Unit 2 notes.pdf

16CS4201 – JAVA PROGRAMMING II YEAR / IV SEM
UNIT 2 - NOTES Page 1
UNIT II – PACKAGES AND INTERFACES
Packages-defining package-access protection-importing packages- interfaces- Defining an interface-
implementing an interface-applying interface-variables in interface-extended interface-Exception
Handling-exception types-uncaught exception-multiple catch-nested try-throw and finally-built-in
exceptions multithreaded programming-java thread model-thread priorities-synchronization thread class
and runnable interface-creating multiple threads- inter thread communication-string-input and output
2.1 PACKAGES
A package in Java is used to group related classes, sub packages and interfaces. Think of it as a folder in
a file directory. Packages are used to avoid name conflicts, and to write a better maintainable code.
Packages are divided into two categories:
 Built-in Packages (packages from the Java API)
 User-defined Packages (create your own packages)
Advantage of Java Package
1) Java package is used to categorize the classes and interfaces so that they can be easily maintained.
2) Java package provides access protection.
3) Java package removes naming collision.
2.1.1 Defining a Package
 To create a package simply include the package command as the first statement in a Java source
file. Any classes declared within that file will belong to the specified package.
 The package statement defines a name space in which classes are stored. If you omit the package
statement, the class names are put into the default package, which has no name.
 This is the general form of the package statement:
package pkg;
Here, pkg is the name of the package. For example, the following statement creates a package called
MyPackage.
package MyPackage;
Example
//save as Simple.java
package MyPack;
public class Simple
{
public static void main(String args[])
{
System.out.println("Welcome to package");
}
}
Save this file Simple.java, and put it in a directory called MyPack. Next, compile the file. Make sure
that the resulting Simple.class file is also in the MyPack directory. Then try executing the Simple class,
using the following command line:
java MyPack.Simple
Remember, you will need to be in the directory above MyPack when you execute this command, or to
have your CLASSPATH environmental variable set appropriately. As explained, Simple is now part of

the package MyPack. This means that it cannot be executed by itself. That is, you cannot use this
command line:
java Simple
Simple must be qualified with its package name.
2.1.2 Access Protection
 Classes and packages both means of encapsulating and containing the name space and scope of
variables and methods.
 Packages acts as a containers for classes and other sub – ordinate packages.
 Classes act as containers for data and code.
 Java address four categories of visibility for class members:
o Sub – classes in the same package.
o Non – sub class in the same package.
o Sub – classes in the different package.
o Classes that are neither in the same package nor subclasses.
 The 3 access specifiers private, public and protected provide a variety of ways to produce the
many levels of access required by these categories.
2.1.3 Importing Packages
There are three ways to import the package from outside the package.
1. import package.*;
2. import package.Classname;
3. fully qualified name.
1) Using packagename.*
If you use package.* then all the classes and interfaces of this package will be accessible but not
subpackages.
The import keyword is used to make the classes and interface of another package accessible to the current
package.
Example of package that import the packagename.*
//save by A.java
package pack1;
public class A
{
public void msg()
{

System.out.println("Hello");
}
}
//save by B.java
package mypack;
import pack1.*;
class B
{
{
A obj = new A();
obj.msg();
}
}
Output:Hello
2) Using packagename.Classname
If you import package.Classname then only declared class of this package will be accessible.
Example of package by import package.Classname
//save by A.java
package pack1;
public class A
{
public void msg()
{
}
}
//save by B.java
package mypack;
import pack1.A;
class B
{
{
A obj = new A();
obj.msg();
}
}
Output:
Hello
3) Using fully qualified name
If you use fully qualified name then only declared class of this package will be accessible. Now there is
no need to import. But you need to use fully qualified name every time when you are accessing the class
or interface.

It is generally used when two packages have same class name e.g. java.util and java.sql packages contain
Date class.
Example of package by import fully qualified name
//save by A.java
package pack1;
public class A
{
public void msg()
{
}
}
//save by B.java
package mypack;
class B
{
{
pack.A obj = new pack1.A(); //using fully qualified name
obj.msg();
}
}
Output:
Hello
Creating Sub package in a package:
We can create sub package in a package in the format:
package packagename.subpackagename;
e.g.: package pack1.pack2;
Here, we are creating pack2 subpackage which is created inside pack1 package. To use the classes and
interfaces of pack2, we can write import statement as: import pack1.pack2;
//Creating a subpackage in a package
package pack1.pack2;
public class Sample
{
public void show ()
{
System.out.println ("Hello Java Learners");
}
}
Built-in Packages
The Java API is a library of prewritten classes that are free to use, included in the Java Development
Environment.

