1. UNIT-4

2. Concepts of Exception handling
Exception hierarchy
Types of exceptions
Usage of try, catch, throw, throws, finally keywords
Built in Exceptions
Creating own Exception sub classes
Concepts of multi threading
Types of multi tasking, uses of multitasking
Thread life cycle
Creating multiple threads by using thread class
Creating multiple threads by using Runnable interface
Synchronization

3. Program Errors
There are basically 3 types of errors in a java program:
1.Compile-time errors
2.Run-time errors
3.Logical errors.

4. D:\>javac Cerror.java CError.java:4: ';' expected } ^ 1 error
1.Compile-time errors:
These are syntactical errors found in the code, due to which a program fails to compile.
For example, forgetting a semicolon at the end of the a java program, or writing a statement without proper syntax will result in compilation-time error.
class CError{
public static void main(String args[]){
System.out.println(“Compilation Error…..”) }
D:\>javac Cerror.java
CError.java:4: ';' expected } ^
1 error

5. D:\>javac RError.java D:\>java RError
2.Run-time errors:
These are the errors which represent inefficiency of the computer system to execute a particular statement, means computer system can not process.
For example, division by zero error occur at run-time.
class RError{
public static void main(String args[]){
int a=10,b=0;
System.out.println(“a/b: ”+(a/b)); }
D:\>javac RError.java
D:\>java RError
Exception in thread "main" java.lang.ArithmeticException: / by zero
at RError.main(RError.java:4)

6. D:\>javac LError.java D:\>java LError Sum is: 10
3.Logical errors:
These errors depict flaws in the logic of the program.
The programmer might be using a wrong formula or the design of the program itself is wrong.
Logical errors are not detected either by the java compiler or JVM.
The programmer is solely responsible for them.
Logical errors means the logic of the program is wrong and these are identified after observing the output.
class LError{
int sum(int a,int b){
return a-b; }
public static void main(String args[]){
LError le=new LError();
System.out.println("Sum is: "+le.sum(20,10));
D:\>javac LError.java
D:\>java LError
Sum is: 10

7. What is an Exception?
An Exception is an abnormal situation (or) unexpected situation in the normal flow of the program execution.
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.
Because of Exceptions the flow of program execution is getting disturbed so that program execution may continue (or) may not be continued.
Examples
Division by zero
Attempts to use an array index out of bounds.
Number input in wrong format
Unable to write output to file
Missing input file

8. Exception Handling
Whenever an exception is occurred, handling those exceptions called as “Exception Handling”.
Performing action in response to exception
Examples
Exit program (abort)
Ignore exception
Deal with exception and continue
Print error message
Request new data
Retry action

9. Benefits of Exception Handling
It allows us to fix the error.
It prevents program from automatically terminating.
Separates Error-Handling Code from Regular Code.
Conventional programming combines error detection, reporting, handling code with the regular code, leads to confusion. 9

10. Exception-Handling Fundamentals
A Java exception is an object that describes an exceptional condition that has occurred in a piece of code.
When an exceptional condition arises, an object representing that exception is created and thrown in the method that caused the error.
An exception can be caught to handle it or pass it on.
Exceptions can be generated by the Java run-time system, or they can be manually generated by your code. 10

11. (contd..)
Java exception handling is managed by via five keywords: try, catch, throw, throws, and finally.
Program statements to monitor are contained within a try block.
If an exception occurs within the try block, it is thrown.
Code within catch block catch the exception and handle it.
System generated exceptions are automatically thrown by the Java run-time system. 11

12. (contd..) To manually throw an exception, use the keyword throw
Any exception that is thrown out of a method must be specified as such by a throws clause.
Statements contained in the finally block will be executed, regardless of whether or not an exception is raised.
An exception is an error which can be handled. It means an exception happens, the programmer can do something to avoid any harm.
But an error is an error which cannot be handled, it happens and the programmer cannot do anything.

