Download Exception Handling and Format output

Exception Handling
and Format output-CS1050-By
Gayani Gupta
Introduction
• Exception – an indication of a problem that
occurs during a program’s execution
• Exception handling – resolving exceptions that
may occur so program can continue or
terminate gracefully
• Exception handling enables programmers to
create programs that are more robust and
fault-tolerant
Introduction
• Examples
– ArrayIndexOutOfBoundsException – an
attempt is made to access an element past the end
of an array
– NullPointerException – when a null
reference is used where an object is expected
Exception-Handling Overview
• Intermixing program logic with error-handling
logic can make programs difficult to read,
modify, maintain and debug
• Exception handling enables programmers to
remove error-handling code from the “main
line” of the program’s execution
• Improves clarity
• Enhances modifiability
Example: Not Catching Exceptions
String inputStr;
int
age;
inputStr = JOptionPane.showInputDialog(null, "Age:");
age
= Integer.parseInt(inputStr);
Error message for invalid input
Exception in thread "main" java.lang.NumberFormatException: For
input string: "ten"
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at Week7.SimpleException.main(SimpleException.java:12)
Catching an Exception
inputStr = JOptionPane.showInputDialog(null, "Age:");
try {
age = Integer.parseInt(inputStr);
try
} catch (NumberFormatException e){
JOptionPane.showMessageDialog(null, "’" + inputStr
catch
}
+
"‘ is invalid\n"
+
"Please enter digits only");
Performance Tip
If the potential problems occur infrequently,
intermixing program and error-handling logic
can degrade a program’s performance, because
the program must perform (potentially
frequent) tests to determine whether the task
executed correctly and the next task can be
performed.
Try-catch: what does that mean?
When an exception occurs, or is thrown, the
normal sequence of flow is terminated. The
exception-handling routine is then executed;
we say the thrown exception is caught.
Try-catch: what does that mean?
• try block – encloses code that might throw
an exception and the code that should not
execute if an exception occurs
• Consists of keyword try followed by a block
of code enclosed in curly braces
Try-catch: what does that mean?
• catch block – catches (i.e., receives) and
handles an exception, contains:
– Begins with keyword catch
– Exception parameter in parentheses –
exception parameter identifies the
exception type and enables catch block to
interact with caught exception object
– Block of code in curly braces that executes
when exception of proper type occurs
Try-catch: what does that mean?
• Matching catch block – the type of the
exception parameter matches the thrown
exception type exactly or is a superclass of it
• Uncaught exception – an exception that
occurs for which there are no matching
catch blocks
– Cause program to terminate if program has
only one thread; Otherwise only current
thread is terminated and there may be
adverse effects to the rest of the program
try-catch Control Flow
try-catch Control Flow
• When an exception occurs:
– try block terminates immediately
– Program control transfers to first matching catch block
• After exception is handled:
– Termination model of exception handling – program
control does not return to the throw point because the
try block has expired; Flow of control proceeds to the
first statement after the last catch block
– Resumption model of exception handling – program
control resumes just after throw point
try-catch Control Flow
• try statement – consists of try block and
corresponding catch and/or finally blocks
Logic errors can occur if you assume that
after an exception is handled, control will
return to the first statement after the throw
point.
Tip
With exception handling, a program can
continue executing (rather than terminating)
after dealing with a problem. This helps
ensure the kind of robust applications that
contribute to what is called mission-critical
computing or business-critical computing.
Review
To catch an exception, code must be enclosed
in a
a. throws block.
b. catch block.
c. try block.
d. finally block.
Review
To catch an exception, code must be enclosed
in a
a. throws block.
b. catch block.
c. try block.
d. finally block.
Review
An uncaught exception:
a. is a possible exception that never actually occurs
during the execution of the program.
b. is an exception that occurs for which the matching
catch clause is empty.
c. is an exception that occurs for which there are no
matching catch clauses.
d. is another term for a thrown exception
Review
An uncaught exception:
a. is a possible exception that never actually occurs
during the execution of the program.
b. is an exception that occurs for which the matching
catch clause is empty.
c. is an exception that occurs for which there are no
matching catch clauses.
d. is another term for a thrown exception
Getting Information
• There are two methods we can call to get
information about the thrown exception:
– getMessage
– printStackTrace
try {
. . .
