10/7/15. MediaItem tostring Method. Objec,ves. Using booleans in if statements. Review. Javadoc Guidelines

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1 Objec,ves Excep,ons Ø Wrap up Files Streams MediaItem tostring Method public String tostring() { String classname = getclass().tostring(); StringBuilder rep = new StringBuilder(classname); return rep.tostring(); Addresses some issues some of you had Can you guess what is happening? What issues does this solve? Why does it work? Oct 7, 2015 Sprenkle - CSCI209 1 Oct 7, 2015 Sprenkle - CSCI209 2 Using booleans in if statements if( incollection == true ) { System.out.println("In collection"); Review What are the two types of excep%ons? What are some ways to handle excep,ons? What does it mean to adver,se an excep,on? if( incollection ) { System.out.println("In collection"); What Eclipse tricks did you figure out? Second: less naïve implementation Oct 7, 2015 Sprenkle - CSCI209 3 Oct 7, 2015 Sprenkle - CSCI209 4 Review: Excep,on Classifica,on Error Unchecked Checked Checked: All non- RuntimeExceptions Throwable IOException Others SQLException Checked Exception Part of java.lang package Unchecked RuntimeException Oct 7, 2015 Sprenkle - CSCI209 5 Javadoc Guidelines Always report if throw checked excep,ons Report any unchecked excep,ons that the caller might reasonably want to catch Ø Excep,on: NullPointerException Ø Allows caller to handle (or not) Ø Document excep,ons that are independent of the underlying implementa,on Errors should not be documented as they are unpredictable Oct 7, 2015 Sprenkle - CSCI

2 Prac,ce: try/catch/finally Blocks statement1; statement2; catch (EOFException e) { statement3; statement4; finally { statement5; Which statements run if: Ø Neither statement1 nor statement2 throws an excep,on Ø statement1 throws an EOFException Ø statement2 throws an EOFException Ø statement1 throws an IOException What to do with a Caught Excep,on? Print the stack aber the excep,on occurs Ø What else can we do? Generally, two op,ons: 1. Catch the excep,on and recover from it 2. Pass excep,on up to whoever called it Oct 7, 2015 Sprenkle - CSCI209 7 Oct 7, 2015 Sprenkle - CSCI209 8 Design Decision: To Throw or Catch? Problem: lower- level excep,on Handled here propagated up to higher- level code Example: user enters account informa,on and gets excep,on message field exceeds allowed length in database What do you think happened? Is this a good solution? GUI Oct 7, 2015 Sprenkle - CSCI209 9 Design Decision: To Throw or Catch? Problem: lower- level excep,on Handled here propagated up to higher- level code Example: user enters account informa,on and gets excep,on message field exceeds allowed length in database Ø Lost context Ø Lower- level detail pollu,ng higher- level API GUI DB Solution: higher-levels should catch lower-level exceptions and throw them in terms of higher-level abstraction Oct 7, 2015 Sprenkle - CSCI Exception here Excep,on Transla,on // Call lower-level abstraction catch (LowerLevelException ex) { // log exception throw new HigherLevelException(); Special case: Excep,on Chaining Ø When higher- level excep,on needs info from lower- level excep,on Most standard Exceptions have this constructor // Call lower-level abstraction catch (LowerLevelException cause) { // log exception throw new HigherLevelException(cause); Oct 7, 2015 Sprenkle - CSCI Guidelines for Excep,on Transla,on Avoidance! Ø Try to ensure that lower- level APIs succeed Ø Ex: verify that your parameters sa,sfy invariants Insulate higher- level from lower- level excep,ons Ø Handle in some reasonable way Ø Always log problem so admin can check If can t do previous two, then use excep,on transla,on Oct 7, 2015 Sprenkle - CSCI

3 Summary: Methods Throwing Excep,ons API documenta,on tells you if a method can throw an excep,on Ø If so, you must handle it If your method could possibly throw an excep,on (by genera,ng it or by calling another method that could), adver,se it! Ø If you can t handle every error, that s OKlet whoever is calling you worry about it Ø However, they can only handle the error if you adver,se the excep,ons you can t deal with Oct 7, 2015 Sprenkle - CSCI Programming with Excep,ons Excep,on handling is slow Use one big try block instead of nes,ng try- catch blocks Ø Speeds up Excep,on Handling Ø Otherwise, code gets too messy Don't ignore excep,ons (e.g., catch block does nothing) Ø Beler to pass them along to higher calls Oct 7, 2015 Sprenkle - CSCI Crea,ng Our Own Excep,on Class Try to reuse an exis,ng excep,on Ø Match in name as well as seman,cs If you cannot find a defined Java Exception class that describes your error, implement a new Exception class! Crea,ng Our Own Excep,on Class public class FileFormatException extends IOException { public FileFormatException() { What happens in this constructor implicitly? public FileFormatException(String message) { super(message); // other 2 standard constructors Is this a checked or unchecked exception? Can now throw excep,ons of type FileFormatException Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI Guidelines for Crea,ng Your Own Excep,on Classes Include accessor methods to get more informa,on about the cause of the excep,on Ø failure- capture informa,on Checked or unchecked excep,on? Ø Checked: forces API user to handle BUT more difficult to use API Has to handle all checked excep,ons Ø Use checked excep,on if excep,onal condi,on cannot be prevented by proper use of API and API user can take a useful ac,on aberward Oct 7, 2015 Sprenkle - CSCI Prac,ce: Designing a New Excep,on Class Scenario: When an alempt to make a purchase with a gib card fails because card doesn t have enough money, throw a new excep,on that you created Recall that all Exceptions are Throwable, so they have the methods: getmessage(), printstacktrace(), getstacktrace() How would someone else use your class? What constructors, additional method(s) may you want to add for your exception class? Oct 7, 2015 Sprenkle - CSCI

