Chapter 10: Exceptions and I/O Streams

Chapter 10: Exceptions and I/O Streams Original Slides by John Lewis and William Loftus Modified significantly by Bob Roggio, July 2007

Exceptions and I/O Streams • Now we can explore two related topics further: exceptions and input/output streams • Chapter 8 focuses on: • Exception Handling • Uncaught Exceptions • The try-catch statement • The finally Clause • Exception propagation • The Exception Class Hierarchy • Checked and unchecked Exceptions • I/O Exceptions • I/O Streams

Exception Handling • An exception is an object that describes an unusual or erroneous situation • Exceptions are thrown by a program or the runtime environment, and may be caught and handled by another part of the program • This means that when an exception occurs, you can sometimes code what you want to happen. • An exception is an object that defines unusual or erroneous situations. • An exception is something that should not normally happen or happen infrequently. • Your book has a list of common exceptions to be thrown. • Array index out of bounds; specified file cannot be found; division by zero, …

Exception Handling • As its name implies, exceptions are ‘exceptions’ to the normal way of doing business and we can design efficient ways to handle them if/when they occur. • 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

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 • You have no doubt seen these! • 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 • We will see these ahead.

Example: Uncaught Exception // Zero.java Author: Lewis/Loftus 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."); }// end main() }// end Zero Output: Exception in thread “main” java.lang.ArithmeticException: / by zero at Zero.main (Zero.java:17) Note: tells problem and class.method where it occurred and statement number! There’s no code to handle this exception explicitly. The second System.out.println does not execute, because the exception occurs first. First line says what exception was thrown and provides some info about why it was thrown. Remaining lines are the call stack trace Can call methods in exception class: getMessage returns a string explaining reason for exception printStackTrace prints the call stack trace. First line says that Exception was thrown. Other lines: Call trace (ahead) Call stack trace problem Exception class Method, file, and line number where exception occurred.

The try-catch Statement • To process an exception when it occurs, the line that can cause (throw) an exception is executed within a try block • A try block is followed by one or morecatch clauses, which contain code to process an exception • The catch clauses are called exception handlers. • The try code is executed. If all is well, execution continues following any catch clauses present. • If an exception occurs, processing continues at the first catch clause that matches the exception type class. • Once the catch is executed, control returns to the next statement following the last catch claus

public class ProductCodes // Author: Lewis / Loftus. Demo use of try-catch block. { public static void main (String[] args) { String code; char zone; int district, valid = 0, banned = 0; Scanner scan – new Scanner (System.in); System.out.print ("Enter product code (XXX to quit): "); code = scan.nextLine(); while (!code.equals ("XXX")) { try { zone = code.charAt(9); district = Integer.parseInt(code.substring(3, 7)); valid++; if (zone == 'R' && district > 2000) banned++; } // end try catch (StringIndexOutOfBoundsException exception) { System.out.println ("Improper code length: " + code); } // end catch catch (NumberFormatException exception) { System.out.println ("District is not numeric: " + code); } // end catch System.out.print ("Enter product code (XXX to quit): "); code = Keyboard.readString(); } // end while System.out.println ("# of valid codes entered: " + valid); System.out.println ("# of banned codes entered: " + banned); } // end main() } // end class Note: try-catch statements identify a block of statements that might cause (throw) an exception. The catch statement (follows the try) tells how a particular kind of exception is handled. catch statements called exception handlers Can have multiple catch clauses. If no problem in try, control resumes after last catch clause.. If an exception occurs and there is an appropriate exception handler for the exception class corresponding to the error. Control resumes following last catch clause. StringIndexOutOFBounds can be thrown by charAt() or substring() methods. NumberFormatException can be thrown by the parseInt method, if substring does not contain a valid integer.

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 • A finally clause is sometimes used to ensure certain code executes no matter what… More later on this.