} catch (NumberFormatException e){
System.out.println(e.getMessage());
e.printStackTrace();
}
printStackTrace, getStackTrace
and getMessage
• Methods in class Throwable retrieve more
information about an exception
– printStackTrace – outputs stack trace to
standard error stream
– getStackTrace – retrieves stack trace
information as an array of StackTraceElement
objects; enables custom processing of the
exception information
– getMessage – returns the descriptive string
stored in an exception
Multiple catch Blocks
• A single try-catch statement can include multiple catch
blocks, one for each type of exception.
try {
. . .
age = Integer.parseInt(inputStr);
// Asuming I have a file I/O here
. . .
} catch (NumberFormatException e){
. . .
} catch (FileNotFoundException file_error){
. . .
}
Multiple catch Control Flow
The finally Block
• There are situations where we need to take
certain actions regardless of whether an
exception is thrown or not.
• We place statements that must be executed
regardless of exceptions in the finally block.
try-catch-finally Control Flow
Propagating Exceptions
• Instead of catching a thrown exception by using the
try-catch statement, we can propagate the thrown
exception back to the caller of our method.
• The method header includes the reserved word throws.
public int getAge( ) throws NumberFormatException {
. . .
int age = Integer.parseInt(inputStr);
. . .
return age;
}
Throwing Exceptions
• We can write a method that throws an exception
directly, i.e., this method is the origin of the exception.
• Use the throw reserved to create a new instance of the
Exception or its subclasses.
• The method header includes the reserved word throws.
public void doWork(int num) throws Exception {
. . .
if (num != val) throw new Exception("Invalid val");
. . .
}
Sample Call Sequence
Exception Thrower
• When a method may throw an exception,
either directly or indirectly, we call the method
an exception thrower.
• Every exception thrower must be one of two
types:
– catcher.
– propagator.
Review
Which of the following is not true regarding the
throw point of an exception?
a. It specifies the point at which the exception
must be handled.
b. It is the initial point at which the exception
occurs.
c. It is specified as the top row of the methodcall stack at the time the exception occurred.
d. All of the above statements are true.
Review
Which of the following is not true regarding the
throw point of an exception?
a. It specifies the point at which the exception
must be handled.
b. It is the initial point at which the exception
occurs.
c. It is specified as the top row of the methodcall stack at the time the exception occurred.
d. All of the above statements are true.
Review
Which of the following is not true regarding the
throw point of an exception?
a. It specifies the point at which the exception
must be handled.
b. It is the initial point at which the exception
occurs.
c. It is specified as the top row of the methodcall stack at the time the exception occurred.
d. All of the above statements are true.
Review question
Which of the following statements is not true?
a. Exception handling enables programmers to
write robust and fault-tolerant programs.
b. Exception handling can only catch the
exception but cannot resolve the exception.
c. Exception handling can resolve exceptions.
d. The Java 2 Platform, Standard Edition,
Version 1.4 introduced the new chained
exception feature.
Review question
Which of the following statements is not true?
a. Exception handling enables programmers to
write robust and fault-tolerant programs.
b. Exception handling can only catch the
exception but cannot resolve the exception.
c. Exception handling can resolve exceptions.
d. The Java 2 Platform, Standard Edition,
Version 1.4 introduced the new chained
exception feature.
Debugging 101
• execute your Java program line by line
• examine the value of variables at different
points in the program
How to do:
set a breakpoint in your code so the debugger
suspends execution (double-click in the gray margin on
the left side of the editor )
choose Run -> Debug As -> Java Applications
Types of Exception Throwers
• An exception catcher is an exception thrower
that includes a matching catch block for the
thrown exception.
• An exception propagator does not contain a
matching catch block.
• A method may be a catcher of one exception
and a propagator of another.
Using the throws Clause
• throws clause – specifies the exceptions a
method may throws
– Appears after method’s parameter list and before
the method’s body
– Contains a comma-separated list of exceptions
– Exceptions can be thrown by statements in
method’s body of by methods called in method’s
body
– Exceptions can be of types listed in throws clause
or subclasses
When to Use Exception Handling
• Exception handling designed to process
synchronous errors
• Synchronous errors – occur when a statement
executes
• Asynchronous errors – occur in parallel with
and independent of the program’s flow of
control
Java Exception Hierarchy
• All exceptions inherit either directly or indirectly from
class Exception
• Exception classes form an inheritance hierarchy that
can be extended
• Class Throwable, superclass of Exception
– Only Throwable objects can be used with the
exception-handling mechanism
– Has two subclasses: Exception and Error
• Class Exception and its subclasses represent exception
situations that can occur in a Java program and that can be
caught by the application
• Class Error and its subclasses represent abnormal situations
that could happen in the JVM – it is usually not possible for a
program to recover from Errors
Portion of class Throwable’s
inheritance hierarchy.
