Practical OOP using Java

Training @ Basis

Faqueer Tanvir Ahmed, 08 Jan 2012

Course objectives• Java programming language basics• Object Oriented Programming concepts and using

OOP in real-life applications• Developing desktop (GUI) applications using Java

SE• Introduction to relational database concepts• Introduction to web application programming

using Java EE

What is a computer?

Input Output

Process

Concurrency• Systems can do more than one thing at a time.– Playing music while displaying video.– Respond to keyboard and mouse while printing a

document.

• Level of multi tasking – Processes: Self contained execution environment.

Use IPC to inter-process communication

Concurrency (Continued)• Level of multi tasking (continued)– Threads: • Light weigh process• Takes less resources • Use shared resources• Context switching inexpensive• Inter-thread communication usually inexpensive• Every process has at least one process

Concurrency in JAVA• Multi-thread support in java is an essential

feature• System thread: By JVM to mange memory,

signalling etc• Typical java program has main thread (main

function)• Main thread can create/manage many more

thread

Concurrency in JAVA (Continued)• Defining and Starting a Thread– Provide a Runnable object by implementing the

Runnable interface.– Subclass Thread. The Thread class itself

implements Runnable.– Invoke Thread.start() in order to start the new

thread.

Concurrency in JAVA (Continued)• Thread objects– Each thread is associated with an instance of the

class Thread.

• Two basic strategies for using Thread– To directly control thread creation and

management– To abstract thread management from the rest of

your application via executor

Exercise 01: Write your first multi-threaded Java application.

Concurrency in JAVA (Continued)• Pausing a Thread– Invoke by Thread.sleep(miliseconds) to suspend

execution for a specified period.

• Interrupts– Stop what is doing and do something else (mostly

exit)– Invoked by thread’s intrrupt() method – Caught by InterruptedException or interrupted

method

Concurrency in JAVA (Continued)• Thread join() method– join method allows one thread to wait for the

completion of another– This causes current method to pause execution– Starts again once target thread terminates– Subject to InterruptedException

Exercise 02: Write sample program to demonstrate sleep and join

Synchronization• Two issues in using shared resources:– Thread Interference (c++/c--)

1. Thread A: Retrieve c.2. Thread B: Retrieve c.3. Thread A: Increment retrieved value; result is 1.4. Thread B: Decrement retrieved value; result is -1.5. Thread A: Store result in c; c is now 1.6. Thread B: Store result in c; c is now -1.

Synchronization– Memory Consistency Errors• int counter = 0; // Shared variable• counter++; // incremented by thread A• System.out.println(counter); // sometime printed by

thread B

Synchronization…• Use keyword synchronized - A thread cannot

acquire a lock owned by another thread.– Synchronized methods • Lock using an object

– Synchronized blocks• Lock using an object• Lock using a class

Synchronization…• Use synchronized - A thread cannot acquire a

lock owned by another thread.– Synchronized methods• lock using an object

– Synchronized blocks• Lock using an object• Lock using a class

Synchronization…• Reentrant Synchronization - A thread can

acquire a lock that it already owns.• Atomic access– Read and write reference variable– Most primitive data type except long and double– Reads and write to all variable declared as volatile– Methods available in high level concurrency objects

Deadlock• Deadlock describes a situation where two or

more threads are blocked forever, waiting for each other.

• Guarded Blocks helps to coordinate actions to avoid deadlock

Immutable Objects• An object is considered immutable if its state

cannot change after it is constructed.– Don’t provide setter/getter and Make fields

private and final– Don’t allow sub class to overwrite method, make

class final. Use instance to create new object.– Don’t allow object to be changed or store

reference to external

High Level Concurrency Object• Lock objects: Supports locking idioms• Executors: For lunching and managing thread.• Concurrent Collections• Atomic variables• ThraedLocal Random (JDK 7)

Exercise 03: Write a simple ticketing system that can sale one

ticket at a time via 5 different sales counter

Thread Priority & Scheduler• The thread scheduler selects the highest

priority thread in the Ready-to-run and gives it CPU control

• Following are the priority levels• MIN_PRIORITY = 1• NORM_PRIORITY = 5 (default)• MAX_PRIORITY = 10

Thread Priority & Scheduler• Preemptive Scheduling:

– If a thread with a higher priority than the current running thread moves to the Ready-to-run state, then the current thread is preempted (moved to Ready-to-run state) to let the higher priority thread execute.

• Time-sliced or Round-robin Scheduling:– A running thread is allowed to execute for a fixed

length of time, after with it moves to the Read-to-run state to await its turn to run again.

High Level Concurrency Objects• Lock objects– Synchronized code relies on a simple reentrant

lock– Supports sophisticated locking– Use implicit locks like Synchronized code– Supports notify/notify all through Condition

objects

Executors• Executor Interfaces define the three executor

object types• Thread Pools are the most common kind of

executor implementation• Fork/Join is a framework (new in JDK 7) for

taking advantage of multiple processors

Executor Interfaces• Executor, a simple interface that supports

launching new tasks• ExecutorService, a subinterface of Executor,

which adds features to manage the lifecycle of individual tasks and of the executor itself

• ScheduledExecutorService, a subinterface of ExecutorService, supports future and/or periodic execution of tasks

Atomic variable• Defined in java.util.concurrent.atomic• AtomicInteger• AtomicIntegerArray• AtomicLong• AtomicLongArray etc

Exercise 04: Write a bouncing ball animation program where you can

add bouncing ball by clicking button and also have a remove ball

button

Exercise 05: Simple animation program by changing image