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
1
Animate objects
Consider the following objects…
• Counter
– Has state and behavior
– Doesn’t do anything unless someone asks it to do so
• using its increment or reset method
• Timer
Animate object
– 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.
Fundamentals of Software
Development I
Lecture 20
2
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
3
Concurrent Threads
• 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
Called
CPU scheduling
Round robin
First-in, First-Out
(FIFO)
Shortest job first
etc.
• The Threads appear to run “at the same time”
– if the time step is small enough
Fundamentals of Software
Development I
Lecture 20
4
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
5
Using the Runnable interface
• An animate object typically:
– Implements the Runnable interface
• Runnable specifies a run method
The animate object
promises to supply
instructions
– 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
Fundamentals of Software
Development I
Lecture 20
The Thread
promises to
execute those
instructions
6
Demo: Threads example
• I’ll show you the Threads project
– 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?
Fundamentals of Software
Development I
Now do the Exercise on Animate Objects and
Threads (associated item on today’s schedule)
Lecture 20
7
Threads
• Threads
– 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
8