Transcript Kickstart Intro to Java

Kickstart Intro to Java
Part II
COMP346/5461 - Operating Systems
Revision 1.5
July 23, 2003
May 14, 2002
Serguei A. Mokhov,
[email protected]
1
Topics
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•
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•
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Arrays
Basics of Inheritance in Java
Threads & Scheduling
Exception Handling
Examples
May 14, 2002
Serguei A. Mokhov,
[email protected]
2
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,
[email protected]
3
“Anonymous” Arrays and
Objects
• “Anonymous” arrays don’t have a name and can
be created dynamically within the initializer:
countArgs( new String[] {“Bar”, “Ban”, “Ech”} );
• Same way any allocated objects can be
anonymous.
• In C/C++ that would be a memory leak.
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[email protected]
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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
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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 null
0 •
1
•
2
•
3
•
4
•
5
•
6
•
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[email protected]
6
Triangle Array Example
Explained (2)
• Then, gradually, with every iteration, the
array becomes multiple-dimensioned:
0
1
2
3
4
5
6
0
•
[0.0] X
1
•
[1.0][2.0]
2
•
[2.0][3.0][4.0]
3
•
[3.0][4.0][5.0][6.0]
4
•
[4.0][5.0][6.0][7.0][8.0]
5
•
[5.0][6.0][7.0][8.0][9.0][10.0]
6
•
[6.0][7.0][8.0][9.0][10.0][11.0][12.0]
May 14, 2002
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[email protected]
7
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
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[email protected]
8
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
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[email protected]
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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
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[email protected]
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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.
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[email protected]
11
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.
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[email protected]
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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.
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[email protected]
13
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
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[email protected]
14
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,
[email protected]
15
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
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[email protected]
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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
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[email protected]
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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,
[email protected]
18
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
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[email protected]
19
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
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[email protected]
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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
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[email protected]
21
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
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[email protected]
22
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
May 14, 2002
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[email protected]
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