The library contains components for managing input, database programming, and much more. For
example,
java.applet
java.awt
java.util
java.lang
java.net
The library is divided into packages and classes. The user can either import a single class (along with its
methods and attributes), or a whole package that contain all the classes that belong to the specified
package.
To use a class or a package from the library, use the import keyword:
Syntax
import package.name.Class; // Import a single class
import package.name.*; // Import the whole package
Example
import java.util.Scanner; // Import a single class
import java.util.*; // Import the whole package
2.2 INTERFACE
An interface in java is a blueprint of a class. It has static constants and abstract methods.
The interface in java is a mechanism to achieve abstraction. There can be only abstract methods in the
java interface not method body. It is used to achieve abstraction and multiple inheritance in Java. It
cannot be instantiated just like abstract class.
 An interface is a collection of abstract methods (i.e. methods without having definition).
 A class that implements an interface inherits abstract methods of the interface.
 An interface is not a class.
 If a class (provided it is not abstract) implements an interface, then all the methods of interface
need to be defined in it.
Why use Java interface?
There are mainly three reasons to use interface. They are given below.
 It is used to achieve abstraction.
 By interface, we can support the functionality of multiple inheritance.
 It can be used to achieve loose coupling.
2.2.1 Declaring Interfaces
The interface keyword is used to declare an interface. Here is a simple example to declare an interface:
access interface name
{
final type varname1 = value;
final type varname2 = value;
return-type method-name1(parameter-list);
return-type method-name2(parameter-list);
}

Interfaces have the following properties:
 An interface is implicitly (i.e. by default) abstract. We do not need to use the abstract keyword
when declaring an interface.
 Each method in an interface is also implicitly (i.e. by default) abstract, so the abstract keyword is
not needed.
 Methods in an interface are implicitly (i.e. by default) public.
The java compiler adds public and abstract keywords before the interface method. More, it adds
public, static and final keywords before data members.
In other words, Interface fields are public, static and final by default, and methods are public and abstract.
2.2.2 Implementing Interfaces
 A class can implement more than one interface at a time.
 A class can extend only one class, but can implement many interfaces.
 An interface itself can extend another interface.
 The general form of a class that includes the implements clause looks like this:
class classname extends superclass implements interface1 , interface2...
{
// class-body
}
Example:
interface Message
{
void message1( );
void message2( );
}
class A implements Message
{
void message1( )
{
System.out.println("Good Morning");
}
void message2( )
{
System.out.println("Good Evening");
}
{

A a=new A();
a.message1();
a.message2();
}
}
Understanding relationship between classes and interfaces
As shown in the figure given below, a class extends another class, an interface extends another interface
but a class implements an interface.
Java Interface Example: Bank
Let's see another example of java interface which provides the implementation of Bank interface.
File: TestInterface2.java
interface Bank
{
float rateOfInterest();
}
class SBI implements Bank
{
public float rateOfInterest()
{ return 9.15f;
}
}
class PNB implements Bank
{
public float rateOfInterest()
{ return 9.7f;
}
}
class TestInterface2
{
public static void main(String[] args)
{
Bank b=new SBI();
System.out.println("ROI: "+b.rateOfInterest());
}
}
Output:
ROI: 9.15

Multiple Inheritance in Java by interface
If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as
multiple inheritance.
Example:
interface Printable
{
void print( );
}
interface Showable
{
void show( );
}
class A implements Printable,Showable
{
public void print( )
{ System.out.println("Hello");
}
public void show( )
{ System.out.println("Welcome");
}
{
A obj = new A();
obj.print( );
obj.show( );
}
}
Output:
Hello
Welcome
Similarities between class and interface
 Both class and interface can contain any number of methods.
 Both class and interface are written in a file with a .java extension, with the name of the interface
matching the name of the file.
 The bytecode of both class and interface appears in a .class file.

Dissimilarities between class and interface
 We cannot instantiate (i.e. create object) an interface but we can instantiate a class.
 An interface does not contain any constructors but a class may contain any constructors.
 All the methods in an interface are abstract (i.e. without definitions) but in a class method may or
may not be abstract.
 An interface cannot contain variables. The only variable that can appear in an interface must be
declared both static and final. But a class can contain any variable.
 An interface is not extended by a class; it is implemented by a class.
 An interface can extend multiple interfaces (i.e. multiple inheritances can be achieved through it).
But a class cannot extend multiple classes (i.e multiple inheritance can not be achieved).
Difference between Class and Interface
Class Interface
In class, you can instantiate variable and create
an object.
In an interface, you can't instantiate variable and
create an object.
Class can contain concrete(with
implementation) methods
The interface cannot contain concrete(with
implementation) methods
The access specifiers used with classes are
private, protected and public.
In Interface only one specifier used is public.
Difference between abstract class and interface
Abstract class and interface both are used to achieve abstraction where we can declare the abstract
methods. Abstract class and interface both can't be instantiated.
But there are many differences between abstract class and interface that are given below.
Abstract class Interface
1) Abstract class can have abstract and non-abstract
methods.
Interface can have only abstract methods.
2) Abstract class doesn't support multiple inheritance. Interface supports multiple inheritance.
3) Abstract class can have final, non-final, static and
non-static variables.
Interface has only static and final variables.
4) Abstract class can provide the implementation of
interface.
Interface can't provide the implementation of
abstract class.
5) The abstract keyword is used to declare abstract
class.
The interface keyword is used to declare
interface.
6) Example:
public abstract class Shape
{
public abstract void draw();
}
Example:
public interface Drawable
{
void draw();
}
Simply, abstract class achieves partial abstraction (0 to 100%) whereas interface achieves fully
abstraction (100%).
Advantages of interface in java:
Advantages of using interfaces are as follows:

