Review. Errors. Ø Users may enter data in the wrong form Ø Files may not exist Ø Program code has bugs!*

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1 Objectives Exceptions Ø Why Exceptions? Ø Throwing exceptions Packages Javadocs Eclipse Log into your machines Review How do we specify that a class or a method cannot be subclassed/overridden? Compare and contrast abstract classes and interfaces When should a class be abstract? Ø If you extend an abstract class, do you have to override all abstract methods? When should you create/use an interface? What is the keyword for defining your class to adhere to an interface? Sept 25, 2013 Sprenkle - CSCI209 1 Sept 25, 2013 Sprenkle - CSCI209 2 Review: Using an Interface or Abstract Class Interfaces ü Any class can use ü Can implement multiple interfaces No implementation - Implementing methods multiple times - Adding a method to interface will break classes that implement Abstract Classes Contain partial implementation - Can t extend/subclass multiple classes ü Add non-abstract methods without breaking subclasses Sept 25, 2013 Sprenkle - CSCI209 3 Iteration over Code Assignment 4 à Assignment 5 Demonstrates typical design/implementation process Ø Start with your original code design Inheritance from GamePiece class Ø Realize it could be designed better Make GamePiece class abstract Use an array of GamePiece objects Easier to add new functionality to Game Major part of problem-solving is figuring out how to break problem into smaler pieces Sept 25, 2013 Sprenkle - CSCI209 4 EXCEPTIONS Errors Programs encounter errors when they run Ø Users may enter data in the wrong form Ø Files may not exist Ø Program code has bugs!* When an error occurs, a program should do one of two things: Ø Revert to a stable state and continue Ø Allow the user to save data and then exit the program gracefully * (Of course, not your programs) Sept 25, 2013 Sprenkle - CSCI209 5 Sept 25, 2013 Sprenkle - CSCI

2 Java Method Behavior Normal/correct case: return specified return type Error case: does not return anything, throws an Exception Ø An exception is an event that occurs during execution of a program that disrupts normal flow of program's instructions Ø Exception: object that encapsulates error information Similar to Python Sept 25, 2013 Sprenkle - CSCI209 7 Handling Exceptions JVM s exception-handling mechanism searches for an exception handler the error recovery code Ø Exception handler deals with a particular exception Ø Searches call stack for a method that can handle (or catch) the exception Call stack Sept 25, 2013 Sprenkle - CSCI Search order for handler Throwable Error Throwable All exceptions indirectly derive from Throwable Ø Child classes: Error and Exception Important Throwable methods Ø getmessage() Detailed message about error Ø printstacktrace() Prints out where problem occurred and path to reach that point Ø getstacktrace() Get the stack in non-text format Exception Sept 25, 2013 Sprenkle - CSCI209 9 Printing Stack Trace Example java.io.filenotfoundexception: fred.txt at java.io.fileinputstream.<init>(fileinputstream.java) at java.io.fileinputstream.<init>(fileinputstream.java) at ExTest.readMyFile(ExTest.java:19) at ExTest.main(ExTest.java:7) How helpful is this output? How user friendly is it? Sept 25, 2013 Sprenkle - CSCI Printing Stack Trace Example java.io.filenotfoundexception: fred.txt at java.io.fileinputstream.<init>(fileinputstream.java) at java.io.fileinputstream.<init>(fileinputstream.java) at ExTest.readMyFile(ExTest.java:19) at ExTest.main(ExTest.java:7) How helpful is this output? How user friendly is it? Useful for debugging your code Generate/display user-friendly errors in finished product Often requires higher-level code to handle exception Sept 25, 2013 Sprenkle - CSCI Exception Classification: Error An internal error Strong convention: reserved for JVM Ø JVM-generated when resource exhaustion or an internal problem Example: Out of Memory error When can that happen in Java? Program s code should not and can not throw an object of this type Unchecked exception Sept 25, 2013 Sprenkle - CSCI