13. Java Exception Class Hierarchy
Object
Error
Throwable
Exception
VirtualMachineError
Ex: Stack overflow
IOException …
RuntimeException
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Unchecked
Checked
NoSuchElementException
FileNotFoundException
EOFException …

14. Exception Types java.lang.Object Throwable Exception Error
Built –in Exception
User-defined Exception
RuntimeException
Checked Exceptions
Checked
Exceptions
Unchecked
Exceptions
(java.lang.Exception)
Un Checked Exceptions
(java.lang.RuntimeException)

15. Exception
Built Exceptions
User Defined Exceptions
Checked Exceptions
Un Checked Exceptions
Built –in Exception
User-defined Exception
Unchecked
Exceptions

16. Exceptions Types
Inside java.lang package, Java defines several exception classes.
All exceptions are subclasses of Exception.
Since java.lang is implicitly imported into all Java programs, most exceptions derived from RuntimeException are automatically available.
Furthermore, they need not be included in any method’s throws list.
These are called unchecked exceptions because the compiler does not check to see if a method handles or throws these exceptions.
Exceptions defined by java.lang those 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.

17. General Form of try-catch block
try{ // block of code to monitor for errors } catch (ExceptionType1 exOb) { // exception handler for ExceptionType1 catch (ExceptionType2 exOb) { // exception handler for ExceptionType2 //Here, ExceptionType is the type of exception that has occurred.

18. Uncaught Exceptions
class Exc0 { public static void main(String args[]) { int d = 0; int a = 42 / d; }
java.lang.ArithmeticException: / by zero
at Exc0.main(Exc0.java:4)
class Exc1 {
static void subroutine() {
int d = 0;
int a = 10 / d; }
public static void main(String args[]) {
Exc1.subroutine();
java.lang.ArithmeticException: / by zero
at Exc1.subroutine(Exc1.java:4)
at Exc1.main(Exc1.java:7)

19. When the Java run-time system detects the attempt to divide by zero, it constructs a new exception object and then throws this exception.
This causes the execution of Exc0 to stop, because once an exception has been thrown, it must be caught by an exception handler and dealt with immediately.
In this example, we haven’t supplied any exception handlers of our own, so the exception is caught by the default handler provided by the Java run-time system.
Any exception that is not caught by your program will ultimately be processed by the default handler.
The default handler displays a string describing the exception, prints a stack trace from the point at which the exception occurred, and terminates the program.

20. Using try and catch
To guard against and handle a run-time error, simply enclose the code that you want to monitor inside a try block.
Immediately following the try block, include a catch clause that specifies the exception type that you wish to catch.
To illustrate how easily this can be done, the following program includes a try block and a catch clause which processes the ArithmeticException generated by the division-by-zero error:

21. Output: Division by zero. After catch statement.
class Exc2 { public static void main(String args[]) { int d, a; try { // monitor a block of code. d = 0; a = 42 / d; System.out.println("This will not be printed."); } catch (ArithmeticException e) { // catch divide-by-zero error System.out.println("Division by zero."); System.out.println("After catch statement.");
Output:
Division by zero.
After catch statement.

22. A try and its catch statement form a unit.
// Handle an exception and move on.
import java.util.Random;
class HandleError {
public static void main(String args[]) {
int a=0, b=0, c=0;
Random r = new Random();
for(int i=0; i<5; i++) {
try {
b = r.nextInt();
c = r.nextInt();
a = / (b/c); }
catch (ArithmeticException e) {
System.out.println("Division by zero.");
a = 0; // set a to zero and continue
System.out.println("b: " + b);
System.out.println("c: " + c);
System.out.println("b/c: " + (b/c));
System.out.println("a: " + a); }
Division by zero. b: c:
b/c: 0
a: 0 b: c:
b/c: -1 a: b: c:
Division by zero. b: c:
b/c: 0
a: 0 b: c:
b/c: 62
a: 199

23. The scope of the catch clause is restricted to those statements specified by the immediately preceding try statement.
A catch statement cannot catch an exception thrown by another try statement (except in the case of nested try statements, described shortly).
Displaying a Description of an Exception: Throwable overrides the toString( ) method (defined by Object) so that it returns a string containing a description of the exception.
You can display this description in a println( ) statement by simply passing the exception as an argument.

