Input/Ouput and Exception Handling

2 Exceptions  An exception is an object that describes an unusual or erroneous situation  Exceptions are thrown by a program, and may be caught and handled by another part of the program  A program can be separated into a normal execution flow and an exception execution flow  An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

3 Exception Handling  Java has a predefined set of exceptions and errors that can occur during execution  A program can deal with an exception in one of three ways: ignore it handle it where it occurs handle it an another place in the program  The manner in which an exception is processed is an important design consideration

4 Exception Handling  If an exception is ignored by the program, the program will terminate abnormally and produce an appropriate message  The message includes a call stack trace that indicates the line on which the exception occurred  The call stack trace also shows the method call trail that lead to the attempted execution of the offending line The getMessage method returns a string explaining why the exception was thrown The printStackTrace method prints the call stack trace  See Zero.java Zero.java

5 Exception Handling public class Zero { // Deliberately divides by zero to produce an exception. public static void main (String[] args) { int numerator = 10; int denominator = 0; System.out.println (numerator / denominator); System.out.println ("This text will not be printed."); } } Exception in thread "main" java.lang.ArithmeticException: / by zero at Zero.main(Zero.java:8)

6 The try Statement  To process an exception when it occurs, the line that throws the exception is executed within a try block  A try block is followed by one or more catch clauses, which contain code to process an exception  Each catch clause has an associated exception type and is called an exception handler  When an exception occurs, processing continues at the first catch clause that matches the exception type  See ProductCodes.java ProductCodes.java

7 The finally Clause  A try statement can have an optional clause following the catch clauses, designated by the reserved word finally  The statements in the finally clause always are executed  If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete  If an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

8 Exception Propagation  An exception can be handled at a higher level if it is not appropriate to handle it where it occurs  Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method  A try block that contains a call to a method in which an exception is thrown can be used to catch that exception  See Propagation.java Propagation.java  See ExceptionScope.java ExceptionScope.java

9 Exception Handling >java Propagation Program beginning. Level 1 beginning. Level 2 beginning. Level 3 beginning. The exception message is: / by zero The call stack trace: java.lang.ArithmeticException: / by zero at ExceptionScope.level3(ExceptionScope.java:54) at ExceptionScope.level2(ExceptionScope.java:41) at ExceptionScope.level1(ExceptionScope.java:18) at Propagation.main(Propagation.java:17) Level 1 ending. Program ending.

10 The throw Statement  A programmer can define an exception by extending the Exception class or one of its descendants  Exceptions are thrown using the throw statement  Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown  When an exception is thrown, method terminates immediately Execution continues with an exception handler

public class BankAccount { public void withdraw(double amount) { if (amount > balance) { IllegalArgumentException exception = new IllegalArgumentException("Amount exceeds balance"); throw exception; } balance = balance - amount; }... } Example

Hierarchy of Exception Classes

13 Checked Exceptions  Two types of exceptions: Checked oThe compiler checks that you don't ignore them oDue to external circumstances that the programmer cannot prevent oMajority occur when dealing with input and output oFor example, IOException Unchecked: oExtend the class RuntimeException or Error oThey are the programmer's fault oExamples of runtime exceptions: NumberFormatException IllegalArgumentException NullPointerException oExample of error: OutOfMemoryError

14 Checked Exceptions  An exception is either checked or unchecked  A checked exception either must be caught by a method, or must be listed in the throws clause of any method that may throw or propagate it  A throws clause is appended to the method header  The compiler will issue an error if a checked exception is not handled appropriately

Unchecked Exceptions  An unchecked exception does not require explicit handling, though it could be processed that way  The only unchecked exceptions in Java are objects of type RuntimeException or any of its descendants  Errors are similar to RuntimeException and its descendants Errors should not be caught Errors do not require a throws clause

I/O Streams  A stream is a sequence of bytes that flow from a source to a destination  In a program, we read information from an input stream and write information to an output stream  A program can manage multiple streams simultaneously

