Chapter 12 Exception Handling And Text IO

animationTrace a Program Executiontry 1 ex) {handling ex;}catch(Exception2 ex) {handling ex;throw ex;}finally {finalStatements;}Execute the final blockNext statement;Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.34

animationTrace a Program Executiontry 1 ex) {handling ex;}catch(Exception2 ex) {handling ex;throw ex;}finally {finalStatements;}Rethrow the exceptionand control istransferred to the callerNext statement;Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.35

Cautions When Using ExceptionsException handling separates error-handlingcode from normal programming tasks, thusmaking programs easier to read and to modify.Be aware, however, that exception handlingusually requires more time and resourcesbecause it requires instantiating a new exceptionobject, rolling back the call stack, andpropagating the errors to the calling methods.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.36

When to Throw ExceptionsAn exception occurs in a method. If you wantthe exception to be processed by its caller, youshould create an exception object and throw it.If you can handle the exception in the methodwhere it occurs, there is no need to throw it.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.37

When to Use ExceptionsWhen should you use the try-catch block in the code?You should use it to deal with unexpected errorconditions. Do not use it to deal with simple, expectedsituations. For example, the following codetry {System.out.println(refVar.toString());}catch (NullPointerException ex) {System.out.println("refVar is null");}Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.38

When to Use Exceptionsis better to be replaced byif (refVar ! tem.out.println("refVar is null");Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.39

Defining Custom Exception ClassesUse the exception classes in the API whenever possible.Define custom exception classes if the predefinedclasses are not sufficient.Define custom exception classes by extendingException or a subclass of Exception.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.40

Custom Exception Class ExampleIn Listing 13.8, the setRadius method throws an exception if theradius is negative. Suppose you wish to pass the radius to thehandler, you have to create a custom exception ionTestCircleWithRadiusExceptionRunLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.41

CompanionWebsiteAssertionsAn assertion is a Java statement that enablesyou to assert an assumption about yourprogram. An assertion contains a Booleanexpression that should be true duringprogram execution. Assertions can be used toassure program correctness and avoid logicerrors.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.42

CompanionWebsiteDeclaring AssertionsAn assertion is declared using the new Java keywordassert in JDK 1.4 as follows:assert assertion; orassert assertion : detailMessage;where assertion is a Boolean expression anddetailMessage is a primitive-type or an Object value.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.43

CompanionWebsiteExecuting AssertionsWhen an assertion statement is executed, Java evaluates theassertion. If it is false, an AssertionError will be thrown. TheAssertionError class has a no-arg constructor and sevenoverloaded single-argument constructors of type int, long, float,double, boolean, char, and Object.For the first assert statement with no detail message, the no-argconstructor of AssertionError is used. For the second assertstatement with a detail message, an appropriate AssertionErrorconstructor is used to match the data type of the message. SinceAssertionError is a subclass of Error, when an assertion becomesfalse, the program displays a message on the console and exits.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.44

CompanionWebsiteExecuting Assertions Examplepublic class AssertionDemo {public static void main(String[] args) {int i; int sum 0;for (i 0; i 10; i ) {sum i;}assert i 10;assert sum 10 && sum 5 * 10 : "sum is " sum;}}Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.45

CompanionWebsiteCompiling Programs withAssertionsSince assert is a new Java keyword introduced inJDK 1.4, you have to compile the program usinga JDK 1.4 compiler. Furthermore, you need toinclude the switch –source 1.4 in the compilercommand as follows:javac –source 1.4 AssertionDemo.javaNOTE: If you use JDK 1.5, there is no need touse the –source 1.4 option in the command.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.46

CompanionWebsiteRunning Programs withAssertionsBy default, the assertions are disabled at runtime. Toenable it, use the switch –enableassertions, or –ea forshort, as follows:java –ea AssertionDemoAssertions can be selectively enabled or disabled atclass level or package level. The disable switch is –disableassertions or –da for short. For example, thefollowing command enables assertions in packagepackage1 and disables assertions in class Class1.java –ea:package1 –da:Class1 AssertionDemoLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.47

CompanionWebsiteUsing Exception Handling orAssertionsAssertion should not be used to replace exceptionhandling. Exception handling deals with unusualcircumstances during program execution. Assertions areto assure the correctness of the program. Exceptionhandling addresses robustness and assertion addressescorrectness. Like exception handling, assertions are notused for normal tests, but for internal consistency andvalidity checks. Assertions are checked at runtime andcan be turned on or off at startup time.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.48

CompanionWebsiteUsing Exception Handling orAssertions, cont.Do not use assertions for argument checking in publicmethods. Valid arguments that may be passed to a publicmethod are considered to be part of the method’scontract. The contract must always be obeyed whetherassertions are enabled or disabled. For example, thefollowing code in the Circle class should be rewrittenusing exception handling.public void setRadius(double newRadius) {assert newRadius 0;radius newRadius;}Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.49

