Liang Chapter 12

Chapter 12 Exceptions and File Motivations
Input/Output When a program runs into a runtime error, the

program terminates abnormally. How can you
handle the runtime error so that the program can
continue to run or terminate gracefully? This is the
CS1: Java Programming subject we will introduce in this chapter.
Colorado State University
Original slides by Daniel Liang

Modified slides by Chris Wilcox
Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All
1 2
rights reserved. rights reserved.
✦
Objectives
To get an overview of exceptions and exception handling (§12.2). Exception-Handling Overview
✦ To explore the advantages of using exception handling (§12.2).
✦ To distinguish exception types: Error (fatal) vs. Exception (nonfatal) and checked vs. unchecked (§12.3).
✦ To declare exceptions in a method header (§12.4.1). Show runtime error
✦ To throw exceptions in a method (§12.4.2).
To write a try-catch block to handle exceptions (§12.4.3).
✦
Quotient Run
✦ To explain how an exception is propagated (§12.4.3).
✦ To obtain information from an exception object (§12.4.4).
✦ To develop applications with exception handling (§12.4.5).
✦ To use the finally clause in a try-catch block (§12.5). Fix it using an if statement
✦ To use exceptions only for unexpected errors (§12.6).
✦ To rethrow exceptions in a catch block (§12.7). QuotientWithIf Run
✦ To create chained exceptions (§12.8).
✦ To define custom exception classes (§12.9).
✦ To discover file/directory properties, to delete and rename files/directories, and to create directories using the
✦
File class (§12.10).
To write data to a file using the PrintWriter class (§12.11.1).
With a method
✦ To use try-with-resources to ensure that the resources are closed automatically (§12.11.2).
✦ To read data from a file using the Scanner class (§12.11.3). QuotientWithMethod Run
✦ To understand how data is read using a Scanner (§12.11.4).
✦ To develop a program that replaces text in a file (§12.11.5).
✦ To read data from the Web (§12.12).
✦ To develop a Web crawler (§12.13).
3 4
Exception Advantages Handling InputMismatchException
QuotientWithException Run InputMismatchExceptionDemo Run
Now you see the advantages of using exception handling. By handling InputMismatchException, your program will
It enables a method to throw an exception to its caller. continuously read an input until it is correct.
Without this capability, a method must handle the
exception or terminate the program.
5 6
Exception Types System Errors

ClassNotFoundException ClassNotFoundException
ArithmeticException ArithmeticException
IOException IOException
Exception NullPointerException Exception NullPointerException

RuntimeException RuntimeException
IndexOutOfBoundsException IndexOutOfBoundsException
Many more classes Many more classes
Object Throwable IllegalArgumentException Object Throwable IllegalArgumentException

System errors are thrown by JVM
LinkageError and represented in the Error class. LinkageError
The Error class describes internal
Error VirtualMachineError system errors. Such errors rarely Error VirtualMachineError
occur. If one does, there is little
Many more classes
you can do beyond notifying the Many more classes
user and trying to terminate the
program gracefully.
7 8
Exceptions Runtime Exceptions
Exception describes errors
caused by your program ClassNotFoundException ClassNotFoundException
and external
ArithmeticException ArithmeticException
circumstances. These IOException IOException
errors can be caught and
Exception NullPointerException Exception NullPointerException
handled by your program.
RuntimeException RuntimeException
IndexOutOfBoundsException IndexOutOfBoundsException
Object Throwable IllegalArgumentException Object Throwable IllegalArgumentException

LinkageError LinkageError
RuntimeException is caused by
programming errors, such as bad
Error VirtualMachineError Error VirtualMachineError casting, accessing an out-of-bounds
array, and numeric errors.
9 10
Checked Exceptions vs. Unchecked Exceptions