3 Exception Classification: Exception 1.RuntimeException: something that happens because of a programming error Ø Unchecked exception Ø Examples: ArrayOutOfBoundsException, NullPointerException, ClassCastException 2. Checked exceptions Ø A well-written application should anticipate and recover from Compiler enforces Ø Examples: IOException, SQLException Sept 25, 2013 Sprenkle - CSCI Exception Classification Error Unchecked Checked Checked: All non- RuntimeExceptions Throwable IOException Others SQLException Checked Exception Part of java.lang package Unchecked RuntimeException Sept 25, 2013 Sprenkle - CSCI Types of Exceptions Unchecked Any exception that derives from Error or RuntimeException Programmer does not create/handle Try to make sure that they don t occur Often indicates programmer error Ø E.g., precondition violations, not using API correctly Checked Any other exception Programmer creates and handles checked exceptions Compiler-enforced checking Ø Improves reliability* For conditions from which caller can reasonably be expected to recover Sept 25, 2013 Sprenkle - CSCI Types of Unchecked Exceptions 1. Derived from the class Error Ø Any line of code can generate because it is an internal error Ø Don t worry about what to do if this happens 2. Derived from the class RuntimeException Ø Indicates a bug in the program Ø Fix the bug Ø Examples: ArrayOutOfBoundsException, NullPointerException, ClassCastException Sept 25, 2013 Sprenkle - CSCI Checked Exceptions Need to be handled by your program Ø Compiler-enforced Ø Improves reliability* For each method, tell the compiler: Ø What the method returns Ø What could possibly go wrong Advertise the exceptions that a method throws Helps users of your interface know what method does and lets them decide how to handle exceptions Discussion: Why Checked and Unchecked Exceptions? Why do we have exceptions that the compiler doesn t force the programmer to check? Ø Think about examples of unchecked exceptions (ArrayOutOfBoundsException, NullPointerException, ClassCastException) and when those exceptions can occur Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI

4 Methods and Exceptions Example BufferedReader has method readline() Ø Reads a line from a stream, such as a file or network connection Method header: Part of Advertising public String readline() throws IOException THROWING EXCEPTIONS Sept 25, 2013 Sprenkle - CSCI Interpreting the header: readline will Ø return a String (if everything went right) Ø throw an IOException (if something went wrong) Sept 25, 2013 Sprenkle - CSCI Advertising Checked Exceptions Advertising: in Javadoc, document under what conditions each exception is thrown tag Examples of when your method should advertise the checked exceptions that it may throw Ø Your method calls a method that throws a checked exception Ø Your method detects an error in its processing and decides to throw an exception Example: Passing an Exception Up public String readdata(bufferedreader in) throws IOException { String str1; Throws an IOException str1 = in.readline(); return str1; readdata() calls readline(), which can throw an IOException If readline() throws this exception to our method Ø readdata() throws the exception as well Ø Whoever calls readdata will handle exception Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI Throwing An Exception We Created if (month < 1 month > 12) { throw new IllegalArgumentException(); 1. Create a new object of class IllegalArgumentException Ø Class derived from RuntimeException 2. throw it Ø Method ends at this point Ø Calling method handles exception Equivalent in Python? Sept 25, 2013 Sprenkle - CSCI A More Descriptive Exception Four constructors for most Exception classes Ø Default (no parameters) Ø Takes a String message Describe the condition that generated this exception more fully Ø 2 more if (month < 1 month > 12) { throw new IllegalArgumentException( "Month is not in valid range (1-12)"); Best messages include all state that could have contributed to the problem Sept 25, 2013 Sprenkle - CSCI

5 Common Exceptions Name Purpose IllegalArgumentException When caller passes in inappropriate argument IllegalStateException Invocation is illegal because of receiving object s state. (Ex: closing a closed window) Both inherit from RuntimeException May seem like these cover everything but only used for certain kinds of illegal arguments and exceptions Not used when Ø A null argument passed in; should be a NullPointerException Ø Pass in invalid index for an array; should be an IndexOutOfBoundsException Sept 25, 2013 Sprenkle - CSCI Factorial Alternatives public static double factorial( int x ) { if( x < 0 ) return 0.0; double fact = 1.0; while( x > 1 ) { fact *= x; x--; return fact; Sept 25, 2013 Sprenkle - CSCI Factorial Alternatives public static double factorial( int x ) { if( x < 0 ) throw new IllegalArgumentException("x" + "must be >= 0"); double fact = 1.0; while( x > 1 ) { fact *= x; x--; return fact; What are the pros and cons of these approaches? Note, no throws clause in method signature. Why? IllegalArgumentException: Thrown to indicate that a method has been passed an illegal or inappropriate argument Goal: Failure Atomicity After an object throws an exception, the object should be in a well-defined, usable state Ø A failed method invocation should leave object in state prior to invocation Approaches: Ø Check parameters/state before performing operation(s) Ø Do the failure-prone operations first Ø Use recovery code to rollback state Ø Apply to temporary object first, then copy over values Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI Practice Practice public void setbirthday(int month, int day) { public void setbirthday(int month, int day) { How should we implement this method? What are some problems we could face? How should we implement this method? Ø Rule of thumb: Handle error checking first Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI

6 PACKAGES Packages Hierarchical structure of Java classes Ø Directories of directories Use import to access packages Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI java lang net Object String util Date Fully qualified name: java.lang.string Standard Practice To reduce chance of a conflict between names of classes, put classes in packages Use package keyword to say that a class belongs to a package: Ø package java.util; Ø First line in class file Typically, use a unique prefix, similar to domain names Ø com.ibm Ø edu.wlu.cs.logic Sept 25, 2013 Sprenkle - CSCI Importing Packages Can import one class at a time or all the classes within a package Examples: import java.util.date; import java.io.*; Ø * form may increase compile time BUT, no effect on run-time performance Import entire package Sept 25, 2013 Sprenkle - CSCI Javadocs Special comments, which are used to generate HTML documentation Syntax: * Comment */ JAVADOCS Documentation is a love letter that you write to your future self. Damian Conway Put before a class, a method, or a field to describe the respective class/method/field Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI

7 Javadoc Can contain HTML syntax in description Example block tags to describe your <paramname> <description> (include special cases) Ø More that we ll get to later Sept 25, 2013 Sprenkle - CSCI Examples * A simple Java class that models a Chicken. The * state of the chicken is its name, height, and weight * Sara Sprenkle */ Tags always go last in Javadoc comment the height of the chicken, in centimeters */ n the String representing the name of the chicken */ Expect these types of comments on all methods from now on Sept 25, 2013 Sprenkle - CSCI Generating Javadocs From command-line: javadoc [options] [packagenames] [sourcefiles] [@files] Or, using Eclipse Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI Software System Award The Software System Award is given to an institution or individuals recognized for developing software systems that have had a lasting influence, reflected in contributions to concepts and/or commercial acceptance. created by IBM. Eclipse changed the way builders think about tools by defining a set of user interaction paradigms for which domain-specific variants are plugged in and customized for their tool. Conceived to address perceived shortcomings in proprietary software development tools, Eclipse allowed developers to seamlessly integrate their own extensions, specializations, and personalizations. Sept 25, 2013 Sprenkle - CSCI Software System Award It revolutionized the notion of an Integrated Development Environment (IDE) by identifying the conceptual kernel underlying any IDE. Eclipse was designed as an open, extensible platform for application development tools with a Java IDE built on top. In 2004 Eclipse became a not-for-profit corporation. The IBM Eclipse team included John Wiegand, Dave Thomson, Gregory Adams, Philippe Mulet, Julian Jones, John Duimovich, Kevin Haaland, Stephen Northover (now with Oracle), and Erich Gamma (now with Microsoft). Sept 25, 2013 Sprenkle - CSCI

8 Open source integrated development environment (IDE) for Java Has market share for Java IDEs Described as an open extensible IDE for anything and nothing in particular Provides a robust Java development environment Incorporates popular software development tools like JUnit and CVS Ø More on those later this semester Plugins allow extensibility Sept 25, 2013 Sprenkle - CSCI Project/Code Organization workspace directory contains all projects Ø Located in your home directory, unless you specified otherwise Use projects to organize your code Within a project Ø src/ directory contains.java files Ø bin/ directory contains.class files Often hidden in GUI Sept 25, 2013 Sprenkle - CSCI Java Made Easier Creating class s basic functionality Ø See Source and Refactor menus Gives you a list of methods for an object Ø After you type object. Ø Then shows parameters for methods Automatically creates template of Javadoc Ø When you type Autocompletion of variables, methods Formatting code Shows unused fields/variables Shows compiler errors Sept 25, 2013 Sprenkle - CSCI Eclipse Demo Why can Eclipse provide this functionality? (but IDLE can t?) Show Birthday class Ø Override equals and tostring methods Create a new class Ø Generate Main method, Comments Create a String object, see methods used Demonstrate refactoring Ø Rename a field Ø Extract a method (month name) Run the Birthday Class (main) Ø Command line arguments Sept 25, 2013 Sprenkle - CSCI Eclipse Hints After you have written a method, type before the method, and then hit enter and the Javadocs template will be automatically generated for you Use command-spacebar for possible completions Your Eclipse Practice Start up Eclipse Create a new Java project: Assign6 Create a new Java class: Test Ø Checkbox: Main method, comments Ø Add an instance field: private int myvar; Ø Use Source menu to generate a constructor Ø Use Source menu to generate tostring Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI

9 Installing at Home Go to Select Downloads Then Eclipse IDE for Java EE Developers Ø For developing web applications and other enterprise applications Ø Requires Java 6 to run We re using Java 7 for the code we write Ø You ll need to install both Java 6 and 7 Eclipse Discussion Helpful hints Ø Control-spacebar Ø Format the file Ø Auto-templates for Javadocs Sept 25, 2013 Sprenkle - CSCI Sept 25, 2013 Sprenkle - CSCI TODO Assignment 6 Ø Practice using Eclipse Ø Creating abstract class, child classes Ø Responses to questions are important Ø Due Monday not difficult but time-consuming Looking ahead: Exam next Friday Sept 25, 2013 Sprenkle - CSCI