4 Discussion: Benefits of Excep,ons Been talking about details Why does Java have excep,ons as part of the language? Why does Java add some features that Python doesn t have? Oct 7, 2015 Sprenkle - CSCI Benefits of Excep,ons Force error checking/handling Ø Otherwise, won t compile Ø Does not guarantee good excep,on handling Ease debugging Ø Stack trace Separates error- handling code from regular code Ø Error code is in catch blocks at end Ø Descrip,ve messages with excep,ons Propagate methods up call stack Ø Let whoever cares about error handle it Group and differen,ate error types Oct 7, 2015 Sprenkle - CSCI java.io.file Class FILES Represents a file or directory Provides func,onality such as Ø Storage of the file on the disk Ø Determine if a par,cular file exists Ø When file was last modified Ø Rename file Ø Remove/delete file Ø Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI Making a File Object Simplest constructor takes full file name (including path) Ø If don t supply path, Java assumes current directory (.) File f1 = new File("chicken.data"); Ø Creates a File object represen,ng a file named chicken.data in the current directory Ø Does not create a file with this name on disk Files, Directories, and Useful Methods A File object can represent a file or a directory Ø Directories are special files in most modern opera,ng systems Use isdirectory() and/or isfile() for type of file File object represents Use exists() method Ø Determines if a file exists on the disk Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI

5 More File Constructors String for the path, String for filename Break any of Java s Principles? File f2 = new File( "/csdept/local/courses/cs209/handouts", "chicken.data"); File for directory, String for filename File dir = new File( "/csdept/local/courses/cs209/handouts"); File f4 = new File(dir, "chicken.data"); Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI Not Portable Accessing the file system is inherently not portable Ø In Windows, paths are c:\\dir Ø In Unix, paths are /home/courses/dir Relies on underlying file system/opera,ng system to perform ac,ons Oct 7, 2015 Sprenkle - CSCI Handling Portability Issues Sta,c fields in File class Ø static separator Unix: "/" Windows: "\\" Ø static pathseparator For separa,ng a list of paths Unix: ":" Windows: ";" Use rela,ve paths, with separators Use configura,on files Why two \\? Oct 7, 2015 Sprenkle - CSCI java.io.file Class 25+ methods Ø Manipulate files and directories Ø Crea,ng and removing directories Ø Making, renaming, and dele,ng files Ø Informa,on about file (size, last modified) Ø Crea,ng temporary files Ø See online API documenta,on STREAMS FileTest.java Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI

6 Streams Java handles input/output using streams, which are sequences of bytes Streams Java handles input/output using streams, which are sequences of bytes input stream: an object from which we can read a sequence of bytes abstract class: java.io.inputstream Oct 7, 2015 Sprenkle - CSCI output stream: an object to which we can write a sequence of bytes abstract class: java.io.outputstream Oct 7, 2015 Sprenkle - CSCI Java Streams MANY (80+) types of Java streams In java.io package Why stream abstrac,on? Ø Informa,on stored in different sources is accessed in essen,ally the same way Example sources: file, on a web server across the network, string Ø Allows same methods to read or write data, regardless of its source Create an InputStream or OutputStream of the appropriate type java.io Classes Overview Two types of stream classes, based on datatype: Byte, Text Abstract base classes for binary data: InputStream Abstract base classes for text data: Reader OutputStream Writer Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI Byte Streams For binary data In java.io package Character Streams For Text In java.io package Handle any character in Unicode set Abstract Base Classes Abstract Base Classes Shaded: Read to/write from data sinks White: Does some processing Shaded: Read to/write from data sinks White: Does some processing Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI

7 Console I/O Output: Ø System.out is a PrintStream object Input Ø System.in is an InputStream object Ø Throws excep,ons if format of input data is not correct Handle in try/catch Opening & Closing Streams Streams are automa%cally opened when constructed Close a stream by calling its close() method Ø Close a stream as soon as object is done with it Ø Free up system resources Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI Reading & Wri,ng Bytes Abstract parent class: InputStream Ø abstract int read() reads one byte from the stream and returns it Concrete input stream classes override read() to provide appropriate func,onality Ø e.g., FileInputStream s read() reads one byte from a file Similarly, OutputStream class has abstract write() to write a byte to the stream Reading & Wri,ng Bytes read() and write() are blocking opera,ons Ø If a byte cannot be read from the stream, the method waits (does not return) un,l a byte is read available() : get the number of bytes that are available for reading Example use: int bytesavailable = System.in.available(); if (bytesavailable > 0) System.in.read(byteBuffer); Oct 7, 2015 Sprenkle - CSCI Oct 7, 2015 Sprenkle - CSCI TODO Assignment 7: Due Monday Midterm: Wednesday Oct 7, 2015 Sprenkle - CSCI