1. Without bothering about the implementation part, we can achieve the security of implementation
2. In java, multiple inheritance is not allowed, however you can use interface to make use of it as
you can implement more than one interface.
2.2.3 Variables in Interfaces
We can declare variables in Java interfaces. By default, these are public, final and static. That is they are
available at all places of the program, we cannot change these values and lastly only one instance of these
variables is created, it means all classes which implement this interface have only one copy of these
variables in the memory.
interface X
{
int max = 10;
}
class Example implements X
{
public void getMax()
{
System.out.println(max);
}
}
class Demo
{
{
Example ob = new Example();
ob.getMax();
}
}
OUTPUT:
10
2.2.4 Extended Interface
 One interface can inherit another by use of the keyword extends.
 The syntax is the same as for inheriting classes.
 When a class implements an interface that inherits another interface, it must provide
implementations for all methods defined within the interface inheritance chain.
interface A
{
void meth1();
void meth2();
}
interface B extends A
{
void meth3();
}
class MyClass implements B
{

public void meth1()
{
System.out.println("Implement meth1().");
}
public void meth2()
{
}
public void meth3()
{
}
}
class IFExtend
{
{
MyClass ob = new MyClass();
ob.meth1();
ob.meth2();
ob.meth3();
}
}
2.3 EXCEPTION HANDLING
Error occurred in a program can be of two types: syntax error or logical error.
 An exception is an abnormal condition that arises in a code sequence at run time. In other words, an
exception is a run-time error.
 A Java exception is an object that describes an exceptional (that is, error) condition that has occurred
in a piece of code.
 Exception Handling is a mechanism by which exception occurred in a program is handled.
 The exception handling in java is one of the powerful mechanism to handle the runtime errors so
that normal flow of the application can be maintained.
 Java exception handling is managed via five keywords: try, catch, throw, throws, and finally.
S.No Keyword/Block Meanings
1 try block The statements which are expected to throw exception are kept
inside try block.
2 catch block If an exception occurs within the try block, it is throws an exception
to the catch block which handle it in some rational manner.
3 throw To manually throw an exception, use the keyword throw.
4 throws A throws clause lists the types of exceptions that a method might
throw.
5 finally Any code that absolutely must be executed before a method returns
is put in a finally block.

The general form of an exception-handling block is:
try
{
// block of code to monitor for errors
}
catch (ExceptionType1 exOb)
{
// exception handler for ExceptionType1
}
catch (ExceptionType2 exOb)
{
// exception handler for ExceptionType2
}
finally
{
// block of code to be executed before try block ends
}
2.3.1 Exception Types
All exception types are subclasses of the built-in class Throwable. Thus, Throwable is at the top
of the exception class hierarchy.
 The Throwable class has two subclasses Error and Exception.
 Error and Exception classes are used for handling errors in java.

2.3.2 Uncaught exceptions
Before we learn how to handle exceptions in our program, it is useful to see what happens when we don’t
handle them. This small program includes an expression that intentionally causes a divide-by-zero error.
class E
{
{
int x = 0;
int y = 1 / x;
System.out.println("This will not be printed.");
}
}
Here is the output generated when this example is executed.
java.lang.ArithmeticException: / by zero
at E.main(E.java:6)
Using try and catch
The following program includes a try block and a catch clause which processes the
ArithmeticException generated by the division-by-zero error.

Demonstration of Division by zero Exception.
class Ex
{
public static void main(String args[ ])
{
int x, y;
try
{
x = 0;
y= 1/ x;
}
catch (ArithmeticException e)
{
System.out.println("Division by zero.");
}
System.out.println("After catch statement.");
}
}
This program generates the following output:
Division by zero.
After catch statement.
Notice that the call to println ( ) inside the try block is never executed. Once an exception is
thrown, program control transfers out of the try block into the catch block. Put differently, catch is not
“called,” so execution never “returns” to the try block from a catch. Thus, the line “This will not be
printed.” is not displayed. Once the catch statement has executed, program control continues with the
next line in the program following the entire try/catch mechanism.
2.3.3 Multiple Catch:
In some cases, more than one exception could be raised by a single piece of code. To handle this type
of situation, we can specify two or more catch clauses, each catching a different type of exception. When
an exception is thrown, each catch statement is inspected in order, and the first one whose type matches
that of the exception is executed. After one catch statement executes, the others are bypassed, and
execution continues after the try/catch block. The following example traps two different exception types:
Solution:
class MultiCatch
{
{
try
{
int a = args.length;
System.out.println("a = " + a);
int b = 1 / a;
int c[ ] = { 1 };

