 42b = 99

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Exploring the String Class• every string you create is actually an object of type String.

Even string constants are actually String objects.System.out.println("This is a String, too");the string “This is a String, too” is a String constant.

• objects of type String are immutable; once a String object is created, its contents cannot be altered

• If you need to change a string, you can always create a new one that contains the modifications.

• Java defines a peer class of String, called StringBuffer, which allows strings to be altered, so all of the normal string manipulations are still available in Java.

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// Demonstrating Strings.class StringDemo {public static void main(String args[]) {String strOb1 = "First String";String strOb2 = "Second String";String strOb3 = strOb1 + " and " + strOb2;System.out.println(strOb1);System.out.println(strOb2);System.out.println(strOb3);}}

Methods of String Class

Javap java.lang.String

boolean equals(String object)int length( )char charAt(int index)

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// Demonstrate String arrays.class StringDemo3 {public static void main(String args[]) {String str[] = { "one", "two", "three" };for(int i=0; i<str.length; i++)System.out.println("str[" + i + "]: " +str[i]);}}

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Using Command-Line Arguments// Display all command-line arguments.class CommandLine {public static void main(String args[]) {for(int i=0; i<args.length; i++)System.out.println("args[" + i + "]: " +args[i]);}}

java CommandLine this is a test 100 -1

args[0]: thisargs[1]: isargs[2]: aargs[3]: testargs[4]: 100args[5]: -1

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Taking Input from Userimport java.io.*;class StringDemo {public static void main(String args[]) throws IOException{System.out.println("Enter");BufferedReader br=new BufferedReader(new InputStreamReader(System.in));String A= br.readLine();int a=Integer.parseInt(br.readLine());char c=(char)br.read();System.out.println(a);System.out.println(A);}}

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Documents

Java Intro