Transcript TRY and Catch lecture

Chapter 17 Exceptions and
Assertions
Chapter 9 Inheritance and Polymorphism
Chapter 17 Exceptions and Assertions
Chapter 18 Binary I/O
Chapter 6 Arrays
Chapter 19 Recursion
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Objectives
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To know what is exception and what is exception handling
(§17.2).
To distinguish exception types: Error (fatal) vs. Exception (nonfatal), and checked vs. uncheck exceptions (§17.2).
To declare exceptions in the method header (§17.3).
To throw exceptions out of a method (§17.3).
To write a try-catch block to handle exceptions (§17.3).
To explain how an exception is propagated (§17.3).
To rethrow exceptions in a try-catch block (§17.4).
To use the finally clause in a try-catch block (§17.5).
To know when to use exceptions (§17.6).
To declare custom exception classes (§17.7 Optional).
To apply assertions to help ensure program correctness (§17.8).
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
2
Syntax Errors, Runtime Errors, and
Logic Errors
You learned that there are three categories of
errors: syntax errors, runtime errors, and logic
errors. Syntax errors arise because the rules of
the language have not been followed. They are
detected by the compiler. Runtime errors occur
while the program is running if the environment
detects an operation that is impossible to carry
out. Logic errors occur when a program doesn't
perform the way it was intended to.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Runtime Errors
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import java.util.Scanner;
public class ExceptionDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter an integer: ");
int number = scanner.nextInt();
If an exception occurs on this
line, the rest of the lines in the
method are skipped and the
program is terminated.
// Display the result
System.out.println(
"The number entered is " + number);
}
}
Terminated.
Run
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Catch Runtime Errors
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import java.util.*;
Run
public class HandleExceptionDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
boolean continueInput = true;
do {
try {
System.out.print("Enter an integer: ");
int number = scanner.nextInt();
If an exception occurs on this line,
the rest of lines in the try block are
skipped and the control is
transferred to the catch block.
// Display the result
System.out.println(
"The number entered is " + number);
continueInput = false;
}
catch (InputMismatchException ex) {
System.out.println("Try again. (" +
"Incorrect input: an integer is required)");
scanner.nextLine(); // discard input
}
} while (continueInput);
}
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
5
Exception Classes
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
6
System Errors
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur. If one does, there is little
you can do beyond notifying the
user and trying to terminate the
program gracefully.
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Exceptions
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Runtime Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-bounds
array, and numeric errors.
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Checked Exceptions vs.
Unchecked Exceptions
RuntimeException, Error and their subclasses are
known as unchecked exceptions. All other
exceptions are known as checked exceptions,
meaning that the compiler forces the programmer
to check and deal with the exceptions.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
10
Unchecked Exceptions
In most cases, unchecked exceptions reflect programming
logic errors that are not recoverable. For example, a
NullPointerException is thrown if you access an object
through a reference variable before an object is
assigned to it; an IndexOutOfBoundsException is
thrown if you access an element in an array outside the
bounds of the array. These are the logic errors that should
be corrected in the program. Unchecked exceptions can
occur anywhere in the program. To avoid cumbersome
overuse of try-catch blocks, Java does not mandate you to
write code to catch unchecked exceptions.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
11
The finally Clause
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
Suppose no
exceptions in the
statements
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement in the
method is executed
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
Suppose an exception
of type Exception1 is
thrown in statement2
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The exception is
handled.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
17
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
18
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The next statement in
the method is now
executed.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
19
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
statement2 throws an
exception of type
Exception2.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
20
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Handling exception
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
21
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Execute the final block
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
22
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Rethrow the exception
and control is
transferred to the caller
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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