24. For example, the catch block in the preceding program can be rewritten like this:
catch (ArithmeticException e) {
System.out.println("Exception: " + e);
a = 0; // set a to zero and continue }
When this version is substituted in the program, and the program is run, each divide-by-zero error displays the following message:
Exception: java.lang.ArithmeticException: / by zero

25. Multiple catch Clauses
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.

26. class MultiCatch { // Demonstrate multiple catch statements
class MultiCatch { // Demonstrate multiple catch statements. public static void main(String args[]) { try { int a = args.length; System.out.println("a = " + a); int b = 42 / 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.");
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

27. time error will result. */ class SuperSubCatch {
/* This program contains an error. A subclass must come before its super class in
a series of catch statements. If not, unreachable code will be created and a compile-
time error will result. */
class SuperSubCatch {
public static void main(String args[]) {
try {
int a = 0;
int b = 42 / a; }
catch(Exception e) {
System.out.println("Generic Exception catch.");
/* This catch is never reached because ArithmeticException is a subclass of Exception. */
catch(ArithmeticException e){ // ERROR - unreachable
System.out.println("This is never reached.");

28. Nested try Statements The try statement can be nested.
That is, a try statement can be inside the block of another try.
Each time a try statement is entered, the context of that exception is pushed on the stack.
If an inner try statement does not have a catch handler for a particular exception, the stack is unwound and the next try statement’s catch handlers are inspected for a match.
This continues until one of the catch statements succeeds, or until all of the nested try statements are exhausted.
If no catch statement matches, then the Java run-time system will handle the exception.

29. class NestTry { // An example of nested try statements
class NestTry { // An example of nested try statements. public static void main(String args[]) { try { int a = args.length; int b = 42 / a; System.out.println("a = " + a); try { // nested try block if(a==1) a = a/(a-a); if(a==2) { int c[] = { 1 }; c[42] = 99; } catch(ArrayIndexOutOfBoundsException e) { System.out.println("Array index out-of-bounds: " + e); catch(ArithmeticException e) { System.out.println("Divide by 0: " + e);
C:\>java NestTry
Divide by 0: java.lang.ArithmeticException: / by zero
C:\>java NestTry One
a = 1
C:\>java NestTry One Two
a = 2
Array index out-of-bounds:
java.lang.ArrayIndexOutOfBoundsException

30. throw
Dealt with catching an exceptions that are thrown by the Java run-time system.
Program can also throw an exception explicitly, using the throw statement.
The general form of throw is shown here:
throw ThrowableInstance;
Here, ThrowableInstance must be an object of type Throwable or a subclass of
Throwable.
There are two ways you can obtain a Throwable object:
using a parameter into a catch clause, or
creating one with the new operator.
The flow of execution stops immediately after the throw statement;
any subsequent statements are not executed.
It finds the catch which is suitable, if it does find a match, control is transferred
to that statement.
If not, then the next enclosing try statement is inspected, and so on.
If no matching catch is found, then the default exception handler halts the
program and prints the stack trace.

31. class ThrowDemo { // Demonstrate throw
class ThrowDemo { // Demonstrate throw. static void demoproc() { try { throw new NullPointerException(); } catch(NullPointerException e) { System.out.println("Caught inside demoproc."); throw e; // rethrow the exception public static void main(String args[]) { demoproc(); System.out.println("Recaught: " + e);

32. throws type method-name(parameter-list) throws exception-list{
If a method is capable of causing an exception that it does not handle, then it must specify some behavior so that callers of the method can guard themselves against that exception.
A throws clause lists the types of exceptions that a method might throw.
You can use throws clause in the method’s declaration.
This is necessary for all exceptions, except those of type Error or RuntimeException, or any of their subclasses.
All other exceptions that a method can throw must be declared in the throws clause.
If they are not, a compile-time error will result.
type method-name(parameter-list) throws exception-list{
// body of method }
Here, exception-list is a comma-separated list of the exceptions that a method can throw.

