1. Threads Chapter 25

2. A PC actually does one task at a time but threads are activities that seem to all run at once Time slicing very tiny amounts of time are used for each activity (thread) then the next one gets a share of the PC’s CPU time. The threads run concurrently

3. Multitasking Things seem to all run at once Share a task Each thread doing one part of a whole task Independent threads: Program: “Bouncer” with old class “Ball” Uses a thread on example “Bounce2” Two threads running concurrently Dying and Killing: “Bounce3” example stop it with [button] at out command

4. The State of a thread(s) new (newly created) running (executing instructions) runnable (currently another thread is running but ours is ready to go) blocked (may be sleeping) dead (exited normally)

5. Scheduling thread priorities and yield(ing) … yield( ); allows another thread to run (our thread is not a hog of resources) The PC’s schedule actually decides what runs and how CPU time is shared (preemptable vs. nonpreemptiable) Mutual exclusion and Synchronization Only one thread allowed into a method that is “synchronized” may cause a bit of a bottleneck so keep “synchronized methods small

6. Program “TwoCounters” vs. “SharedNumber” Public void synchronized increment ( ) { Thread interaction – want and notify both threads want same resources (CRT) at same time Program “Poets” example of both accessing CRT writing lines mixed… A solution “Mowing” requires a request to use screen See program “ScreenController” – each gets five sconds “Clock” digital screen used, displays hours, minutes wakes after 60 second ticks

7. See program “Minute” and “TickTock” Class “Café” - “You want fries with that”? Buttons for placing order and timing cooking – each part of the total task “food cooked and to customer” Class “Queue” Possible problems:  Interruptions (happen in real world too)  Deadlock See “philosophers” I heard it with 5 golden forks – same problem – they need two forks to eat and have to cooperate (five forks and 5 philosophers) Mean philosophers could starve the 5 th out if they teamed up

8. Deadlock: “two threads waiting for something that the other thread is holding” Common causes: Ignore each other (bounce balls) Share data (..counting threads) Collaborate (café) Compete for resources (poets)