CIS219

1
CIS219

• Last lecture
• -
• 16 December 2005

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Threads

• What are they?
• How do we make them in Java?
• Interface Runnable
• Class Thread
• Controlling threads?
• Start
• Stop
• Sleep

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What are threads in Java

• If you need several things happening at once
• Example Listeners
• Threads are processes
• Think about Windows
• You can play music and write Java at the same
  time

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How to make a Object a Thread?

• There are two basic ways
• With the Interface Runnable
• Class Thread
• Since Class Thread needs to be extended and
  inheritance via extension is a scarce resource we
  prefer the use of Interface Runnable
• However to extend a class makes some thing
  easier.
• Example Sleep function

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Class Thread

• Check API !!
• Class Rect extends Thread
• // This is all you need to give a class the
  ability to become a thread
• If we read the API we see
• public class Thread extends Object implements
  Runnable
• So what happens when we make a thread?

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Thats what happens

• class ThreadRect extends Thread
• public void run()
• ....            
• Everything we want to be done in the thread we
  put into the run() method
• It will get called when the Thread gets started
• We will learn later how to start a thread

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So lets see how to start a Thread

• ThreadRect p new ThreadRect()
• p.start()
• Now we started the Thread
• Everything in the run method will get executed

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Runnable interface

• Generally this method of making threads is used
  since we can only extend from one class but
  implement many
• Trade of
• We cannot use all the methods from Class thread
  as easily later more
• Let see how to make Threads in this way

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Using Runnable

• Again check the API !!
• There we see
• public interface Runnable
• It has only one method
•  void run()
• // since it is not specified, its public
• // since all methods in an interface are abstract
  and not
• // implemented, we need to do that and implement
  them
• So our class would look like this

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Using Runnable

• class ThreadRect implements Runnable
• public void run()
• ....            
• So how can we start this Thread?

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One last thing

• Boolean keepAlive true
• ...
• public void run()
• while(keepAlive)
• // keep on running
• ...
• Note
• We need to hava condition on what we can
  terminate the thread and that keeps the thread
  alive or running. So a boolean is a neat
  solution for this problem.

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Sarting A Thread II

• ThreadRect p new ThreadRect()
• new Thread(p).start()
• Note that essentially we also start a thread here
  but that we loose the reference to the object
  as soon as it has been created
• This makes it hard to apply methods such as sleep
  from the Thread class a a later point