33. class ThrowsDemo { // This is correct.
// This program contains an error and will not compile. class ThrowsDemo { static void throwOne() { System.out.println("Inside throwOne."); throw new IllegalAccessException("demo"); } public static void main(String args[]) { throwOne();
class ThrowsDemo { // This is correct.
static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo"); }
public static void main(String args[]) {
try {
throwOne();
catch (IllegalAccessException e) {
System.out.println("Caught " + e);
inside throwOne
caught java.lang.IllegalAccessException: demo

34. 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.
This can be useful for closing file handles and freeing up any other resources.
The finally clause is optional.
However, each try statement requires at least one catch or a finally clause.
Any time a method is about to return to the caller from inside a try/catch block, via an uncaught exception or an explicit return statement, the finally clause is also executed just before the method returns.

35. inside procA procA’s finally Exception caught inside procB
class FinallyDemo { static void procA() { try { System.out.println("inside procA"); throw new RuntimeException("demo"); } finally { System.out.println("procA's finally"); static void procB() { System.out.println("inside procB"); return; System.out.println("procB's finally");
static void procC() {
try {
System.out.println("inside procC"); }
finally {
System.out.println("procC's finally");
public static void main(String args[]) {
procA();
catch (Exception e) {
System.out.println("Exception caught");
procB();
procC();
inside procA
procA’s finally
Exception caught
inside procB
procB’s finally
inside procC
procC’s finally

36. Java’s Built-in Exceptions
Inside java.lang package, Java defines several exception classes.
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.
Furthermore, they need not be included in any method’s throws list.
These are called unchecked exceptions because the compiler does not check to see if a method handles or throws these exceptions.
Exceptions defined by java.lang those 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.

37. Java’s Unchecked RuntimeException Subclasses

39. Java’s Checked Exceptions Defined in java.lang

40. Creating Your Own Exception Subclasses
You can create your own exception types to handle situations specific to your applications.
To do this, define a subclass of Exception (which is, of course, a subclass
of Throwable).
The Exception class does not define any methods of its own. It does, of
course, inherit those methods provided by Throwable.
Thus, all exceptions, including those that you create, have the methods
defined by Throwable available to them.
You may also wish to override one or more of these methods in exception
classes that you create.

41. Every user defined Exception last word must be Exception since every exception is a class.
E.g., AgeException
MarksOutOfBoundsException
Whenever a superclass is Exception then that type of User defined Exception is Checked Exception.
E.g., class XYZException extends Exception{
// body of the class }
If the superclass is RuntimeException then that user defined Exception is unchecked Exception.
E.g., class XYZException extends RunTimeException{
//body of the class

42. Called compute(1) Normal exit Called compute(20)
class MyException extends Exception { // This program creates a custom exception type. private int detail; MyException(int a) { detail = a; } public String toString() { return "MyException[" + detail + "]"; class ExceptionDemo { 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"); public static void main(String args[]) { try { compute(1); compute(20); catch (MyException e) { System.out.println("Caught " + e);
Called compute(1)
Normal exit
Called compute(20)
Caught MyException[20]

43. The Methods Defined by Throwable

45. Single Tasking A task means doing some calculation, processing, etc.
Generally, a task involves execution of a group of statements, for example executing a program.
In single tasking environment, only one task is given to processor at a time.
This means we are wasting a lot of processor time and processor has to sit idle without any job for a long time.
This is the drawback in single tasking.

46. Multi Tasking
In multi tasking environment, several tasks are given to processor at a time.
Multi tasking can be done by scheduling algorithms.
In this, most of the processor time is getting engaged and it is not sitting idle.
Uses:
To use the processor time in a better way.
To achieve good performance.
To make processor not to sit idle for long time.

47. Concepts of Multithreading
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.
There are two distinct types of multitasking:
 Process-based and
 Thread-based.

48. Process-based vs. Thread-based
1. A process is a program that is executing.
Processes are heavy weight tasks that require their own separate address spaces.
3. Interprocess communication is expensive and limited.
4. Context switching from one process to another is also costly.
1. A thread is a separate path of execution.
2. Threads are light weight process. They share the same address space and cooperatively share the same heavy weight process.
3. Inter thread communication is inexpensive.
4. Context switching from one thread to the next is low cost.

