The document discusses multi-threading in Java. It defines a thread as a portion of a program that can run independently and concurrently with other portions. Multi-threading allows a program to manage multiple tasks simultaneously. The document outlines thread states like new, runnable, waiting, timed waiting, blocked, and dead. It also discusses advantages like easier programming and better performance, and disadvantages like potential deadlocks. Finally, it explains two ways to create new threads by extending the Thread class or implementing the Runnable interface.

1. CS3135/CS2135
Object Oriented Programming (Java)
BSCS-3 / MCS-3
Lecture # 20
Multi Threading

2. Concurrency
Thread
• A portion of a program that can run independently of and concurrently with other portions of the
program.
• Multithreading is the ability to do multiple things at once within the same application.
• Individual and separate unit of execution that is part of a process.
• Multiple threads can work together to accomplish a common goal
Multi Threading
• It is easy to confuse multithreading with multitasking or multiprogramming, which are somewhat
different ideas.
• Multithreading is the ability of a program or an operating system process to manage its use by more
than one user at a time and to even manage multiple requests by the same user without having
multiple copies of the program running in the computer.
• Java includes built-in support for threading.
Instructor: Tanzila Kehkashan
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3. MultiThreading
Advantages
• Easier to program. (1 thread per task)
• Can provide better performance
• Thread only runs when needed
• No polling to decide what to do
• Multiple threads can share resources
• Utilize multiple processors if available
Disadvantages
• Multiple threads can lead to deadlock
• Overhead of switching between threads
Instructor: Tanzila Kehkashan
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4. Thread States: Life Cycle of Thread
New
• Life cycle started. Just created but not started execution.
Runnable
• Created, started, and executing.
Waiting
• Sometimes a runnable thread transitions to waiting state while it
waits for another thread to perform a task. A waiting thread
transitions back to the runnable state only when another thread
notifies it to continue executing.
• Cannot use processor.
Timed Waiting
• Also called Sleep state.
• A runnable thread can enter timed waiting state for specified interval of time (sleep interval) . It transitions back to
runnable state when that time interval expires.
• Cannot use processor.
Blocked
• In block state, a runnable thread is unable to run because it is waiting for some event to occur. (for example I/O)
Dead / Terminated
• Runnable thread has successfully completed its task or otherwise terminates perhaps due to error.
Instructor: Tanzila Kehkashan
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5. Multi-Threading
• There are two ways to create a new thread of execution.
• One is to declare a class to be a subclass of Thread.
• This subclass should override the run method of class Thread.
• An instance of the subclass can then be allocated and started.
• For example;
class myClass extends Thread
{
long minPrime;
myClass(long minPrime)
{
this.minPrime = minPrime;
}
public void run()
{
// compute primes larger than minPrime
. . .
}
}
myClass p = new myClass(143);
p.start();
Instructor: Tanzila Kehkashan
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6. Multi-Threading
• The other way is to declare a class that implements the Runnable interface.
• That class then implements the run method.
• An instance of the class can then be allocated, passed as an argument when creating Thread, and
started.
• For example; class myClass implements Runnable
{
long minPrime;
PrimeRun(long minPrime)
{
this.minPrime = minPrime;
}
public void run()
{
// compute primes larger than minPrime
. . .
}
}
myClass obj = new myClass(143);
Thread t=new Thread(obj);
t.start(); //new Thread(obj).start();
Instructor: Tanzila Kehkashan
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7. Multi-Threading
java.lang.Thread
• Thread()
• Thread(String) //name of thread
• long getId() //returns ID of thread
• String getName() //name of thread
• void run()
• static void sleep(long) //milliseconds //temporarily cease execution
• void start() //calls run method
Instructor: Tanzila Kehkashan
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8. Instructor: Tanzila Kehkashan
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9. Instructor: Tanzila Kehkashan
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