CMSC 132 Object Oriented Programming II Exceptions Effective Java Department of Computer Science University of Maryland College Park 1 Overview Exceptions Motivation Representation in Java Effective Java 2 Exception Handling Performing action in response to exception Example actions Ignore exception Print error message Request new data Retry action Approaches 1 Exit program 2 Exit method returning error code 3 Throw exception 3 Problem May not be able to handle error locally Not enough information in method class Need more information to decide action Handle exception in calling function s instead Decide at application level instead of library Examples Incorrect data format ask user to reenter data Unable to open file ask user for new filename Insufficient disk space ask user to delete files Will need to propagate exception to caller s 4 Exception Handling Exit Program Approach Exit program with error message error code Example if error System err println Error found System exit 1 message error code Problem Drastic solution Event must be handled by user invoking program Program may be able to deal with some exceptions 5 Exception Handling Error Code Approach Exit function with return value error code Example A if error return 1 B if retval A 1 return 1 Problems Calling function must check process error code May forget to handle error code May need to return error code to caller Agreement needed on meaning of error code Error handling code mixed with normal code 6 Exception Handling Throw Exception Approach Throw exception caught in parent s catch block Example A if error throw new ExceptionType Java exception backtracks to B caller s until matching catch try block found A catch ExceptionType e action 7 Exception Handling Throw Exception Advantages Compiler ensures exceptions are caught eventually No need to explicitly propagate exception to caller Backtrack to caller s automatically Class hierarchy defines meaning of exceptions No need for separate definition of error codes Exception handling code separate clearly marked 8 Representing Exceptions Exceptions represented as Objects derived from class Throwable Code public class Throwable extends Object No error message Throwable Throwable String mesg Error message String getMessage Return error mesg void printStackTrace Record methods called location 9 Representing Exceptions Java Exception class hierarchy Two types of exceptions checked unchecked 10 Representing Exceptions Java Exception class hierarchy ClassNotFoundException ClassNotFoundException Exception Exception CloneNotSupportedException CloneNotSupportedException IOException IOException AWTException AWTException ArithmeticException ArithmeticException NullPointerException NullPointerException RuntimeException RuntimeException Object Object Throwable Throwable IndexOutOfBoundsException IndexOutOfBoundsException NoSuchElementException NoSuchElementException LinkageError LinkageError VirtualMachoneError VirtualMachoneError Error Error AWTError AWTError Checked Checked Unchecked Unchecked 11 Unchecked Exceptions Class Error RunTimeException Serious errors not handled by typical program Usually indicate logic errors Example NullPointerException IndexOutOfBoundsException Catching unchecked exceptions is optional Handled by Java Virtual Machine if not caught 12 Checked Exceptions Class Exception except RunTimeException Errors typical program should handle Used for operations prone to error Example IOException ClassNotFoundException Compiler requires catch or declare Catch and handle exception in method OR Declare method can throw exception force calling function to catch or declare exception in turn Example void A throws ExceptionType 13 Designing Using Exceptions Use exceptions only for rare events Not for common cases checking end of loop High overhead to perform catch Place statements that jointly accomplish task into single try catch block Use existing Java Exceptions if possible 14 Designing Using Exceptions Avoid simply catching ignoring exceptions Poor software development style Example try throw new ExceptionType1 throw new ExceptionType2 throw new ExceptionType3 catch Exception e catches all exceptions ignores exception returns 15 Overview Exceptions Effective Java Puzzlers Principles 16 Effective Java Title Effective Java Programming Language Guide Author Joshua Bloch Contents Useful tips for Java programming 17 Java Puzzlers By J Bloch Java Simple and elegant Need to avoid some sharp corners Puzzlers Java code fragments Expose some tricky aspects of Java Effective Java Ways of avoiding Java programming problems 18 What s In A Name public class Name private String myName public Name String n myName n public boolean equals Object o if o instanceof Name return false Name n Name o return myName equals n myName public static void main String args Set s new HashSet s add new Name Donald System out println s contains new Name Donald Output 1 True 2 False 3 It Varies Name class violates Java hashCode contract If you override equals must also override hashCode 19 You re Such A Character public class Trivial Output public static void main String args 1 Ha System out print H a 2 HaHa 3 Neither System out print H a Prints Ha169 H a evaluated as int then converted to String Use string concatenation with care At least one operand must be a String 20 The Confusing Constructor public class Confusing public Confusing Object o System out println Object public Confusing double dArray System out println double array public static void main String args new Confusing null Output 1 Object 2 double array 3 Neither When multiple overloadings apply the most specific wins Avoid overloading If you overload avoid ambiguity 21 Time For A Change Problem If you pay 2 00 for a gasket that costs 1 10 how much change do you get Output public class Change 1 0 9 2 0 90 public static void main String args 3 Neither System out println 2 00 1 10 Prints 0 8999999999999999 Decimal values can t be represented exactly by float or double Avoid float or double where exact answers are required Use BigDecimal int or long instead 22 A Private Matter class Base public String name Base class Derived extends Base private String name Derived public class PrivateMatter public static void main String args System out println new Derived name Output 1 Derived 2 Base 3 Neither Compiler error in class PrivateMatter Can t access name Private field can hide public Avoid hiding public fields 23 Effective Java Topics 1 Creating
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