49. Thread Life Cycle Thread can exist in several states: New State:
After the creations of Thread instance the thread is in this state but before the start() method invocation. At this point, the thread is considered not alive.
Ready (Runnable) State:
A thread start its life from Runnable state.
A thread first enters runnable state after the invoking of start() method but a thread can return to this state after either running, waiting, sleeping or coming back from blocked state also.
On this state a thread is waiting for a turn on the processor.

50. Thread Life Cycle (Contd..)
Running State:
A thread is in running state that means the thread is currently executing.
There are several ways to enter in Runnable state but there is only one way to enter in Running state: the scheduler select a thread from runnable pool.
Blocked State:
A thread can enter in this state because of waiting the resources that are
hold by another thread.
Dead State:
A thread can be considered dead when its run() method completes.
If any thread comes on this state that means it cannot ever run again.

51. start()
Schedular dispatch

52. Thread Life Cycle (Contd..)
Starting from the birth of a thread, till its death, a thread exists in different states which are collectively called “Thread Life Cycle”.
A thread will be born when it is created using Thread class as:
Thread obj=new Thread();
A thread goes into runnable state when start() method is applied on it.
That is void start() method is used to call the public void run() method.
From runnable state, a thread may get into not-runnable state, when sleep() or wait() or suspend() methods act on it.
notify() and resume() methods are used to restart waiting and suspended threads respectively.

53. Thread Life Cycle (Contd..)
yield() method causes the currently executing thread object to temporarily pause and allow other threads to execute.
ThreadDeath class will be invoked whenever the stop() method is called.

54. Single Thread System Vs. Multi Thread System
Single-thread System use an approach called an event loop with pooling.
A Single thread of control runs in an infinite loop, pooling a single event queue to decide what to do next.
In a single-threaded environment, when a thread blocks because it is waiting for some resources the entire program stops running.
The advantage of multi threading is that the main loop, pooling mechanism is eliminated.
Multithreading enables us to write very efficient programs that make maximum use of CPU. Because ideal time can be kept to a minimum.
One thread can pause with out stopping other parts of the program. Java supports multi threading.

55. Thread Class Constructors
1. Thread() Allocates a new Thread Object.
E.g., Thread t=new Thread();
2. Thread(String name) Allocates a new thread.
E.g., Thread t= new Thread( “FirstChild”);
3. Thread(Runnable target) Allocates a new Thread object.
E.g., SecondThread st= new SecondThread();
Thread t=new Thread(st);
4. Thread(Runnable target, String name)
           Allocates a new Thread object.
E.g., SecondThread st=new SecondThread();
Thread t= new Thread(st, ”Secondchild”);

56. 5. Thread(ThreadGroup group, String name) Allocates a new Thread object. E.g., ThreadGroup tg=new ThreadGroup(“Image Group”); Thread t1= new Thread(tg,”frist child”); Thread t2= new Thread(tg,”second child”); 6. Thread(ThreadGroup group, Runnable target) Allocates a new thread object. Where group is the thread group, target is the object whose run method is called. SecondThread st= new SecondThread(); Thread t1= new Thread(tg, st); Thread t2= new Thread(tg, st);

57. Thread Class Methods
public final boolean isAlive()      Tests if this thread is alive.
boolean isDaemon()    Tests if this thread is a daemon thread.
public final void join() throws InterruptedException
Waits for this thread to die.
public final String getName()  Returns this thread's name.  
public static void sleep(long millis) throws InterruptedException
Causes the currently executing thread to sleep for the specified number of milliseconds.

58. Thread Class Methods
public static void sleep(long millis, int nanos) throws InterruptedException
Causes the currently executing thread to sleep for the specified number of milliseconds plus the specified number of nanoseconds.
public static void yield()
Causes the currently executing thread object to temporarily pause and allow other threads to execute.
public static Thread currentThread()
Returns a reference to the currently executing thread object.
public final void setName(String name)
Changes the name of this thread to be equal to the argument name.