Exception Propagation • An exception can be handled at a higher level if it is not appropriate to handle it where it occurs • Exceptions propagateup through the method-calling hierarchy until they are caught and handled or until they reach the level of the 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(page 546) • See ExceptionScope.java(page 547) • Let’s look at some code…

Propagating the Exception and Exception Scope (next slide) //************************************************************ // Propagation.java Author: Lewis/Loftus // // Demonstrates exception propagation. //************************************************************ public class Propagation { //----------------------------------------------------------------- // Invokes the level1 method to begin the exception demonstration. //----------------------------------------------------------------- static public void main (String[] args) { ExceptionScope demo = new ExceptionScope(); // creates object of type demo. System.out.println("Program beginning."); demo.level1(); // invokes demo.level1() System.out.println("Program ending."); }// end main() }// end Propagation class.

// ExceptionScope.java // Demonstrates exception propagation. public class ExceptionScope { // Catches and handles the exception that is thrown in level3. public void level1() { System.out.println("Level 1 beginning."); try { level2(); // transfers control to level2(). }// end try catch (ArithmeticException problem) { System.out.println (); System.out.println ("The exception message is: " + problem.getMessage()); System.out.println (); System.out.println ("The call stack trace:"); problem.printStackTrace(); System.out.println (); }//end catch() System.out.println("Level 1 ending."); }// end level1() // Serves as an intermediate level. The exception propagates through this method back to level1 public void level2() { System.out.println("Level 2 beginning."); level3 (); // transfers control to level 3(). System.out.println("Level 2 ending."); }// end level 2() // Performs a calculation to produce an exception. It is not caught and handled at this level. public void level3 () { int numerator = 10, denominator = 0; // OK. Here’s the obvious error. Note: no exception handler in level 3() System.out.println("Level 3 beginning."); // To catch an exception at a higher level, the method with the exception int result = numerator / denominator; // must be invoked inside a try block that has a catch clause to handle exception. System.out.println("Level 3 ending."); // Control is passed up to level2(). But no handler here; control passes to level1() } // end level3() // Because level2() is invoked inside a try block that has a catch clause to handle }// end class // the exception, the exception is caught and handled now. // Note that the last System.out.println statements are not executed! Once caught, the level1() ‘ending’ message executes. An exception can be caught within the method where the exception occurs. (try clause is found within a method; ditto for catch clauses. But, if a method calls another method and an exception occurs there, that called method can handle the exception OR control can return to the calling method that contains a try…catch pair and be handled up there. ========================================== Notice that we have catch(ArithmeticException problem) An exception (recall) is an object and thus problem is an object of type ArithmeticException. So, we can call: problem.getMessage() and problem.printStaceTrace()

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) Level1 ending Program ending. Note: control does not revert to the interrupted methods.!

The Exception Class Hierarchy • Important to note that all the classes that define the various exceptions are related by inheritance. (p.549) • See Object note: Throwable is parent to both Error and Exceptin! and many types of exceptions are derived from Exception class. Throwable Error Exception <Others> RunTimeException ArithmeticException IndexOutOFBoundsException NullPointerException <Others> There are many other child classes that define specific exceptions are part of other packages. Inheritance relationships can span package boundaries.

The throw Statement • Because Throwable is a base class and Exception inherits from it, a programmer can define his/her own exceptions 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 • Let’s see how we can create our own! We must derive our own class. • See CreatingExceptions.java(page 550) • See OutOfRangeException.java(page 551)

//********************************************************************//******************************************************************** // CreatingExceptions.java Author: Lewis/Loftus // // Demonstrates the ability to define an exception via inheritance. //******************************************************************** public class CreatingExceptions { // Creates an exception object and possibly throws it. public static void main (String[] args) throws OutOfRangeException { // This is not part of the standard Java library! final int MIN = 25, MAX = 40; Scanner scan = new Scanner (System.in); OutOfRangeException problem = // object is created (inheritance) and initialized. new OutOfRangeException ("Input value is out of range."); System.out.print ("Enter an integer value between " + MIN + " and " + MAX + ", inclusive: "); int value = scan.nextInt(); // Determines if the exception should be thrown if (value < MIN || value > MAX) throw problem; System.out.println ("End of main method."); // may never reach }// end main() }// end CreatingEXceptions