c[42] = 99;
}
catch(ArithmeticException e)
{
System.out.println("Divide by 0: " + e);
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("Array index oob: " + e);
}
System.out.println("After try/catch blocks.");
}
}
This program will cause a division-by-zero exception if it is started with no command line parameters,
since a will equal zero. It will survive the division if you provide a command-line argument, setting a to
something larger than zero. But it will cause an ArrayIndexOutOfBoundsException, since the int array
c has a length of 1, yet the program attempts to assign a value to c[42].
Here is the output generated by running it both ways:
C:>java MultiCatch
a = 0
Divide by 0: java.lang.ArithmeticException: / by zero
After try/catch blocks.
C:>java MultiCatch TestArg
a = 1
Array index oob: java.lang.ArrayIndexOutOfBoundsException
After try/catch blocks.
2.3.4 Nested try
The try block within a try block is known as nested try block in java.
Why use nested try block
Sometimes a situation may arise where a part of a block may cause one error and the entire block itself
may cause another error. In such cases, exception handlers have to be nested.
Syntax:
try
{
statement 1;
try
{
statement 1;
}
catch(Exception e)
{ }
}
catch(Exception e)
{ }

Example :
class Excep6
{
{
try
{ try
{
System.out.println("going to divide");
int b =39/0;
}
{ System.out.println(e);
}
try
{ int a[]=new int[5];
a[5]=4;
}
}
System.out.println("other statement);
}
catch(Exception e)
{ System.out.println("handeled");
}
System.out.println("normal flow..");
}
}
OUTPUT:
going to divide
java.lang.ArithmeticException: / by zero
java.lang.ArrayIndexOutOfBoundsException: 5
other statement
normal flow..
2.3.5 throw
Instead of exceptions thrown by the Java run-time system, throw keyword helps to throw an exception
explicitly. The general form of throw is shown here:
throw new ThrowableInstance;
Here, ThrowableInstance must be an object of type Throwable or a subclass of Throwable. throw is a
keyword. This keyword is mainly used in user defined exception were we throw an exception manually
and terminate the execution abnormally.
The function of throw keyword and default exceptional handling is same.
Example:
class ThrowDemo
{

{
int x=2, y=0;
try
{
if(y==0)
throw new ArithmeticException();
}
{
System.out.println("Exception Occurred: Division by 0 " + e);
}
}
}
User Defined Exceptions
Although Java’s built-in exceptions handle most common errors, we will probably want to create our
own exception types to handle situations specific to your applications. This is quite easy to do: just define
a subclass of Exception (which is, of course, a subclass of Throwable).
Example:
class MyException extends Exception
{
private int detail;
MyException(int a)
{
detail = a;
}
public String toString()
{
return "MyException[" + detail + "]";
}
}
class Samp1
{
static void compute(int a) throws MyException
{
System.out.println("Called compute(" + a + ")");
if(a > 10)
throw new MyException(a);
System.out.println("Normal exit");
}
{
try
{
compute(1);
compute(20);
}
catch (MyException e)

{
System.out.println("Caught " + e);
}
}
}
OUTPUT:
Called compute(1)
Normal exit
Called compute(20)
Caught MyException[20]
2.3.6 throws
If a method is capable of causing an exception that it does not handle, it must specify this behavior so that
callers of the method can guard themselves against that exception. We do this by including throws clause
in the method's declaration. A throws clause lists the types of exceptions that a method might throw.
This is the general form of a method declaration that includes a throws clause:
return_type method-name (parameter-list) throws exception-list
{
// body of method
}
Example:
class ThrowsDemo
{
static void throwOne() throws IllegalAccessException
{
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
{
try
{
throwOne();
}
catch (IllegalAccessException e)
{
System.out.println("Caught " + e);
}
}
}
OUTPUT:
Inside throwOne.
Caught java.lang.IllegalAccessException: demo
2.3.7 finally
 finally creates a block of code that will be executed after a try/catch block has completed and
before the code following the try/catch block.

 The finally block will execute whether or not an exception is thrown.
 If an exception is thrown, the finally block will execute even if no catch statement matches the
exception.
Example 1:
class Ex
{
{
int x, y;
try
{
x = 0;
y= 1/ x;
}
catch (ArithmeticException e)
{
System.out.println("Division by zero.");
}
finally
{
System.out.println("End of Try/Catch Block");
}
System.out.println("End of program");
}
}
Output:
Division by zero.
End of Try/Catch Block
End of program
Example 2:
Let's see the java finally example where exception doesn't occur.
class TestFinallyBlock
{
{
try
{
int data=25/5;
System.out.println(data);
}
catch(NullPointerException e)
}
finally
{ System.out.println("finally block is always executed");