59. Thread Class Methods
public final int getPriority()     Returns this thread's priority.
public final void setPriority(int newPriority)    
Changes the priority of this thread.
NOTE 1: The value of newPriority must be within the range MIN_PRIORITY and MAX_PRIORITY (1 and 10), for default priority, specify NORM_PRIORITY, which is currently 5. These priorities are defined as final variables within Thread.
NOTE 2:A thread’s priority is used to decide when to switch from one running thread to the next. This is called a context switch.
A thread can voluntarily relinquish control.
A thread can be preempted by a higher-priority thread.
public final ThreadGroup getThreadGroup()
Returns the thread group to which this thread belongs. This method returns null if this thread has died.

60. main Thread
In every java program, there is always a thread running internally. This thread is used by JVM to execute the program statements.
When a Java program starts up, one thread begins running immediately.
This is usually called the main thread of your program, because it is the one that is executed when your program begins.
The main thread is important for two reasons:
It is the thread from which other “child” threads will be spawned.
Often it must be the last thread to finish execution because it performs various shutdown actions.

61. Let us find the current thread….. Current Thread= Thread[main,5,main]
class CurrentThreadDemo{ public static void main(String sree[]){ System.out.println(“Let us find the current thread…..”); Thread t = Thread.currentThread(); System.out.println(“Current Thread= ”+t); System.out.println(“It’s name= ”+t.getName()); }
Let us find the current thread…..
Current Thread= Thread[main,5,main]
It’s name= main
Which thread always runs in a java program by default?
main thread

62. Creating a Thread and Running it
Create a class that extends Thread class or implements Runnable interface. Both are found in java.lang package.
class Myclass1 extends Thread{ class Myclass2 implements Runnable{
//methods (or) //methods
//statements; //statements;
} }
Write a run() method in a class.
Myclass1 obj=new Myclass1(); Myclass2 obj=new Myclass2();
obj.start(); Thread t = new Thread(obj);
t.start();

63. class One extends Thread{ //Creating thread using “extends Thread” public void run(){ for(int i=0;i<5;i++){ System.out.println("i value: "+i); } class ThreadDemo{ public static void main(String sree[]){ One o=new One(); o.start();
i value: 0
i value: 1
i value: 2
i value: 3
i value: 4

64. //By extending Thread class
interface Runnable{
public void run(); }
//By extending Thread class
class MyThread extends Thread{
public void run(){
Syste.out.println("MyThread"); } 3
class Thread implements Runnable{
private Runnable target;
private boolean started;
private ThreadGroup group;
public synchronized void start() {
if (started)
throw new IllegalThreadStateException();
started = true;
group.add(this);
start0(); }
private native void start0();
public void run() {
if (target != null) {
target.run();
class Demo{
public static void main(String args[]){
MyThread mt = new MyThread();
mt.start(); } 1 2 x

65. class One implements Runnable{ //Creating thread using “Runnable interface” public void run(){ for(int i=0;i<5;i++){ System.out.println("i value: "+i); } class RunnableDemo{ public static void main(String sree[]){ One o=new One(); Thread t1=new Thread(o); t1.start();
i value: 0
i value: 1
i value: 2
i value: 3
i value: 4

66. //By implementing Runnable interface
interface Runnable{
public void run(); }
//By implementing Runnable interface
class MyThread implements Runnable{
public void run(){
Syste.out.println("MyThread"); } 4
class Thread implements Runnable{
private Runnable target;
private boolean started;
private ThreadGroup group;
public synchronized void start() {
if (started)
throw new IllegalThreadStateException();
started = true;
group.add(this);
start0(); }
private native void start0();
public void run() {
if (target != null) {
target.run();
class Demo{
public static void main(String args[]){
MyThread mt = new MyThread();
Thread t=new Thread(mt);
t.start(); } 1 2 3
Here target = mt
mt != null
mt.run();