Unchecked Exceptions In most cases, unchecked exceptions reflect programming
logic errors that are not recoverable. For example, a
RuntimeException, Error and their subclasses are NullPointerException is thrown if you access an object
known as unchecked exceptions. All other through a reference variable before an object is assigned to
it; an IndexOutOfBoundsException is thrown if you access
exceptions are known as checked exceptions, an element in an array outside the bounds of the array.
meaning that the compiler forces the programmer These are the logic errors that should be corrected in the
to check and deal with the exceptions. 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.
11 12
Unchecked Exceptions Declaring, Throwing, and
Catching Exceptions
ClassNotFoundException
ArithmeticException
IOException
Exception NullPointerException
RuntimeException method1() { declare exception
method2() throws Exception {
IndexOutOfBoundsException try {
invoke method2; if (an error occurs) {
Many more classes
}
Object Throwable IllegalArgumentException catch exception catch (Exception ex) { throw new Exception(); throw exception
Process exception; }
} }
Many more classes }
LinkageError
Error VirtualMachineError Unchecked

exception.
Many more classes
13 14
Declaring Exceptions Throwing Exceptions

Every method must state the types of checked When the program detects an error, the program
exceptions it might throw. This is known as can create an instance of an appropriate exception
declaring exceptions. type and throw it. This is known as throwing an
exception. Here is an example,
public void myMethod()
throws IOException throw new TheException();
public void myMethod() TheException ex = new TheException();

throws IOException, OtherException throw ex;
15 16
Throwing Exceptions Example Catching Exceptions
try {
/** Set a new radius */ statements; // Statements that may throw exceptions
public void setRadius(double newRadius) }
catch (Exception1 exVar1) {
throws IllegalArgumentException {
handler for exception1;
if (newRadius >= 0)
}
radius = newRadius; catch (Exception2 exVar2) {
else handler for exception2;
throw new IllegalArgumentException( }
"Radius cannot be negative"); ...
} catch (ExceptionN exVar3) {
handler for exceptionN;
}
17 18
Catch or Declare Checked Exceptions

Catching Exceptions
main method { method1 { method2 { An exception
Suppose p2 is defined as follows:
... ... ... is thrown in
try { try { try { method3
... ... ...
invoke method1; invoke method2; invoke method3;
statement1; statement3; statement5;
} } }
catch (Exception1 ex1) { catch (Exception2 ex2) { catch (Exception3 ex3) {
Process ex1; Process ex2; Process ex3;
} } }
statement2; statement4; statement6;
} } }
Call Stack void p2() throws IOException {

method3 if (a file does not exist) {
throw new IOException("File does not exist");
method2 method2
}
method1 method1 method1
main method main method main method main method ...

}
19 20
Catch or Declare Checked Exceptions Example: Declaring, Throwing, and
Java forces you to deal with checked exceptions. If a method declares a
checked exception (i.e., an exception other than Error or
Catching Exceptions
RuntimeException), you must invoke it in a try-catch block or declare to
throw the exception in the calling method. For example, suppose that
✦ Objective: This example demonstrates
method p1 invokes method p2 and p2 may throw a checked exception (e.g., declaring, throwing, and catching exceptions
IOException), you have to write the code as shown in (a) or (b). by modifying the setRadius method in the
Circle class defined in Chapter 9. The new
void p1() {
try {
void p1() throws IOException {
setRadius method throws an exception if
p2();
}
catch (IOException ex) {
p2();
radius is negative.
}
...
}
} CircleWithException
(a) (b) Run
TestCircleWithException
21 22
Rethrowing Exceptions The finally Clause