}
System.out.println("rest of the code...");
}
}
Output:
5
finally block is always executed
rest of the code...
2.3.8 Built-in Exceptions
 Inside the standard package java.lang, Java defines several exception classes.
 The most general of these exceptions are subclasses of the standard type RuntimeException.
 Since java.lang is implicitly imported into all Java programs, most exceptions derived from
RuntimeException are automatically available.
 These exceptions need not be included in any method’s throws list. In the language of Java, these
are called unchecked exceptions because the compiler does not check to see if a method handles
or throws these exceptions.
Unchecked Exceptions
 Those exceptions defined by java.lang that must be included in a method’s throws list if that
method can generate one of these exceptions and does not handle it itself. These are called
checked exceptions.
Checked Exceptions
Exception Meaning
ClassNotFoundException Class not found.
IllegalAccessException Access to a class is denied.
InstantiationException Attempt to create an object of an abstract class or interface.
InterruptedException One thread has been interrupted by another thread.
NoSuchFieldException A requested field does not exist.
NoSuchMethodException A requested method does not exist.
Below is the list of important built-in exceptions in Java.
1. Arithmetic Exception
It is thrown when an exceptional condition has occurred in an arithmetic operation.
class ArithmeticException_Demo

{
{
try
{ int a = 30, b = 0;
int c = a/b; // cannot divide by zero
System.out.println ("Result = " + c);
}
{ System.out.println ("Can't divide a number by 0");
}
}
}
Output: Can't divide a number by 0
2. ArrayIndexOutOfBoundException
It is thrown to indicate that an array has been accessed with an illegal index. The index is
either negative or greater than or equal to the size of the array.
class ArrayIndexOutOfBound_Demo
{
{
try
{ int a[] = new int[5];
a[6] = 9; // accessing 7th element in an array of
// size 5
}
{ System.out.println ("Array Index is Out Of Bounds");
}
}
}
Output: Array Index is Out Of Bounds
3. ClassNotFoundException
This Exception is raised when we try to access a class whose definition is not found
4. FileNotFoundException
This Exception is raised when a file is not accessible or does not open.
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
class File_notFound_Demo
{
{
try
{ // Following file does not exist

File file = new File("E://file.txt");
FileReader fr = new FileReader(file);
}
catch (FileNotFoundException e)
{ System.out.println("File does not exist");
}
}
}
Output: File does not exist
5. IOException
It is thrown when an input-output operation failed or interrupted
6. InterruptedException
It is thrown when a thread is waiting , sleeping , or doing some processing , and it is
interrupted.
7. NoSuchFieldException
It is thrown when a class does not contain the field (or variable) specified
8. NoSuchMethodException
It is thrown when accessing a method which is not found.
9. NullPointerException
This exception is raised when referring to the members of a null object. Null represents
nothing
class NullPointer_Demo
{
{
try
{ String a = null; //null value
System.out.println(a.charAt(0));
}
catch(NullPointerException e)
{ System.out.println("NullPointerException..");
}
}
}
Output:
NullPointerException..
10. NumberFormatException
This exception is raised when a method could not convert a string into a numeric format.
class NumberFormat_Demo
{
{

try
{ // "akki" is not a number
int num = Integer.parseInt ("akki") ;
System.out.println(num);
}
catch(NumberFormatException e)
{ System.out.println("Number format exception");
}
}
}
Output: Number format exception
11. RuntimeException
This represents any exception which occurs during runtime.
12. StringIndexOutOfBoundsException
It is thrown by String class methods to indicate that an index is either negative than the
size of the string
class StringIndexOutOfBound_Demo
{
{
try
{ String a = "This is like chipping "; // length is 22
char c = a.charAt(24); // accessing 25th element
System.out.println(c);
}
catch(StringIndexOutOfBoundsException e)
{ System.out.println("StringIndexOutOfBoundsException");
}
}
}
Output: StringIndexOutOfBoundsException
Difference between final, finally and finalize
There are many differences between final, finally and finalize. A list of differences between final, finally
and finalize are given below:
S.No. final finally finalize
1) final is used to apply restrictions
on class, method and variable.
Final class can't be inherited, final
method can't be overridden and
final variable value can't be
changed.
finally is used to place
important code, it will be
executed whether
exception is handled or
not.
finalize is used to
perform clean up
processing just before
object is garbage
collected.
2) final is a keyword. finally is a block. finalize is a method.

2.4 MULTITHREADING
Multithreading in java is a process of executing multiple threads simultaneously. A multithreaded
program contains two or more parts that can run concurrently. Each part of such a program is called a
thread, and each thread defines a separate path of execution. Thus, multithreading is a specialized form of
multitasking.
Multiprocessing and multithreading, both are used to achieve multitasking. However, we use
multithreading than multiprocessing because threads use a shared memory area. They don't allocate
separate memory area so saves memory, and context-switching between the threads takes less time than
process.
Java Multithreading is mostly used in games, animation, etc.
What is a Thread?
A thread is a lightweight subprocess, the smallest unit of processing. It is a separate path of execution.
Threads are independent. If there occurs exception in one thread, it doesn't affect other threads. It uses a
shared memory area.
As shown in the above figure, a thread is executed inside the process. There is context-switching between
the threads. There can be multiple processes inside the OS, and one process can have multiple threads.
Advantages of Java Multithreading
1) It doesn't block the user because threads are independent and you can perform multiple operations at
the same time.
2) You can perform many operations together, so it saves time.
3) Threads are independent, so it doesn't affect other threads if an exception occurs in a single thread.
Multitasking
Multitasking is a process of executing multiple tasks simultaneously. We use multitasking to utilize the
CPU. Multitasking can be achieved in two ways:
 Process-based Multitasking (Multiprocessing)
 Thread-based Multitasking (Multithreading)
1) Process-based Multitasking (Multiprocessing)

