1. Animate objects and threads
Outline:
What is an animate object?
What is a Thread?
How are animate objects and Threads related?
Apply these ideas:
Paper-and-pencil exercise
Threads program example
Fundamentals of Software Development I
Lecture 20

2. Animate objects Consider the following objects… Counter Timer
Has state and behavior
Doesn’t do anything unless someone asks it to do so
using its increment or reset method
Timer
Has state and behavior, much like a Counter
Has a traditional, passive reset method
But a Timer acts by itself!
Its increment method is called by the Timer itself on a regular basis.
Animate object
Fundamentals of Software Development I
Lecture 20

3. Threads In Day 1 we saw two prerequisites for computation:
Instructions for the computation must be present
The instructions must be executed
In Java, objects that execute instructions are called Threads
Each Thread follows the instructions it is given
No object can act except a Thread
There can be many Threads executing concurrently
How can an ordinary computer execute many Threads “at the same time”?
Answer on next slide
Fundamentals of Software Development I
Lecture 20

4. Concurrent Threads A computer repeatedly:
Called CPU scheduling
Round robin
First-in, First-Out (FIFO)
Shortest job first
etc.
A computer repeatedly:
Picks a Thread
Restores the system to the state when the Thread last ran
Which statement was it doing?
What values were bound to the objects/variables at that time?
Runs the Thread for a little while
Saves the state of the system
The Threads appear to run “at the same time”
if the time step is small enough
Fundamentals of Software Development I
Lecture 20

5. Runnable: connecting Animate objects and Threads
What are they?
A Thread is, by definition, an instruction-follower
An animate object is, by definition, an object with its own Thread (i.e., its own instruction-follower)
What do they promise?
The animate object promises to supply instructions
The Thread promises to execute them
How are they connected?
through the Runnable interface (see next slide)
Fundamentals of Software Development I
Lecture 20

6. Using the Runnable interface
An animate object typically:
Implements the Runnable interface
Runnable specifies a run method
Constructs and starts a Thread
In its constructor or soon thereafter
Exercise: Write a single expression that constructs a Thread and asks it to start
The constructor needs an argument – the object doing the constructing
Answer: (new Thread(this)).start()
Supplies a method called run: public void run()
run often contains a loop that goes for a long time
The Thread:
executes the animate object’s run method
The animate object promises to supply instructions
The Thread promises to execute those instructions
Fundamentals of Software Development I
Lecture 20

7. Demo: Threads example I’ll show you the Threads project Note:
There is a link to it on Angel (under Exercise link in the Schedule page)
Note:
Timer class: an animate object!
Implements Runnable
Constructs and starts a new Thread
run method
Creation of Timers in Threads constructor
Interaction of Timers with Panels
We’ll use this same example later to demo a GUI (graphical user interface). Ignore details of panels, etc. for now
Interaction of Reset button and Timers
Independent objects!
What does (purposely) NOT work quite right? That is, what needs to be synchronized?
Now do the Exercise on Animate Objects and Threads (associated item on today’s schedule)
Fundamentals of Software Development I
Lecture 20

8. Threads Threads There can be many Threads executing concurrently
Each Thread follows the instructions it is given
No object can act except a Thread
executes the animate object’s run method
There can be many Threads executing concurrently
Fundamentals of Software Development I
Lecture 20