Bài giảng điện tử môn tin học: Developing Comprehensive Projects pps

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Part IV: Developing Comprehensive
Projects

Chapter 10: Exception Handling

Chapter 11: Multithreading

Chapter 12: Multimedia

Chapter 13: Input and Output

Chapter 14: Networking
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Chapter 10
Exception Handling

What exceptions are for

What exceptions are NOT for

Catching & Throwing exceptions

Exception Specifications

Standard Java Exceptions


Exceptions and Polymorphism

The finally clause

Resource Management

Uncaught Exceptions
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What Exceptions are For

To handle Bad Things

I/O errors, other
runtime
errors

when a function fails to fulfill its
specification

so you can restore program stability (or
exit gracefully)
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What Exceptions are For, cont

To
force you

to handle Bad Things

because return codes can be tedious

and sometimes you’re lazy
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Example
File I/O
public FileReader(String fileName)
throws FileNotFoundException
public void close() throws IOException
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import java.io.*;
class OpenFile
{
public static void main(String[] args)
{
if (args.length > 0)
{
try
{
// Open a file:
FileReader f =
new FileReader(args[0]);
System.out.println(args[0]
+ " opened");

f.close();
}
catch (IOException x)
{
System.out.println(x);
}
}
}
}
Example
File I/O, cont
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What Exceptions are For, cont

To signal errors from constructors

because constructors have no return value
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What Exceptions are NOT For

NOT For Alternate Returns:

e.g., when end-of-file is reached:
while ((s = f.readLine()) != null) …

Exceptions are only for the exceptional!

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Catching Exceptions

Wrap code to be checked in a try-block

checking occurs all the way down the
execution stack

try-blocks can be nested

control resumes at most enclosed matching
handler
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Catching Exceptions, cont

Place one or more catch-clauses after try-
block

runtime system looks back up the call stack
for a matching handler

subclass types match superclass types

catching Exception catches everything (almost)

handlers are checked

in the order they
appear

place most derived types first!

execution resumes after last handler

if you let it (could branch or throw)
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Throwing Exceptions

Must throw objects derived (ultimately) from
Throwable

Usually derive from java.lang.Exception

The class name is the most important
attribute of an exception

Can optionally include a message

Provide two constructors:

MyException( )

MyException(String s)
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Throwing Exceptions, cont

Control is passed up the execution
stack to a matching handler

Various methods exist for processing
exceptions:

getMessage( )

toString( ) (class name + message)

printStackTrace( )
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Throwing Exceptions, cont

Functions must “advertise” their
exceptions

every function must specify the “checked”
exceptions it (or its callees!) may throw

Callers must do one of two things:

handle your exceptions with try-catch, or

advertise your exceptions along with theirs

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Sample Program

FixedStack

implements a stack with an array of Object

various methods throw exceptions

class StackException

StackTest

must handle StackExceptions
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class StackException extends Exception
{
StackException()
{}
StackException(String msg)
{
super(msg);
}
}
Sample Program, cont
Sample Program, cont

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class FixedStack
{
private int capacity;
private int size;
private Object[] data;
public FixedStack(int cap)
{
data = new Object[cap];
capacity = cap;
size = 0;
}
public void push(Object o)
throws StackException
{
if (size == capacity)
throw new StackException("overflow");
data[size++] = o;
}
Sample Program, cont
Sample Program, cont
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public Object pop()
throws StackException
{
if (size <= 0)

throw new StackException("underflow");
return data[ size];
}
public Object top()
throws StackException
{
if (size <= 0)
throw new StackException("underflow");
return data[size-1];
}
public int size()
{
return this.size;
}
}
Sample Program, cont
Sample Program, cont
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class StackTest
{
public static void main(String[] args)
{
FixedStack s = new FixedStack(3);
doTest(s);
}
public static void doTest(FixedStack s)
{
try

{
System.out.println("Initial size = "
+ s.size());
s.push("one");
s.push(new Integer(2));
s.push(new Float(3.0));
s.push("one too many"); // error!
}
catch(StackException x)
{
System.out.println(x);
}
Sample Program, cont
Sample Program, cont
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try
{
System.out.println("Top: " + s.top());
System.out.println("Popping ");
while (s.size() > 0)
System.out.println(s.pop());
}
catch(StackException x)
{
throw new InternalError(x.toString());
}
}
}

/* Output:
Initial size = 0
StackException: overflow
Top: 3.0
Popping
3.0
2
one
*/
Sample Program, cont
Sample Program, cont
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Using printStackTrace( )
catch(StackException x)
{
x.printStackTrace(System.out);
}
…
StackException: overflow
at FixedStack.push(FixedStack.java:18)
at StackTest.doTest(StackTest.java, Compiled Code)
at StackTest.main(StackTest.java:6)
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Standard Java Exceptions
Throwable
Exception Error

RuntimeException
IOException
. . .
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Class java.lang.Exception

The one you usually derive from

“Checked Exceptions”

specifications checked at
compile
time

you must either catch or advertise these

Used for recoverable errors

Not programmer errors
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java.lang.Exception Subclasses

AWTException

ClassNotFoundException


CloneNotSupportedException

IOException

NoSuchFieldException
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Class java.lang.Error

For JVM Failures and other Weird
Things

let program terminate

InternalError is one of these

Don’t catch them

you don’t know what to do!

These are “unchecked exceptions”

not required to advertise
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java.lang.Error Subclasses

AWTError


LinkageError

…

ThreadDeath

VirtualMachineError

InternalError, OutOfMemoryError,
StackOverflowError, UnknownError