67. //Multi thread example using “Thread Class”
class One extends Thread{ public void run(){ for(int i=0;i<5;i++){ System.out.println("i value: "+i); }
class ThreadDemo{
public static void main(String sree[]){
One t1=new One();
One t2=new One();
One t3=new One();
t1.start();
t2.start();
t3.start(); }
D:\>java ThreadDemo
i value: 0
i value: 1
i value: 2
i value: 3
i value: 4

68. //Another Multithreading example using “Thread Class”
class MultiThreadDemo{
public static void main(String sree[]){
One o=new One();
o.start();
Two t=new Two();
t.start(); }
class One extends Thread{
public void run(){
for(int i=0;i<5;i++){
System.out.println("i value: "+i); }
class Two extends Thread{
for(int j=10;j>4;j--){
System.out.println(“j value: "+j);
i value: 0
j value: 5
i value: 1
j value: 4
i value: 2
i value: 0
i value: 1
i value: 2
j value: 5
j value: 4

69. //Multi thread example using “Runnable interface”
class One implements Runnable{ public void run(){ for(int i=0;i<4;i++){ System.out.println("i value: "+i); }
class RunnableDemo{
public static void main(String sree[]){
One o=new One();
Thread t1=new Thread(o);
t1.start();
Thread t2=new Thread(o);
t2.start(); }
D:\>java RunnableDemo
i value: 0
i value: 1
i value: 2
i value: 3

70. //Another Multithreading example using “Runnable interface”
class MultiThreadDemo{
public static void main(String sree[]){
One o=new One();
Thread t1=new Thread(o);
t1.start();
Two t=new Two();
Thread t2=new Thread(t);
t2.start(); }
class One implements Runnable{
public void run(){
for(int i=0;i<5;i++){
System.out.println("i value: "+i); }
class Two implements Runnable{
for(int j=10;j>4;j--){
System.out.println(“j value: "+j);
i value: 0
j value: 5
i value: 1
j value: 4
i value: 2
i value: 0
i value: 1
i value: 2
j value: 5
j value: 4

71. Difference between “extends Thread” and “implements Runnable” :
Both are functionally same.
But when we write extends Thread, there is no scope to extend another class, as multiple inheritance is not supported in java.
class MyClass extends Thread, AnotherClass //invalid
If we write implements Runnable, then still there is scope to extend another class.
class MyClass extends AnotherClass implements Runnable //valid
This is definitely advantageous when the programmer wants to use threads and also wants to access the features of another class.

72. //Program to display the name of the threads
class Demo extends Thread{
public void run(){
System.out.println(getName()); }
class ThreadInfo{
public static void main(String args[]){
Thread t=Thread.currentThread();
System.out.println(“Main Thread is: "+t.getName());
Demo obj1=new Demo();
Demo obj2=new Demo();
Demo obj3=new Demo();
obj1.start();
obj2.start();
obj3.start(); }
Main Thread is: main
Thread-0
Thread-1
Thread-2

73. Thread-0 :0 Thread-0 :1 Thread-0 :2 Thread-2 :0 Thread-2 :1
class One extends Thread{ //Program to demonstrate Thread Priorities
public void run(){
for(int i=0;i<3;i++)
System.out.println(getName()+" :"+i); }
class ThreadPriority1{
public static void main(String args[]){
One t1=new One();
One t2=new One();
One t3=new One();
t1.setPriority(Thread.MAX_PRIORITY);
t2.setPriority(Thread.MIN_PRIORITY); t3.setPriority(Thread.NORM_PRIORITY);
t1.start();
t2.start();
t3.start(); }
Thread-0 :0
Thread-0 :1
Thread-0 :2
Thread-2 :0
Thread-2 :1
Thread-2 :2
Thread-1 :0
Thread-1 :1
Thread-1 :2

74. join() Often we want the main thread to finish last.
It does not happen automatically.
We uses join( ) to ensure that the main thread is the last to stop.
final void join( ) throws InterruptedException
This method waits until the thread on which it is called terminates.