 A process-based multitasking is the feature that allows our computer to run two or more
programs concurrently. For example, process-based multitasking enables us to run the Java
compiler at the same time that we are using a music player.
 Each process has an address in memory. In other words, each process allocates a separate memory
area.
 A process is heavyweight.
 Cost of communication between the process is high.
 Switching from one process to another requires some time for saving and loading registers,
memory maps, updating lists, etc.
2) Thread-based Multitasking (Multithreading)
 In a thread-based multitasking environment, the thread is the smallest unit of dispatchable code.
This means that a single program can perform two or more tasks simultaneously.
 Threads share the same address space.
 A thread is lightweight.
 Cost of communication between the thread is low.
2.4.1 Java Thread Model
One thread can pause without stopping other parts of your program. For example, the idle time created
when a thread reads data from a network or waits for user input can be utilized elsewhere.When a thread
blocks in a Java program, only the single thread that is blocked pauses. All other threads continue to run.
Life Cycle of a Thread
A thread can be in one of the five states. The life cycle of the thread in java is controlled by JVM. The
java thread states are as follows:
1. New
2. Runnable
3. Running
4. Non-Runnable (Blocked)
5. Terminated

1) New
The thread is in new state if you create an instance of Thread class but before the invocation of start()
method.
2) Runnable
The thread is in runnable state after invocation of start() method, but the thread scheduler has not selected
it to be the running thread.
3) Running
The thread is in running state if the thread scheduler has selected it.
4) Non-Runnable (Blocked)
This is the state when the thread is still alive, but is currently not eligible to run.
5) Terminated
A thread is in terminated or dead state when its run() method exits.
2.4.2 Thread Priority
 Java assigns to each thread a priority that determines how that thread should be treated with
respect to the others.
 Priorities are represented by a number between 1 and 10.
 In most cases, thread schedular schedules the threads according to their priority (known as
preemptive scheduling). But it is not guaranteed because it depends on JVM specification that
which scheduling it chooses
 Java priorities are in the range between MIN_PRIORITY (a constant of 1) and MAX_PRIORITY
(a constant of 10). By default, every thread is given priority NORM_PRIORITY (a constant of 5).
 Threads with higher priority are more important to a program and should be allocated processor
time before lower-priority threads. However, thread priorities cannot guarantee the order in which
threads execute and very much platform dependent.
 A thread’s priority is used to decide when to switch from one running thread to the next. This is
called a context switch.
The rules that determine when a context switch takes place are simple:
 A thread can voluntarily relinquish control. This is done by explicitly yielding, sleeping, or
blocking on pending I/O. In this scenario, all other threads are examined, and the highest priority
thread that is ready to run is given the CPU.
 A thread can be preempted by a higher-priority thread. In this case, a lower-priority thread
that does not yield the processor is simply preempted—no matter what it is doing— by a higher-
priority thread. Basically, as soon as a higher-priority thread wants to run, it does. This is called
preemptive multitasking.
To set a thread’s priority, use the setPriority( ) method, which is a member of Thread. This is its general
form:
final void setPriority(int level)
Here, level specifies the new priority setting for the calling thread. The value of level must be within the
range MIN_PRIORITY and MAX_PRIORITY. Currently, these values are 1 and 10, respectively. To
return a thread to default priority, specify NORM_PRIORITY, which is currently 5. These priorities are
defined as static final variables within Thread. You can obtain the current priority setting by calling the
getPriority() method of Thread, shown here:
final int getPriority( )

2.4.3 The Thread Class and the Runnable Interface
There are two ways to create a thread:
1. By extending Thread class
2. By implementing Runnable interface.
2.4.3.1 Creating Thread by extending Thread class:
To create a thread is to create a new class that extends Thread, and then to create an instance of that
class. The extending class must override the run ( ) method, which is the entry point for the new thread.
It must also call start ( ) to begin execution of the new thread.
Thread class provide constructors and methods to create and perform operations on a thread. Thread class
extends Object class and implements Runnable interface.
The Thread class defines several methods that help manage threads.
Method Meaning
getName Obtain a thread’s name.
getPriority Obtain a thread’s priority.
isAlive Determine if a thread is still running.
join Wait for a thread to terminate.
run Entry point for the thread.
sleep Suspend a thread for a period of time.
start Start a thread by calling its run method.
Example:
class Multi extends Thread
{
public void run()
{
System.out.println("thread is running...");
}
{
Multi t1=new Multi();
t1.start();
}
}
Output: thread is running...
2.4.3.2 Creating Thread by Implementing Runnable interface
The easiest way to create a thread is to create a class that implements the Runnable interface.
 To implement Runnable, a class need only implement a single method called run( ), which is
declared like this:
public void run( )
We will define the code that constitutes the new thread inside run() method. It is important to understand
that run() can call other methods, use other classes, and declare variables, just like the main thread can.

