may 14, 2002serguei a. mokhov, mokhov@cs.concordia.ca 1 kickstart intro to java part ii comp346/5461...

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Kickstart Intro to JavaPart II

COMP346/5461 - Operating Systems

Revision 1.5

July 23, 2003

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Topics• Arrays

• Basics of Inheritance in Java

• Threads & Scheduling

• Exception Handling

• Examples

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Creating Arrays

• Two ways:– Using new:int aMyIntArray[] = new int[10];

• NOTE: creating an array does not imply creating objects for the elements of the array! Objects must be explicitly created!

• In our case, we have a primitive data type int, so it gets initialized to 0, in case of an object it is null.

– Using initializer:int aMyIntArray[] = {1,2,3,4,5,6,7,8,9,10};

• This creates an array dynamically and initializes it with the values.

• NOTE: this is different from C/C++ because values in the initializer in Java can be dynamic, whereas in C/C++ they are always constant.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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“Anonymous” Arrays and Objects

• “Anonymous” arrays don’t have a name and can be created dynamically within the initializer:

• Same way any allocated objects can be anonymous.

• In C/C++ that would be a memory leak.

countArgs( new String[] {“Bar”, “Ban”, “Ech”} );

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Multidimensional Arrays• Implemented as arrays-of-arrays, so arrays aren’t

necessarily rectangular.• Syntax:byte a64bytes[][] = new byte[8][8];

• No need to specify all dimensions right away:int aCube[][][] = new int[7][][];– Basically, we have 7 arrays of type int[][].– Later on the new dimensions will be there after proper

initialization.

• This is illegal, however:int aCube[][][] = new int[5][][2];

• Example: TriangleArray.java

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Triangle Array Example Explained

• After we execute this line:float aTriangle[][] = new float[7][];

we have the following: where a dot (•) stands for null0 •

1 •

2 •

3 •

4 •

5 •

6 •

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Triangle Array Example Explained (2)

• Then, gradually, with every iteration, the array becomes multiple-dimensioned:

0 •

1 •

2 •

3 •

4 •

5 •

6 •

[0.0] X

[1.0][2.0]

[2.0][3.0][4.0]

[3.0][4.0][5.0][6.0]

[4.0][5.0][6.0][7.0][8.0]

[5.0][6.0][7.0][8.0][9.0][10.0]

[6.0][7.0][8.0][9.0][10.0][11.0][12.0]

0 1 2 3 4 5 6

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Accessing Array Elements

• Just like in C/C++

• If an attempt is made to access the array outside boundaries, an exception ArrayIndexOutOfBoundsException is thrown, and can be caught and dealt with during run-time.

• More about exceptions later in the tutorial.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Basics of Inheritance in Java

• No multiple inheritance.• class Foo extends Bar {} means Foo inherits Bar.

• If no extends given, then the object is always assumed to inherit from the Object class.

• By using super() in the child’s class we can call parent’s constructor.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Threads & Scheduling

• Recall from COMP229 what threads are– One parent process, thus code and resources are the same.– Own stack and registers state, perhaps some private data.

• Java is a multithreaded language and has a very easy to use thread package (java.lang.Thread class) unlike C/C++

• JVM 1.2.* and earlier use priority-based scheduling and a Round Robin policy for the threads of the same priority. Later versions of the JVM let the OS schedule the threads (which is particularly useful on multiple CPUs)

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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java.lang.Thread

• This class provides facilities to start and stop threads, assign priorities, etc.http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Thread.html

• There’s a built-in support for the synchronized methods and blocks of code using the synchronized keyword. This means only one thread at a time should run the code marked as synchronized.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Most Often Used Method of the Thread class (1)

• start()When you create an object of a subclass Thread it doesn’t do anything useful, just wastes space. In order for the created thread to begin execution and do some useful work, the parent thread has to invoke the start() method.

• run()Upon call to start() by the parent, the created thread starts execution the body of the run() method, which is provided by the user in the subclassed thread object.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Most Often Used Method of the Thread Class (2)

• yield()Among the threads of the same priority, if one thread wishes voluntarily relinquish the CPU and let another thread go ahead, it calls yield(). Using yield() instruction in any form is sometimes called cooperative scheduling. We might use it to simulate a some sort of an “interrupt” at a given point of the thread’s execution path.

• join()An analogy to the wait() system call in C. A parent thread calls join() on a child thread object if it wants to pause and wait until the child terminates.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Threads Example

• Debrief threads operate on the common stack concurrently. Concurrency implies atomicity; otherwise, we get unexpected results.

• Pseudo code:acquire block: {block = stack[top]; (!) top--;}release block: {top++; (!) stack[top] = block;}

• Block.java – explained

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Another Way of Creating Threads

• … is to implement interface Runnable.• When is it needed? Just when you need a Thread behaviour,

but you cannot subclass Thread because your object already extends from something.

• Syntax: class myClass extends OtherClass implements Runnable {}

• You will need to define run() with no arguments of that class.

• Then do:– Thread myObj = new myClass();– myObj.start();

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Java’s Exception Handling Mechanism

• A very significant feature of Java. Beware: It is similar in a way to that of C++, but not the same!

• Some terminology:– Exception is a signal indicating that something

exceptional happened (whatever exceptional means), usually an error.

– Throwing an exception means signaling that exceptional condition.

– Catching an exception is to react to the signal an do whatever necessary to recover from it (i.e. to handle an exception).

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Exception Propagation

• Lexical block structure of Java method first;

• Up the method call stack

• Next higher enclosing code block

• …

• Invoking method

• If never caught – to main() => JVM exits with an error message and a stack trace.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Advantages of Using Exceptions

• Logical way of dealing with regular exceptions and errors by grouping the handling code in one place.

• Clarity of algorithms and code in your programs.

• Less error-prone code (sometimes catching an exception is mandatory and enforced by Java).

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Exception Objects

• All Exceptions are derived from the class Throwable (java.lang.Throwable), which has two standard subclasses:– Error (usually fatal exceptions, which should

not/cannot be caught, e.g. out of memory, linkage, dyn. loading, etc)

– Exception (usually recoverable, file ops, array bounds, etc)

• The Exceptions are also Objects thus have some methods and data members. E.g. getMessage() returns a human-readable error that occurred.

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Handling Exceptions

• Three statements: try / catch / finally• try {...}

Does nothing but indicates a block of code which is more likely to have exceptions.

• catch(Exception e) {...}try is followed by zero or more catch statements to handle specified exceptions

• finally {...}catch is optionally followed by one finally block, which does clean up and is always guaranteed to execute regardless how the try and catch blocks exit (only System.exit() is not part of that rule).

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Declaring Exceptions

• To throw “normal exceptions”:public void writeFile() throws IOException {} public void break_window() throws BrokenWindowException {}

• “Normal” means not subclasses of either Error or RuntimeException

• C++ diff: Java uses throws and not throw

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Defining and Generating Exceptions

• throw new MyException(“oh dear…”);

• The application stops and looks for catch statements; if not found, it propagates the exception to higher levels as described before.

• ExceptionExample.java

May 14, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

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Links and References

• Official Java site: http://java.sun.com

• Java in a Nutshell, Second Edition by David Flanagan, (C) 1997 O’Reily & Associates, Inc. ISBN: 1-56592-262-X