75. Creates Thread t1
Main thread
Calls t1.join()
Main thread suspends
execution
Thread t1 terminates
Main thread resumes

76. // Using join() to wait for threads to finish.
class NewThread implements Runnable {
String name; // name of thread
Thread t;
NewThread(String threadname) {
name = threadname;
t = new Thread(this, name);
System.out.println("New thread: " + t);
t.start(); // Start the thread }
// This is the entry point for thread.
public void run() {
try {
for(int i = 5; i > 0; i--) {
System.out.println(name + ": " + i);
Thread.sleep(1000);
catch (InterruptedException e) {
System.out.println(name + " interrupted.");
System.out.println(name + " exiting.");
class DemoJoin {
public static void main(String args[]) {
NewThread ob1 = new NewThread("One");
NewThread ob2 = new NewThread("Two");
NewThread ob3 = new NewThread("Three");
System.out.println("Thread One is alive: "+ ob1.t.isAlive());
System.out.println("Thread Two is alive: "+ ob2.t.isAlive());
System.out.println("Thread Three is alive: "+ ob3.t.isAlive());
// wait for threads to finish
try {
System.out.println("Waiting for threads to finish.");
ob1.t.join();
ob2.t.join();
ob3.t.join(); }
catch (InterruptedException e) {
System.out.println("Main thread Interrupted");
System.out.println("Main thread exiting.");

77. 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.
Synchronization is a mechanism to ensure that a resource is shared by one thread at a time.
Synchronization can be achieved in two ways:
1) Method level synchronization or synchronized method
2) Block level synchronization or synchronized statements
In method level synchronization the total data of method is sensitive.
Where as in block level synchronization part of the method is sensitive.

78. A monitor is an object that is used as a mutually exclusive lock.
Only one thread can own a monitor at a given time.
When a thread enters into the monitor all other threads attempting to enter the locked monitor will be suspended until the first thread exits the monitor.

79. Seat reservation database
Ex:-
Booking agent 1
Booking agent 2
…..
Booking agent n
Check availability
Reserve seat
Check
availability
Reserve seat
Check availability
Reserve seat
Seat reservation database 79

80. [One[Three[Two] ] //Without Synchronization class Callme{
void call(String msg){
System.out.print("["+msg);
try {
Thread.sleep(1000); }
catch(InterruptedException e) {
System.out.println("Interrupted");
System.out.println("]");
class Caller extends Thread{
Callme target;
String msg;
Thread t;
Caller(Callme tar,String str){
target=tar;
msg=str;
t = new Thread(this);
t.start(); }
public void run(){
target.call(msg);
class SyncDemo{
public static void main(String args[]){
Callme cr=new Callme();
Caller t1=new Caller(cr,"One");
Caller t2=new Caller(cr,"Two");
Caller t3=new Caller(cr,"Three");
[One[Three[Two] ]

81. //Synchronization Example class Callme{ public synchronized void call(String msg){ System.out.print("["+msg); try { Thread.sleep(1000); } catch(InterruptedException e) { System.out.println("Interrupted"); System.out.println("]");
class Caller extends Thread{
Callme target;
String msg;
Thread t;
Caller(Callme tar,String str){
target=tar;
msg=str;
t = new Thread(this);
t.start(); }
public void run(){
target.call(msg);
class SyncDemo{
public static void main(String args[]){
Callme cr=new Callme();
Caller t1=new Caller(cr,"One");
Caller t2=new Caller(cr,"Two");
Caller t3=new Caller(cr,"Three");
[One]
[Three]
[Two]

82. first second i value is:0 i value is:1 i value is:2
class BlockSyn implements Runnable{ /Example for block level synchronization public void run(){ display(); } public void display(){ System.out.println(Thread.currentThread().getName()); synchronized(this){ for(int i=0;i<3;i++){ try{ Thread.sleep(1000); System.out.println("i value is:"+i); catch(Exception e){ System.out.println(e);
first
second
i value is:0
i value is:1
i value is:2
public static void main(String args[]){
BlockSyn s= new BlockSyn();
Thread t1= new Thread(s,"first");
Thread t2= new Thread(s,"second");
t1.start();
t2.start(); }