 After we create a class that implements Runnable, you will instantiate an object of type Thread
from within that class. Thread defines several constructors. The one that we will use is shown
here:
Thread(Runnable threadOb, String threadName);
Here threadOb is an instance of a class that implements the Runnable interface and the name of the new
thread is specified by threadName.
 After the new thread is created, it will not start running until you call its start( ) method, which is
declared within Thread. The start( ) method is shown here:
void start( );
Here is an example that creates a new thread and starts it running:
Example:
class Multi implements Runnable
{
public void run()
{
System.out.println("thread is running...");
}
{
Multi obj=new Multi();
Thread t1 =new Thread(obj);
t1.start();
}
}
Output: thread is running...
2.4.3 Creating Multiple Threads
We have been using only two threads: the main thread and one child thread. However, our program can
spawn as many threads as it needs. For example, the following program creates three child threads:
If you have to perform single task by many threads, have only one run() method. For example:
Program of performing single task by multiple threads
class TestMultitasking1 extends Thread
{
public void run()
{
System.out.println("task one");
}
{
TestMultitasking1 t1=new TestMultitasking1();
t1.start();
t2.start();
t3.start();
}

}
Output:
task one
task one
task one
If you have to perform multiple tasks by multiple threads,have multiple run() methods.For example:
Program of performing two tasks by two threads
class Simple1 extends Thread
{
public void run()
{
System.out.println("task one");
}
}
class Simple2 extends Thread
{
public void run()
{
System.out.println("task two");
}
}
class TestMultitasking3
{
{
Simple1 t1=new Simple1();
Simple2 t2=new Simple2();
t1.start();
t2.start();
}
}
Output:
task one
task two
Example:
class A implements Runnable
{
public void run()
{
for(int i=1;i<=5;i++)
{
System.out.println("A = "+i);
}
}
}
class B implements Runnable

{
synchronized public void run()
{
{
System.out.println("B = "+i);
}
}
}
class C implements Runnable
{
public void run()
{
{
System.out.println("C = "+i);
}
}
}
class ExampleThread
{
public static void main(String[] args)
{
A obj1 = new A();
B obj2 = new B();
C obj3 = new C();
Thread t1 = new Thread(obj1);
t3.setPriority(7);
t2.setPriority(Thread.MAX_PRIORITY);
int n = t3.getPriority();
System.out.println("Priority of Thread t3 is " + n);
t1.start();
t2.start();
t3.start();
}
}
OUTPUT:
Priority of Thread t3 is 7
A = 1
B = 1
A = 2
B = 2
B = 3
A = 3
B = 4
C = 1

B = 5
A = 4
C = 2
A = 5
C = 3
C = 4
C = 5
Using isAlive( ) and join( )
Two ways exist to determine whether a thread has finished. First, we can call isAlive( ) on the thread.
This method is defined by Thread, and its general form is shown here:
final boolean isAlive( )
The isAlive( ) method returns true if the thread upon which it is called is still running. It returns false
otherwise. While isAlive( ) is occasionally useful, the method that you will more commonly use to wait
for a thread to finish is called join( ), shown here:
final void join ( ) throws InterruptedException
This method waits until the thread on which it is called terminates. Its name comes from the concept of
the calling thread waiting until the specified thread joins it. Additional forms of join ( ) allow you to
specify a maximum amount of time that you want to wait for the specified thread to terminate.
class TestJoinMethod1 extends Thread
{
public void run()
{
{
try
{
Thread.sleep(500);
}
catch(Exception e)
{ System.out.println(e); }
System.out.println(i);
}
}
{
TestJoinMethod1 t1=new TestJoinMethod1();
t1.start();
try
{
t1.join();
}
catch(Exception e)
{System.out.println(e);}

t2.start();
t3.start();
}
}
Output:
1
2
3
4
5
1
1
2
2
3
3
4
4
5
5
2.4.4 Synchronization
When two or more threads need access to a shared resource, they need some way to ensure that the
resource will be used by only one thread at a time. The process by which this is achieved is called
synchronization.
Key to synchronization is the concept of the monitor (also called a semaphore). A monitor is an object
that is used as a mutually exclusive lock, or mutex. Only one thread can own a monitor at a given time.
When a thread acquires a lock, it is said to have entered the monitor. All other threads attempting to enter
the locked monitor will be suspended until the first thread exits the monitor. These other threads are said
to be waiting for the monitor. A thread that owns a monitor can reenter the same monitor if it so desires.
2.4.4.1 Using Synchronized Methods:
Synchronization is easy in Java, because all objects have their own implicit monitor associated
with them. To enter an object’s monitor, just call a method that has been modified with the synchronized
keyword. While a thread is inside a synchronized method, all other threads that try to call it (or any other
synchronized method) on the same instance have to wait. To exit the monitor and relinquish control of
the object to the next waiting thread, the owner of the monitor simply returns from the synchronized
method.
Example 1:
class Table
{
synchronized void printTable(int n)
{
{
System.out.println(n*i);
try
{