try { try {
statements; statements;
} }
catch(TheException ex) { catch(TheException ex) {
handling ex;
perform operations before exits; }
throw ex; finally {
} finalStatements;
}
23 24
animation animation
Trace a Program Execution Trace a Program Execution
Suppose no
exceptions in the
statements The final block is
try { try { always executed
statements; statements;
} }
catch(TheException ex) { catch(TheException ex) {
handling ex; handling ex;
} }
finally { finally {
finalStatements; finalStatements;
} }
Next statement; Next statement;
25 26
animation animation
Next statement in the try { Suppose an exception
method is executed statement1; of type Exception1 is
try { statement2; thrown in statement2
statements; statement3;
} }
catch(TheException ex) { catch(Exception1 ex) {
handling ex;
handling ex;
}
} finally {
finally { finalStatements;
finalStatements; }
}
Next statement;
Next statement;
27 28
animation animation
try { The exception is try { The final block is
statement1; handled. statement1; always executed.
statement2; statement2;
} }
catch(Exception1 ex) { catch(Exception1 ex) {
} }
finally { finally {
} }
29 30
animation animation
try {
try { The next statement in statement1; statement2 throws an
statement1; the method is now statement2; exception of type
statement2; executed. statement3; Exception2.
statement3; }
catch(Exception1 ex) {
} handling ex;
catch(Exception1 ex) { }
handling ex; catch(Exception2 ex) {
} handling ex;
finally { throw ex;
finalStatements; }
} finally {
finalStatements;
Next statement; }
Next statement;
31 32
animation animation
try { try {
statement1; Handling exception statement1; Execute the final block
} }
} }
throw ex; throw ex;
} }
finally { finally {
} }
33 34
animation
Trace a Program Execution Cautions When Using Exceptions
try {
statement1; Rethrow the exception
statement2; ✦ Exception handling separates error-handling
and control is
}
statement3; transferred to the caller code from normal programming tasks, thus
catch(Exception1 ex) { making programs easier to read and to modify.
handling ex;
} Be aware, however, that exception handling
catch(Exception2 ex) {
handling ex; usually requires more time and resources
}
throw ex;
because it requires instantiating a new exception
finally { object, rolling back the call stack, and
finalStatements;
} propagating the errors to the calling methods.
Next statement;
35 36
When to Throw Exceptions When to Use Exceptions
✦ An exception occurs in a method. If you want When should you use the try-catch block in the code?
You should use it to deal with unexpected error
the exception to be processed by its caller, you conditions. Do not use it to deal with simple, expected
should create an exception object and throw it. situations. For example, the following code
If you can handle the exception in the method
try {
where it occurs, there is no need to throw it.
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
37 38
Defining Custom Exception Classes

When to Use Exceptions
✦ Use the exception classes in the API whenever possible.
is better to be replaced by ✦ Define custom exception classes if the predefined
classes are not sufficient.
if (refVar != null)
System.out.println(refVar.toString()); ✦ Define custom exception classes by extending
Exception or a subclass of Exception.
else
System.out.println("refVar is null");
39 40
Companion
Custom Exception Class Example Website
Assertions
In Listing 13.8, the setRadius method throws an exception if the An assertion is a Java statement that enables
radius is negative. Suppose you wish to pass the radius to the
handler, you have to create a custom exception class.
you to assert an assumption about your
program. An assertion contains a Boolean
expression that should be true during
program execution. Assertions can be used to
assure program correctness and avoid logic
InvalidRadiusException
errors.
CircleWithRadiusException
TestCircleWithRadiusException Run
41 42
Companion Companion
Website
Declaring Assertions Website
Executing Assertions
An assertion is declared using the new Java keyword When an assertion statement is executed, Java evaluates the
assert in JDK 1.4 as follows: assertion. If it is false, an AssertionError will be thrown. The
AssertionError class has a no-arg constructor and seven
overloaded single-argument constructors of type int, long, float,
assert assertion; or double, boolean, char, and Object.
assert assertion : detailMessage;
For the first assert statement with no detail message, the no-arg
where assertion is a Boolean expression and constructor of AssertionError is used. For the second assert
detailMessage is a primitive-type or an Object value. statement with a detail message, an appropriate AssertionError
constructor is used to match the data type of the message. Since
AssertionError is a subclass of Error, when an assertion becomes
false, the program displays a message on the console and exits.
43 44
Companion Companion Compiling Programs with
Website
Executing Assertions Example Website
Assertions
public class AssertionDemo {
public static void main(String[] args) {
int i; int sum = 0;
Since assert is a new Java keyword introduced in
for (i = 0; i < 10; i++) { JDK 1.4, you have to compile the program using
}
sum += i;
a JDK 1.4 compiler. Furthermore, you need to
assert i == 10; include the switch –source 1.4 in the compiler
}
assert sum > 10 && sum < 5 * 10 : "sum is " + sum;
command as follows:
}
javac –source 1.4 AssertionDemo.java
NOTE: If you use JDK 1.5, there is no need to