CompanionWebsiteUsing Exception Handling orAssertions, cont.Use assertions to reaffirm assumptions. This gives youmore confidence to assure correctness of the program. Acommon use of assertions is to replace assumptions withassertions in the code.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.50

CompanionWebsiteUsing Exception Handling orAssertions, cont.Another good use of assertions is place assertions in aswitch statement without a default case. For example,switch (month) {case 1: . ; break;case 2: . ; break;.case 12: . ; break;default: assert false : "Invalid month: " month}Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.51

The File ClassThe File class is intended to provide an abstraction thatdeals with most of the machine-dependent complexitiesof files and path names in a machine-independentfashion. The filename is a string. The File class is awrapper class for the file name and its directory path.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.52

Obtaining file properties and manipulating fileLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.53

Problem: Explore File PropertiesObjective: Write a program that demonstrates how tocreate files in a platform-independent way and use themethods in the File class to obtain their properties. Thefollowing figures show a sample run of the program onWindows and on Unix.TestFileClassLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.Run54

Text I/OA File object encapsulates the properties of a file or a path,but does not contain the methods for reading/writing datafrom/to a file. In order to perform I/O, you need to createobjects using appropriate Java I/O classes. The objectscontain the methods for reading/writing data from/to a file.This section introduces how to read/write strings andnumeric values from/to a text file using the Scanner andPrintWriter classes.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.55

Writing Data Using PrintWriterjava.io.PrintWriter PrintWriter(filename: String)Creates a PrintWriter for the specified file. print(s: String): voidWrites a string. print(c: char): voidWrites a character. print(cArray: char[]): voidWrites an array of character. print(i: int): voidWrites an int value. print(l: long): voidWrites a long value. print(f: float): voidWrites a float value. print(d: double): voidWrites a double value. print(b: boolean): voidWrites a boolean value.Also contains the overloadedprintln methods.A println method acts like a print method; additionally itprints a line separator. The line separator string is definedby the system. It is \r\n on Windows and \n on Unix.The printf method was introduced in §3.6, “FormattingConsole Output and Strings.”Also contains the overloadedprintf methods.WriteDataLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.Run56

Try-with-resourcesProgrammers often forget to close the file. JDK 7 providesthe followings new try-with-resources syntax thatautomatically closes the files.try (declare and create resources) {Use the resource to process the file;}WriteDataWithAutoCloseRunLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.57

Reading Data Using Scannerjava.util.Scanner Scanner(source: File) Scanner(source: String)Creates a Scanner object to read data from the specified file.Creates a Scanner object to read data from the specified string. close() hasNext (): booleanClos es th is scanner.Returns true if this scanner has another token in its input. next(): StringReturns next token as a stri ng. nextByte(): byte nextShort(): shortReturns next token as a b yt e.Returns next token as a short. nextInt(): int nextLong(): longReturns next token as an int.Returns next token as a long. nextFloat(): floatReturns next token as a float. nextDouble(): double useDelimiter(pattern: String):ScannerReturns next token as a d ouble.Sets this scanner’s delimiting pattern.ReadDataLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.Run58

Problem: Replacing TextWrite a class named ReplaceText that replaces a string in a textfile with a new string. The filename and strings are passed ascommand-line arguments as follows:java ReplaceText sourceFile targetFile oldString newStringFor example, invokingjava ReplaceText FormatString.java t.txt StringBuilder StringBufferreplaces all the occurrences of StringBuilder by StringBuffer inFormatString.java and saves the new file in t.txt.ReplaceTextRunLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.59

Reading Data from the WebJust like you can read data from a file on yourcomputer, you can read data from a file on theWeb.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.60

Reading Data from the WebURL url new URL("www.google.com/index.html");After a URL object is created, you can use theopenStream() method defined in the URL class to open aninput stream and use this stream to create a Scanner objectas follows:Scanner input new Scanner(url.openStream());ReadFileFromURLRunLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.61

Case Study: Web CrawlerThis case study develops a program that travels theWeb by following hyperlinks.Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.62

Case Study: Web CrawlerThe program follows the URLs to traverse the Web. Toavoid that each URL is traversed only once, the programmaintains two lists of URLs. One list stores the URLspending for traversing and the other stores the URLs thathave already been traversed. The algorithm for thisprogram can be described as follows:Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.63

Case Study: Web CrawlerAdd the starting URL to a list named listOfPendingURLs;while listOfPendingURLs is not empty {Remove a URL from listOfPendingURLs;if this URL is not in listOfTraversedURLs {Add it to listOfTraversedURLs;Display this URL;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 {Add it to listOfPendingURLs if it is not is listOfTraversedURLs;}}}WebCrawlerRunLiang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. Allrights reserved.64

Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rig