Thread.sleep(400);
}
catch(Exception e)
{ System.out.println(e); }
}
}
}
class MyThread1 extends Thread
{
Table t;
MyThread1(Table t)
{ this.t=t;
}
public void run()
{
t.printTable(5);
}
}
{
Table t;
MyThread2(Table t)
{ this.t=t;
}
public void run()
{
t.printTable(100);
}
}
public class TestSynchronization2
{
{
Table obj = new Table();//only one object
MyThread1 t1=new MyThread1(obj);
t1.start();
t2.start();
}
}
Output:
5
10
15
20
25
100
200

300
400
500
2.4.4.2 Using synchronized Statement:
While creating synchronized methods within classes that you create is an easy and effective means of
achieving synchronization, it will not work in all cases. To understand why, consider the following.
Imagine that you want to synchronize access to objects of a class that was not designed for
multithreaded access. That is, the class does not use synchronized methods. Further, this class was not
created by you, but by a third party and you do not have access to the source code. Thus, you can’t
add synchronized to the appropriate methods within the class. How can access to an object of this class
be synchronized?
Fortunately, the solution to this problem is quite easy: You simply put calls to the methods defined by
this class inside a synchronized block. This is the general form of the synchronized statement:
synchronized (object)
{
// statements to be synchronized
}
Here, object is a reference to the object being synchronized. A synchronized block ensures that a call to a
method that is a member of object occurs only after the current thread has successfully entered object’s
monitor.
class Table
{
void printTable(int n)
{
{
System.out.println(n*i);
try
{
Thread.sleep(400);
}
catch(Exception e)
{System.out.println(e);}
}
}
}
{
Table t ; int N;
MyThread1(Table t,int n)
{
this.t=t;
N=n;
}
public void run()
{

synchronized(t) //synchronized block
{
t.printTable(n);
}
}
}
public class Test
{
{
Table obj = new Table();//only one object
t1.start();
t2.start();
}
}
Output:
5
10
15
20
25
100
200
300
400
500
Example:
class A extends Thread
{
int total;
synchronized public void run()
{
total = 0;
{
total=total + i;
}
notify();
}
}
public class Example3Thread {
public static void main(String[] args) {
A obj = new A();
try

{
obj.start();
synchronized(obj)
{
System.out.println("Waiting....");
obj.wait();
}
}
catch(Exception e)
{
}
System.out.println("Total = " + obj.total);
}
}
OUTPUT:
Waiting....
Total = 15
2.4.5 Inter Thread Communication using Wait, notify and notifyAll
Inter-thread communication or Co-operation is all about allowing synchronized threads to
communicate with each other.
Cooperation (Inter-thread communication) is a mechanism in which a thread is paused running in its
critical section and another thread is allowed to enter (or lock) in the same critical section to be
executed.It is implemented by following methods of Object class:
 wait()
 notify()
 notifyAll()
wait( ) tells the calling thread to give up the monitor and go to sleep until some other thread enters the
same monitor and calls notify( ). notify( ) wakes up the first thread that called wait( ) on the same object.
notifyAll( ) wakes up all the threads that called wait( ) on the same object. The highest priority thread
will run first. These methods are declared within Object, as shown here:
final void wait( ) throws InterruptedException
final void notify( )
final void notifyAll( )
Steps for Inter thread communication:
1. Threads enter to acquire lock.
2. Lock is acquired by on thread.
3. Now thread goes to waiting state if you call wait() method on the object. Otherwise it releases the
lock and exits.
4. If you call notify() or notifyAll() method, thread moves to the notified state (runnable state).
5. Now thread is available to acquire lock.
6. After completion of the task, thread releases the lock and exits the monitor state of the object.

class Customer
{
int amount=10000;
synchronized void withdraw(int amount)
{
System.out.println("going to withdraw...");
if(this.amount<amount)
{
System.out.println("Less balance; waiting for deposit...");
try
{
wait();
}
catch(Exception e)
{}
}
this.amount-=amount;
System.out.println("withdraw completed...");
}
synchronized void deposit(int amount)
{
System.out.println("going to deposit...");
this.amount+=amount;
System.out.println("deposit completed... ");
notify();
}
}
class Samp1
{
{
final Customer c=new Customer();
new Thread()
{
public void run()

{
c.withdraw(15000);
}
}.start();
new Thread()
{
public void run()
{
c.deposit(10000);
}
}.start();
}
}
Output:
going to withdraw...
Less balance; waiting for deposit...
going to deposit...
deposit completed...
withdraw completed

Unit 2 notes.pdf

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Unit 2 notes.pdf