Exception Handling and Format output

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Exception Handlingand Format output
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Midterm exam

• Date Oct 31
• Content
• Week 1 to Week 8
• Format
• Multiple choice
• Determine the results of the code
• Write simple code

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Introduction

• Exception an indication of a problem that
  occurs during a programs 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

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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

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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
  programs execution
• Improves clarity
• Enhances modifiability

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Example Not Catching Exceptions
Error message for invalid input
Exception in thread "main" java.lang.NumberFormatE
xception For input string "ten" at
java.lang.NumberFormatException.forInputString(Unk
nown Source) at java.lang.Integer.parseInt(Unknown
Source) at java.lang.Integer.parseInt(Unknown
Source) at Week7.SimpleException.main(SimpleExcept
ion.java12)
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Catching an Exception
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Performance Tip

• If the potential problems occur infrequently,
  intermixing program and error-handling logic can
  degrade a programs performance, because the
  program must perform (potentially frequent) tests
  to determine whether the task executed correctly
  and the next task can be performed.

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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.

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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

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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

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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

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try-catch Control Flow
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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

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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.
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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.

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Review

• To catch an exception, code must be enclosed in
  a
• a. throws block.
• b. catch block.
• c. try block.
• d. finally block.

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Review

• To catch an exception, code must be enclosed in
  a
• a. throws block.
• b. catch block.
• c. try block.
• d. finally block.

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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

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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

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Getting Information

• There are two methods we can call to get
  information about the thrown exception
• getMessage
• printStackTrace

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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

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Multiple catch Blocks

• A single try-catch statement can include multiple
  catch blocks, one for each type of exception.

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Multiple catch Control Flow
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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.

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try-catch-finally Control Flow
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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.

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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.

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Sample Call Sequence
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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.

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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
  method-call stack at the time the exception
  occurred.
• d. All of the above statements are true.

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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
  method-call stack at the time the exception
  occurred.
• d. All of the above statements are true.

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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
  method-call stack at the time the exception
  occurred.
• d. All of the above statements are true.

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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.

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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.

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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 -gt Debug As -gt Java Applications

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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.

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Using the throws Clause

• throws clause specifies the exceptions a method
  may throws
• Appears after methods parameter list and before
  the methods body
• Contains a comma-separated list of exceptions
• Exceptions can be thrown by statements in
  methods body of by methods called in methods
  body
• Exceptions can be of types listed in throws
  clause or subclasses

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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 programs flow of control

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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

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Portion of class Throwables inheritance
hierarchy.
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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.

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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.

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Handling Checked/Compiling Exceptions
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Handling Unchecked/Runtime Exceptions
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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.

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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

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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
  classs subclasses will perform the same
  functionality

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Outline
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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.

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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.

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Review

• In the catch block below, what is
  nullPointerException?
• catch (NullPointerException nullPointerExceptio
  n )
• System.err.println(nullPointerException)
  
• // end catch
• a. The type of the exception being caught.
• b. The name of catch blocks exception parameter.
• c. A finally block.
• d. An exception handler.

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Review

• In the catch block below, what is
  nullPointerException?
• catch (NullPointerException nullPointerExceptio
  n )
• System.err.println(nullPointerException)
  
• // end catch
• a. The type of the exception being caught.
• b. The name of catch blocks exception parameter.
• c. A finally block.
• d. An exception handler.

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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

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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

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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.

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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.

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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

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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

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Preconditions and Postconditions

• Preconditions and postconditions are the states
  before and after a methods execution
• Used to facilitate debugging and improve design
• You should state the preconditions and
  postconditions in a comment before the method
  declaration

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Preconditions and Postconditions

• Preconditions
• Condition that must be true when the method is
  invoked
• Describe method parameters and any other
  expectations the method has about the current
  state of a program
• If preconditions not met, methods behavior is
  undefined

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Preconditions and Postconditions

• Postconditions
• Condition that is true after the method
  successfully returns
• Describe the return value and any other
  side-effects the method may have
• When calling a method, you may assume that a
  method fulfills all of its postconditions

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Assertions

• Assertions are conditions that should be true at
  a particular point in a method
• Help ensure a programs validity by catching
  potential bugs
• Preconditions and Postconditions are two kinds of
  assertions
• Assertions can be stated as comments or
  assertions can be validated programmatically
  using the assert statement

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Assertions

• The syntax for the assert statement is
• assert ltboolean expressiongt
• where ltboolean expressiongt represents the
  condition that must be true if the code is
  working correctly.
• If the expression results in false, an
  AssertionError (a subclass of Error) is thrown.

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Sample Use 1
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Second Form

• The assert statement may also take the form
• assert ltboolean expressiongt ltexpressiongt
• where ltexpressiongt represents the value passed
  as an argument to the constructor of the
  AssertionError class. The value serves as the
  detailed message of a thrown exception.

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Sample Use 2
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Compiling Programs with Assertions

• Before Java 2 SDK 1.4, the word assert is a valid
  nonreserved identifier. In version 1.4 and after,
  the word assert is treated as a regular
  identifier to ensure compatibility.
• To enable the assertion mechanism, compile the
  source file using
• javac source 1.4 ltsource filegt