COMP201 Java Programming Topic 7: Exceptions Reading: Chapter 11.
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Transcript of COMP201 Java Programming Topic 7: Exceptions Reading: Chapter 11.
COMP201 Topic 7 / Slide 2
Outline
Introduction
Java exception classes
Dealing with exceptions
Throwing exceptions
Catching exceptions
COMP201 Topic 7 / Slide 3
Introduction
Causes of errors User input errors:
typos, malformed URL, wrong file name, wrong info in file…
Hardware errors:Disk full, printer out of paper or down, web page
unavailable…
Code errors:invalid array index, bad cast, read past end of file, pop empty stack, null object reference…
COMP201 Topic 7 / Slide 4
Introduction
Goals of error handling
Don’t want:
Want: Return to a safe state and enable user to execute other
commands
Allow user to save work and terminate program gracefully.
COMP201 Topic 7 / Slide 5
Introduction
Java exception handling mechanism: Every method is allowed to have two exit paths
No errors occur– Method exits in the normal way
– Returns a value
– Control passed to the calling code. If errors occur
– Method exits via an alternative exit path
– Throws an object that encapsulates the error information
– Control passed to exception mechanism that searches for an appropriate exception handler to deal with the error condition
COMP201 Topic 7 / Slide 6
Throw & Catch an Exception
Exception object is created when an error occurs Exception object contains information about the exception The object is handed off to the runtime system Runtime system searches for the method that contains an
appropriate exception handler
COMP201 Topic 7 / Slide 7
Advantage
Separate error handling code from regular code Sample code without error handling
traditional error detection, reporting, and handling often lead to messy code
readFile { open the file; determine its size; allocate that much memory; read the file into memory; close the file;}
Traditional ApproacherrorCodeType readFile { initialize errorCode = 0; open the file; if (theFileIsOpen) { determine the length of the file; if (gotTheFileLength) { allocate that much memory; if (gotEnoughMemory) { read the file into memory; if (readFailed) { errorCode = -1; } } else { errorCode = -2; } } else { errorCode = -3; } close the file; if (theFileDidntClose && errorCode == 0) { errorCode = -4; } else { errorCode = errorCode and -4; } } else { errorCode = -5; } return errorCode;}
Java Exception Handling ApproachreadFile { try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; } catch (fileOpenFailed) { doSomething; } catch (sizeDeterminationFailed) { doSomething; } catch (memoryAllocationFailed) { doSomething; } catch (readFailed) { doSomething; } catch (fileCloseFailed) { doSomething; }}
It does NOT spare your effort to detect, report and handle the exception, but provide an elegant framework.
COMP201 Topic 7 / Slide 10
Java Exception Classes Java has many classes for error handling.
They are called exception classes
java.io.IOException
java.lang.VirtualMachineError
java.lang.LinkageError
java.lang.Exception
java.lang.Error
java.lang.Throwable
java.lang.NullPointerException
java.lang.ArithmeticException
java.lang.RuntimeException
java.lang.ClassCastException
unchecked
unchecked
checked
java.io.EOFException…
COMP201 Topic 7 / Slide 11
Unchecked exceptions Error: For internal errors in JVM.
RuntimeException: Logical errors in program (C++ logical-error). Bad cast Out-of-bound array access Null reference access
Those two exceptions are unchecked JVM internal errors beyond your control You should not allow logical errors at the first place
COMP201 Topic 7 / Slide 12
All other exceptions (C++ runtime_error) are checked, i.e. you have to explicitly handle them. Otherwise compiler errors results in. Trying to read pass end of file Open a malform URL …
AclNotFoundException, ActivationException, AWTException, BadLocationException, ClassNotFoundException, CloneNotSupportedException, DataFormatException, ExpandVetoException, GeneralSecurityException, IllegalAccessException, InstantiationException, InterruptedException, IntrospectionException, InvocationTargetException, IOException, LastOwnerException, …
Checked exceptions
Can be found in online API
COMP201 Topic 7 / Slide 13
Java Exception Classes You can define new Exception classes.
class FileFormatException extends IOException{ // default constructor public FileFormatException() {} //constructor contains a detailed message public FileFormatException(String message) { super( message );}New Exception class must be subclass of Throwable Most programs
throw and catch objects that derive from the Exception class
COMP201 Topic 7 / Slide 14
Dealing with Exceptions Need to consider exceptions when writing each method
Identifying possible exceptions Check the API of each method you use to see if it throws an
exceptions Consider whether your own codes might produce exceptions.
Dealing with exceptions Catching exceptions: Do this if you know how to handle an exception
in the current method. Throwing exceptions: Do this if you don’t know how to handle an
exception in the current method and need the caller method to deal with it.
The most important slide
COMP201 Topic 7 / Slide 15
Example: Consider reading Employee information from a text file: File format (employeeGood.dat)
3
Harry Hacker|37363.75|1989|10|1
Carl Cracker|78937.5|1987|12|15
Tony Tester|39995.0|1990|3|15 Some thing might go wrong (employeeBad.dat)
3
Harry Hacker|37363.75|1989|10|1
Carl Cracker|78937.5|1987|12|15
Tony Tester|39995.0|1990|3|
Throwing Exceptions
Missing one item here
COMP201 Topic 7 / Slide 16
Throw an exception generated by method call: public void readData(BufferedReader in)throws IOException
{ String s = in.readLine(); StringTokenizer t = new StringTokenizer(s, "|"); name = t.nextToken(); salary = Double.parseDouble(t.nextToken()); int y = Integer.parseInt(t.nextToken()); int m = Integer.parseInt(t.nextToken()); int d = Integer.parseInt(t.nextToken()); GregorianCalendar calendar = new GregorianCalendar(y, m - 1, d); // GregorianCalendar uses 0 = January hireDay = calendar.getTime(); }//DataFileTest.java
Throwing Exceptions
COMP201 Topic 7 / Slide 17
The method readLine of BufferedReader throws an IOExpcetion. We do not deal with this exception in the current method. So we state that the readData method might throw IOException.