use the –source 1.4 option in the command.
45 46
Companion
Website
Running Programs with Companion
Website Using Exception Handling or
Assertions Assertions
By default, the assertions are disabled at runtime. To Assertion should not be used to replace exception
enable it, use the switch –enableassertions, or –ea for handling. Exception handling deals with unusual
short, as follows: circumstances during program execution. Assertions are
to assure the correctness of the program. Exception
java –ea AssertionDemo
handling addresses robustness and assertion addresses
Assertions can be selectively enabled or disabled at correctness. Like exception handling, assertions are not
class level or package level. The disable switch is – used for normal tests, but for internal consistency and
disableassertions or –da for short. For example, the validity checks. Assertions are checked at runtime and
following command enables assertions in package can be turned on or off at startup time.
package1 and disables assertions in class Class1.
java –ea:package1 –da:Class1 AssertionDemo
47 48
Companion
Website Using Exception Handling or Companion
Website Using Exception Handling or
Assertions, cont. Assertions, cont.
Do not use assertions for argument checking in public Use assertions to reaffirm assumptions. This gives you
methods. Valid arguments that may be passed to a public more confidence to assure correctness of the program. A
method are considered to be part of the method’s common use of assertions is to replace assumptions with
contract. The contract must always be obeyed whether
assertions in the code.
assertions are enabled or disabled. For example, the
following code in the Circle class should be rewritten
using exception handling.
public void setRadius(double newRadius) {

assert newRadius >= 0;
radius = newRadius;
}
49 50
Companion
Website
Using Exception Handling or The File Class
Assertions, cont. The File class is intended to provide an abstraction that
deals with most of the machine-dependent complexities
Another good use of assertions is place assertions in a
switch statement without a default case. For example, of files and path names in a machine-independent
fashion. The filename is a string. The File class is a
switch (month) { wrapper class for the file name and its directory path.
case 1: ... ; break;
...
default: assert false : "Invalid month: " + month
}
51 52
Obtaining file properties and manipulating file
Problem: Explore File Properties
Objective: Write a program that demonstrates how to
create files in a platform-independent way and use the
methods in the File class to obtain their properties. The
following figures show a sample run of the program on
Windows and on Unix.
TestFileClass Run
53 54
Text I/O Writing Data Using PrintWriter