I/O Streams  The java.io package contains many classes that allow us to define various streams with particular characteristics  Some classes assume that the data consists of characters  Others assume that the data consists of raw bytes of binary information  Streams can be further subdivided as follows: data stream, which acts as either a source or destination processing stream, which alters or manipulates the basic data in the stream

I/O Streams Character Streams Byte Streams Data Streams Processing Streams Input Streams Output Streams

Character vs. Byte Streams  A character stream manages 16-bit Unicode characters  A byte stream manages 8-bit bytes of raw binary data A program must determine how to interpret and use the bytes in a byte stream Typically they are used to read and write sounds and images  The InputStream and OutputStream classes (and their descendants) represent byte streams  The Reader and Writer classes (and their descendants) represent character streams

Data vs. Processing Streams  A data stream represents a particular source or destination such as a string in memory or a file on disk  A processing stream (also called a filtering stream) manipulates the data in the stream It may convert the data from one format to another It may buffer the stream

The IOException Class  Operations performed by the I/O classes may throw an IOException A file intended for reading or writing might not exist Even if the file exists, a program may not be able to find it The file might not contain the kind of data we expect  An IOException is a checked exception

Standard I/O  There are three standard I/O streams: standard input – defined by System.in standard output – defined by System.out standard error – defined by System.err  System.in typically represents keyboard input  System.out and System.err typically represent a particular window on the monitor screen  We use System.out when we execute println statements

Writing Text Files  To write to a file, construct a PrintWriter object PrintWriter out = new PrintWriter("output.txt");  If file already exists, it is emptied before the new data are written into it  If file doesn't exist, an empty file is created  Use print and println to write into a PrintWriter : out.println(29.95); out.println("Hello, World!");  You must close a file when you are done processing it: out.close(); Otherwise, not all of the output may be written to the disk file  When the input or output file doesn't exist, a FileNotFoundException can occur

Reading Text Files  Information can be read from and written to text files by declaring and using the correct I/O streams  The FileReader class represents an input file containing character data  The FileReader and BufferedReader classes together create a convenient text file output stream  See CheckInventory.java CheckInventory.java  See InventoryItem.java InventoryItem.java

Reading Text Files  Simplest way to read text: use Scanner class  To read from a disk file, construct a FileReader  Then, use the FileReader to construct a Scanner object FileReader reader = new FileReader("input.txt"); Scanner in = new Scanner(reader);  Use the Scanner methods to read data from file next, nextLine, nextInt, and nextDouble

A Sample Program  Reads all lines of a file and sends them to the output file, preceded by line numbers  Sample input file: Mary had a little lamb Whose fleece was white as snow. And everywhere that Mary went, The lamb was sure to go!  Program produces the output file: /* 1 */ Mary had a little lamb /* 2 */ Whose fleece was white as snow. /* 3 */ And everywhere that Mary went, /* 4 */ The lamb was sure to go!  Program can be used for numbering Java source files

fileio/LineNumberer.java 01: import java.io.FileReader; 02: import java.io.FileNotFoundException; 03: import java.io.PrintWriter; 04: import java.util.Scanner; 05: 06: public class LineNumberer 07: { 08: public static void main(String[] args) 09: throws FileNotFoundException 10: { 11: Scanner console = new Scanner(System.in); 12: System.out.print("Input file: "); 13: String inputFileName = console.next(); 14: System.out.print("Output file: "); 15: String outputFileName = console.next(); 16: 17: FileReader reader = new FileReader(inputFileName); 18: Scanner in = new Scanner(reader); 19: PrintWriter out = new PrintWriter(outputFileName); 20: int lineNumber = 1;

fileio/LineNumberer.java (cont.) 21: 22: while (in.hasNextLine()) 23: { 24: String line = in.nextLine(); 25: out.println("/* " + lineNumber + " */ " + line); 26: lineNumber++; 27: } 28: 29: out.close(); 30: } 31: }

File Dialog Boxes Continued

File Dialog Boxes (cont.)  JFileChooser chooser = new JFileChooser(); FileReader in = null; if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) { File selectedFile = chooser.getSelectedFile(); reader = new FileReader(selectedFile);... }