If you simply ignore it, compiler error results in. Try this.
Throwing Exceptions
COMP201 Topic 7 / Slide 18
The nextToken method might produce an NoSuchElementException.
But this exception is unchecked, so we don’t have to worry about it. No compiler errors result in because of this.
This leads to ungraceful behavior when exception is thrown (Run ReadTextTest2 on employeeBad.dat)
Throwing Exceptions
COMP201 Topic 7 / Slide 19
Create and throw your own exceptionpublic void readData(BufferedReader in) throws IOException, FileFormatException { String s = in.readLine(); StringTokenizer t = new StringTokenizer(s, "|");
… int d;
if (t.hasMoreTokens()) { d = Integer.parseInt(t.nextToken()); } else { FileFormatException e = new FileFormatException ("Last item missing in:\n" + s +"\n" ); throw e; } … } //ExceptionTest.java
Throwing Exceptions
COMP201 Topic 7 / Slide 20
Notes: Can throw multiple types of exceptions
public void readData(BufferedReader in)
throws IOException, FileFormatException Overriding method in subclass cannot throw more exceptions than
corresponding method in superclass– If method in superclass does not throw any exceptions, overriding
method in subclass cannot either Suppose method1 calls method2. If method2 throws someException, method1 must
– Either catch someException– Or declare someException in its header
Throwing Exceptions
COMP201 Topic 7 / Slide 21
Catching Exceptions Catch exceptions with try/catch block
try{ code more code}catch( ExceptionType e){ handler for this exception} If a statement in the try block throws an exception
The remaining statements in the block are skipped Handler code inside catch block executed.
If no exceptions are thrown by codes in the try block, the catch block is skipped.
COMP201 Topic 7 / Slide 22
Catching Exceptionspublic static void main(String[] args)
{ try
{ BufferedReader in = new BufferedReader(
new FileReader(args[0]));
Employee[] newStaff = readData(in);
…
}
catch(IOException exception)
{ exception.printStackTrace();
exit(0);
}} //ExceptionTest.java This code will exit right away with an error message if something goes
wrong in readData or in the constructor of FilerReader (an IOException will be thrown)
COMP201 Topic 7 / Slide 23
An more interesting exception handlerpublic void readData(BufferedReader in) throws IOException { String s = in.readLine(); StringTokenizer t = new StringTokenizer(s, "|");
… int d;
try { d = Integer.parseInt(t.nextToken());
} catch (NoSuchElementException e) { String input = JOptionPane.showInputDialog
("Hire day for "+ name +"not available from file.\n“
+ "please provide from keyboard"); // convert string to integer value
d = Integer.parseInt(input); …
}} //ExceptionTest1.java
Catching Exceptions
COMP201 Topic 7 / Slide 24
Catching Multiple Exceptions Can have multiple catchers for multiple types of exceptions:
public static void main(String[] args)
{ try
{ BufferedReader in = new BufferedReader(
new FileReader(args[0]));
Employee[] e = readData(in);
…
}
catch(IOException e1)
{ exception.printStackTrace(); }
catch(ArrayIndexOutOfBoundsException e2)
{ System.out.print("No file name provided " );
System.exit(1);
}
} // ExceptionTest2.java
Might throw ArrayIndexOutOfBoundsExpection
COMP201 Topic 7 / Slide 25
Dealing with Exceptions Example:
try {
average = total/count;
System.out.println(“Average is ” + average); }
catch (ArithmeticException e) {
System.out.println(“Oops: ”+ e);
average = -1;}
If count is 0, this code will print out something like “Oops: division by zero”.
COMP201 Topic 7 / Slide 26
Dealing with Exceptions Can have multiple catchers for multiple types of exceptions:
try {…}
catch (ArithmeticException e1){…}
catch (IOException e2) {…}
catch (Exception e3) {…} //ANY exception
COMP201 Topic 7 / Slide 27
Catching Exceptions Catchers can also re-throw an exception or throw
exception that is different from the exception caught.
graphics g = image.getGraphics();
try { …}
catch (MalformedURLException e)
{ g.dispose();
throw e;
}
We wish to dispose the graphics object g, but we don’t know how to deal with the exception.
COMP201 Topic 7 / Slide 28
The finally clausetry{ code more code}catch( ExceptionType e){ handler for this exception}finally{ .. }
The finally block is executed regardless whether exceptions are thrown in the try block.
Useful in situations where resources must be released no matter what happened
COMP201 Topic 7 / Slide 29
A caution about the finally clause: Codes in the finally block are executed even there are return
statements in the try block
public static int f(int n) { try { return n* n; } finally { if ( n==2) return 0; }
}f(2) return 0 instead of 4!
The finally clause
COMP201 Topic 7 / Slide 30
Dealing with Exceptions Search for handler: Steps:
– Tries to find a handler in the Catch block for the current exception in the current method. Considers a match If the thrown object can legally be assigned to the exception handler’s argument.
– If not found, move to the caller of this method
– If not there, go another level upward, and so on. (Reason for the last item on slide 20.)
– If no handler found, program terminates.
COMP201 Topic 7 / Slide 31
C++ Notes Java exception handling similar to that of C++,
except:– C++ enforces the throw specifier at run time, while
Java checks Throw specifier at compile time.
– In C++ a function can throw an exception in case of no throw specification, while in Java a method can only throw exceptions advertised.
– In C++ you can throw values of any type, while in Java you can only throw objects of a subclass of Throwable.