Threads in Java

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Threads in Java

• By William Francis

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Concurrency

• Concurrency is defined as software that can do
  multiple things at once
• Threads and processes are two examples of
  concurrency, we will be talking about threads

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What are they?

• Threads are useful for running multiple sections
  of code simultaneously, for example, sorting an
  array while printing time out to the screen
• Threads become more important in advanced
  applications, where there is a possibility for
  multiple actions at once

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Date Example

• http//www.realapplets.com/tutorial/ThreadExample.
  html
• Note, you need an html with this code
• lthtmlgt
• ltbodygt
• ltapplet codeThreadExample height 300 width
  300gt lt/bodygtlt/htmlgt

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How do they work?

• Quite simply, the CPU allots a very brief slot of
  time to each thread, and it only appears to be
  simultaneous to us, the code within the thread is
  executed as if it was the only process running

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Why are they useful?

• Think of a standard text editor, in your text
  editor the software must be capable of listening
  to keyboard input, while updating to the screen
  at the same time!
• If your program could only do one thing at a
  time, your program would be constantly
  interrupted while trying to update to the screen,
  and nothing would be accomplished

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Text Editor contd

• Without using threads
• Things would be constantly stopped by events
• The user must wait until the computer has updated
  to the screen character, by character

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Why havent I seen these before?

• Actually, you have!
• Every single program has at least one thread, a
  main thread, even the most basic of programs
  has at least one thread
• Thread.sleep(1000) is a command that pauses the
  current thread

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How do I make them?

• Threads are most useful when they are actually
  separate classes
• Use
• threadName.start()
• This will begin the run() method in the
  threadName class

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Basic Commands

• Thread.start()
• Public void run()
• Thread.sleep(int)
• Thread.getName
• Public static void yield()
• Public void destroy() especially important for
  threads running endlessly!

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See ThreeThread example

• This thread example can be accessed by going to
  http//www.science.uva.nl/ict/ossdocs/java/tutoria
  l/java/threads/simple.html
• Download ThreeThreadsTest.java and
  SimpleThread.java

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How do you do that?

• IN A SEPARATE CLASS
• public class example implements Runnable
• public void run()
• System.out.println(hello)
• Output hello by typing
• (new Thread(new example())).start()
• In your other program

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Explained

• The previous example showed a thread as an object
• The
• (new Thread(new example())).start()
• Starts the run method in that class IF Thread has
  been extended
• The code inside the run method is what is
  executed when .start() is called

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Thats too complicated

• It doesnt have to be a separate class
• Implement Runnable
• public class thread_example Implements Runnable
• Include a run method without parameters
• public void run()
• System.out.println(hello)

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Lets Make Our Own!

• Lets try a simple Thread example

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Isnt there an easier way?

• Yes, but
• causes problems
• Limits the programmer
• since youre copying and pasting anyways

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What can happen to a thread?

• Threads can be
• interrupted
• paused
• made high/low priority
• blocked from certain methods
• many other features to prevent crashes
• Wait for other methods

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Links

• http//webcem01.cem.itesm.mx8005/web/java/tutoria
  l/essential/concurrency/index.html
• http//www.cs.clemson.edu/cs428/resources/java/tu
  torial/JTThreadEx.html
• http//www.science.uva.nl/ict/ossdocs/java/tutoria
  l/java/threads/simple.html