Checked/Compiling vs.
Unchecked/Running time
• There are two types of exceptions:
– Checked/Compiling time
– Unchecked/Run time
• A checked exception is an exception that is
checked at compile time.
• All other exceptions are unchecked, or
runtime, exceptions. As the name suggests,
they are detected only at runtime.
Different Handling Rules
• When calling a method that can throw
checked exceptions
– use the try-catch statement and place the call in
the try block, or
– modify the method header to include the
appropriate throws clause.
• When calling a method that can throw
runtime exceptions, it is optional to use the
try-catch statement or modify the method
header to include a throws clause.
Handling Checked/Compiling Exceptions
Handling Unchecked/Runtime Exceptions
Java Exception Hierarchy
• Two categories of exceptions: checked and
unchecked
• Checked exceptions
– Exceptions that inherit from class Exception but
not from RuntimeException
– Compiler enforces a catch-or-declare requirement
– Compiler checks each method call and method
declaration to determine whether the method
throws checked exceptions. If so, the compiler
ensures that the checked exception is caught or is
declared in a throws clause. If not caught or
declared, compiler error occurs.
Java Exception Hierarchy
• Unchecked exceptions
– Inherit from class RuntimeException or
class Error
– Compiler does not check code to see if
exception is caught or declared
– If an unchecked exception occurs and is
not caught, the program terminates or runs
with unexpected results
– Can typically be prevented by proper
coding
Java Exception Hierarchy
• catch block catches all exceptions of its type
and subclasses of its type
• If there are multiple catch blocks that match
a particular exception type, only the first
matching catch block executes
• It makes sense to use a catch block of a
superclass when all the catch blocks for that
class’s subclasses will perform the same
functionality
Outline
try
{
statements
} // end try
catch ( AKindOfException exception1 )
{
exception-handling statements
}
.
.
.
catch ( AnotherKindOfException exception2 )
{
exception-handling statements
}
finally
{
statements
}
Review
Exceptions can occur:
a. from the Java Virtual Machine.
b. through explicitly mentioned code in a try
block.
c. through calls to other methods made in a try
block.
d. All of the above.
Review
Exceptions can occur:
a. from the Java Virtual Machine.
b. through explicitly mentioned code in a try
block.
c. through calls to other methods made in a try
block.
d. All of the above.
Review
In the catch block below, what is nullPointerException?
catch (NullPointerException nullPointerException )
{
System.err.println(“nullPointerException”);
} // end catch
a. The type of the exception being caught.
b. The name of catch block’s exception parameter.
c. A finally block.
d. An exception handler.
Review
In the catch block below, what is nullPointerException?
catch (NullPointerException nullPointerException )
{
System.err.println(“nullPointerException”);
} // end catch
a. The type of the exception being caught.
b. The name of catch block’s exception parameter.
c. A finally block.
d. An exception handler.
Throwing Exceptions Using the throw
Statement
• throw statement – used to throw exceptions
• Programmers can thrown exceptions
themselves from a method if something has
gone wrong
• throw statement consists of keyword throw
followed by the exception object
Rethrowing Exceptions
• Exceptions are rethrown when a catch block
decides either that it cannot process the
exception or that it can only partially process
it
• Exception is deferred to outer try statement
• Exception is rethrown by using keyword
throw followed by a reference to the
exception object
Stack Unwinding
• Stack unwinding – When an exception is
thrown but not caught in a particular scope,
the method-call stack is “unwound,” and an
attempt is made to catch the exception in the
next outer try block.
Stack Unwinding
• When unwinding occurs:
– The method in which the exception was not caught
terminates
– All local variables in that method go out of scope
– Control returns to the statement that originally
invoked the method – if a try block encloses the
method call, an attempt is made to catch the
exception.
Declaring New Exception Types
• You can declare your own exception classes
that are specific to the problems that can
occur when another program uses your
reusable classes
• New exception class must extend an existing
exception class
Declaring New Exception Types
• Typically contains only two constructors
– One takes no arguments, passes a default
exception messages to the superclass constructor
– One that receives a customized exception message
as a string and passes it to the superclass
constructor