//********************************************************************//******************************************************************** // OutOfRangeException.java Author: Lewis/Loftus // // Represents an exceptional condition in which a value is out of // some particular range. //******************************************************************** public class OutOfRangeException extends Exception Here we are creating OUR OWN { Exception Handler (like ArrayIndexOutOfRange) //----------------------------------------------------------------- by extending the object, Exception. // Sets up the exception object with a particular message. //----------------------------------------------------------------- OutOfRangeException (String message) { super (message); } // end of OUtOFRangeException method() }//end class OutOfRangeException This is a simple example, but is very typical when we want to trap and process our own potential errors. But it is important to note that this class is derived from Exception which is derived from Throwable.. Throwable provides the ability to use the throw statement! There are other simple ways to accommodate an input error such as this. A simple if statement can check and provide a message; alternatively, you may design your own Exceptions!

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 • We are saying that if a problem occurs, the method will throw an exception. • 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 to not require a throws clause

Next three slides moved from last slides to here because we have used these objects and methods. • They do deal with the various kinds of streams that follow this discussion though…

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 • In the next program, we create a file of test data from our program. • We use the FileWriter class to represent a text output file. • PrintWriter provides print and println methods to help FileWriter, which has minimal method support for manipulating data.

// TestData.java // Demonstrates the use of a character file output stream. import java.util.Random; import java.io.*; public class TestData { // Creates a file of test data that consists of ten lines each containing ten integer values in range 10-99. public static void main (String[] args) throws IOException { final int MAX = 10; int value; Random rand = new Random(); String file = "test.dat"; // creating a String object, file, that contains the name of the external file. FileWriter fw = new FileWriter (file); BufferedWriter bw = new BufferedWriter (fw); PrintWriter outFile = new PrintWriter (bw); // PrintWriter contains methods print(), println(), close()… // all these work together. Have added a layer in the file stream configuration to include a //BufferedWriter object, bw. This simply gives the output stream buffering capabilities which makes the processing more efficient. Buffering is always recommended in writing text files. Note how the objects from one action are fed as input parameters to the next!! for (int line=1; line <= MAX; line++) { for (int num=1; num <= MAX; num++) { value = rand.nextInt (90) + 10; outFile.print (value + " "); }// end inner for outFile.println (); // note we have not supplied an IOException handler. Thus, if anything goes } // end outer for // wrong, we will allow the program to terminate. outFile.close(); // Because all IOExcpetions areh checked exceptions, we must include the System.out.println ("Output file has been created: " + file); // throws clause on the method header to indicate }// end main() // that they may be thrown. } // end class TestData

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 just as we used FileWriter, BufferedWriter, and PrintWriter in the previous program.

I/O Streams and I/O Exceptions • A stream is a sequence of bytes that flow from a source to a destination • In a program, we read information from an inputstream and write information to an outputstream • A program can manage multiple streams simultaneously • There are three streams considered standard I/O streams. • System class contains three object reference variables: in, out, err that represent the three standard I/O streams. (public, static, so they can be accessed through the System class) • Have used standard output stream with calls to System.out.println and standard input stream when we used a Scanner object. • By default, they represent particular I/O devices: keyboard input, monitor screen output. (These could be changed)

I/O Streams • The java.io package contains many classes that allow us to define various other streams with particular characteristics • Some classes assume that the data consists of characters; other classes assume the data consists of raw bytes of binary information. • Some classes allow us to manipulate the data in the stream, such as buffering info or numbering it. By judiciously combining classes, we can represent a stream of information that has the characteristics we wish. • This is a huge topic!!!

Data Streams Processing Streams Input Streams Output Streams Character Streams Byte Streams I/O Streams - Subdivisions • Data stream acts as either a source or destination • Processing stream - alters/manipulates basic data in the stream • All kinds of opportunities for Input / Output Exceptions!!!

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 OutputStreamclasses (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 •  We have used data streams when we have read files and processing streams, when we have buffered the input streams! • We must design our programs to be as resilient to these kinds of errors as possible. They will occur! So we need to be able to handle them!

Chapter 10: Exceptions and I/O Streams