java.io.PrintWriter
A File object encapsulates the properties of a file or a path, +PrintWriter(filename: String) Creates a PrintWriter for the specified file.
but does not contain the methods for reading/writing data +print(s: String): void Writes a string.
+print(c: char): void Writes a character.
from/to a file. In order to perform I/O, you need to create +print(cArray: char[]): void Writes an array of character.
objects using appropriate Java I/O classes. The objects +print(i: int): void Writes an int value.
+print(l: long): void Writes a long value.
contain the methods for reading/writing data from/to a file. +print(f: float): void Writes a float value.
This section introduces how to read/write strings and +print(d: double): void Writes a double value.
+print(b: boolean): void Writes a boolean value.
numeric values from/to a text file using the Scanner and
Also contains the overloaded A println method acts like a print method; additionally it
PrintWriter classes. println methods. prints a line separator. The line separator string is defined
Also contains the overloaded by the system. It is \r\n on Windows and \n on Unix.
printf methods. The printf method was introduced in §4.6, “Formatting
Console Output and Strings.”
.
WriteData Run
55 56
Try-with-resources Reading Data Using Scanner
Programmers often forget to close the file. JDK 7 provides java.util.Scanner
the followings new try-with-resources syntax that +Scanner(source: File) Creates a Scanner object to read data from the specified file.
+Scanner(source: String) Creates a Scanner object to read data from the specified string.
automatically closes the files. +close() Clos es th is scanner.
+hasNext(): boolean Returns true if this scanner has another token in its input.
try (declare and create resources) { +next(): String Returns next token as a stri ng.
Use the resource to process the file; +nextByte(): byte Returns next token as a b yte.
+nextShort(): short Returns next token as a short.
} +nextInt(): int Returns next token as an int.
+nextLong(): long Returns next token as a long.
+nextFloat(): float Returns next token as a float.
+nextDouble(): double Returns next token as a d ouble.
+useDelimiter(pattern: St ring): Sets this scanner’s delimiting pattern.
Scanner
WriteDataWithAutoClose Run
ReadData Run
57 58
Problem: Replacing Text Reading Data from the Web

Write a class named ReplaceText that replaces a string in a text Just like you can read data from a file on your
file with a new string. The filename and strings are passed as computer, you can read data from a file on the
command-line arguments as follows:
java ReplaceText sourceFile targetFile oldString newString Web.
For example, invoking
java ReplaceText FormatString.java t.txt StringBuilder StringBuffer
replaces all the occurrences of StringBuilder by StringBuffer in
FormatString.java and saves the new file in t.txt.
ReplaceText Run
59 60
Reading Data from the Web Case Study: Web Crawler
URL url = new URL("www.google.com/index.html"); This case study develops a program that travels the
Web by following hyperlinks.
After a URL object is created, you can use the
openStream() method defined in the URL class to open an
input stream and use this stream to create a Scanner object
as follows:
Scanner input = new Scanner(url.openStream());

ReadFileFromURL Run
61 62
Case Study: Web Crawler Case Study: Web Crawler

Add the starting URL to a list named listOfPendingURLs;
The program follows the URLs to traverse the Web. To while listOfPendingURLs is not empty {
avoid that each URL is traversed only once, the program Remove a URL from listOfPendingURLs;
maintains two lists of URLs. One list stores the URLs if this URL is not in listOfTraversedURLs {
Add it to listOfTraversedURLs;
pending for traversing and the other stores the URLs that Display this URL;
have already been traversed. The algorithm for this Exit the while loop when the size of S is equal to 100.
Read the page from this URL and for each URL contained in the page {
program can be described as follows: Add it to listOfPendingURLs if it is not is listOfTraversedURLs;
}
}
}
WebCrawler Run
63 64

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Chapter 12 Exceptions and File Motivations

Input/Output When a program runs into a runtime error, the

Original slides by Daniel Liang

QuotientWithException Run InputMismatchExceptionDemo Run

Exception Types System Errors

Exception NullPointerException Exception NullPointerException

Many more classes Many more classes

Many more classes Many more classes

Checked Exceptions vs. Unchecked Exceptions

Error VirtualMachineError Unchecked

Many more classes

Declaring Exceptions Throwing Exceptions

public void myMethod() TheException ex = new TheException();

Catch or Declare Checked Exceptions

Call Stack void p2() throws IOException {

main method main method main method main method ...

Rethrowing Exceptions The finally Clause

Next statement; Next statement;

Next statement; Next statement;

Next statement; Next statement;

Defining Custom Exception Classes

javac –source 1.4 AssertionDemo.java

NOTE: If you use JDK 1.5, there is no need to

public void setRadius(double newRadius) {

Text I/O Writing Data Using PrintWriter

Problem: Replacing Text Reading Data from the Web

Scanner input = new Scanner(url.openStream());

Case Study: Web Crawler Case Study: Web Crawler

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