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Understanding Java Exceptions. Outline What exceptions are for What exceptions are NOT for Catching & Throwing exceptions Exception Specifications Standard.

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Presentation on theme: "Understanding Java Exceptions. Outline What exceptions are for What exceptions are NOT for Catching & Throwing exceptions Exception Specifications Standard."— Presentation transcript:

1 Understanding Java Exceptions

2 Outline What exceptions are for What exceptions are NOT for Catching & Throwing exceptions Exception Specifications Standard Java Exceptions Exceptions and Polymorphism The finally clause Resource Management Uncaught Exceptions

3 What Exceptions are For To handle Bad Things I/O errors, other runtime errors when a function fails to fulfill its specification so you can restore program stability (or exit gracefully)

4 What Exceptions are For ~ continued ~ To force you to handle Bad Things because return codes can be tedious and sometimes you’re lazy

5 Example File I/O public FileReader(String fileName)String throws FileNotFoundExceptionFileNotFoundException public void close() throws IOExceptionIOException

6 import java.io.*; class OpenFile { public static void main(String[] args) { if (args.length > 0) { try { // Open a file: FileReader f = new FileReader(args[0]); System.out.println(args[0] + " opened"); f.close(); } catch (IOException x) { System.out.println(x); }

7 What Exceptions are For ~ continued ~ To signal errors from constructors because constructors have no return value

8 What Exceptions are NOT For NOT For Alternate Returns: e.g., when end-of-file is reached: while ((s = f.readLine()) != null) … Exceptions are only for the exceptional!

9 Catching Exceptions Wrap code to be checked in a try-block checking occurs all the way down the execution stack try-blocks can be nested control resumes at most enclosed matching handler

10 Catching Exceptions ~ continued ~ Place one or more catch-clauses after try- block runtime system looks back up the call stack for a matching handler subclass types match superclass types catching Exception catches everything (almost) handlers are checked in the order they appear place most derived types first! execution resumes after last handler if you let it (could branch or throw)

11 Throwing Exceptions Must throw objects derived (ultimately) from Throwable Usually derive from java.lang.Exception The class name is the most important attribute of an exception Can optionally include a message Provide two constructors: MyException( ) MyException(String s)

12 Throwing Exceptions ~ continued ~ Control is passed up the execution stack to a matching handler Various methods exist for processing exceptions: getMessage( ) toString( )(class name + message) printStackTrace( )

13 Throwing Exceptions ~ continued ~ Functions must “advertise” their exceptions every function must specify the “checked” exceptions it (or its callees!) may throw Callers must do one of two things: handle your exceptions with try-catch, or advertise your exceptions along with theirs

14 Sample Program FixedStack implements a stack with an array of Object various methods throw exceptions class StackException StackTest must handle StackExceptions

15 class StackException extends Exception { StackException() {} StackException(String msg) { super(msg); }

16 class FixedStack { private int capacity; private int size; private Object[] data; public FixedStack(int cap) { data = new Object[cap]; capacity = cap; size = 0; } public void push(Object o) throws StackException { if (size == capacity) throw new StackException("overflow"); data[size++] = o; }

17 public Object pop() throws StackException { if (size <= 0) throw new StackException("underflow"); return data[--size]; } public Object top() throws StackException { if (size <= 0) throw new StackException("underflow"); return data[size-1]; } public int size() { return this.size; }

18 class StackTest { public static void main(String[] args) { FixedStack s = new FixedStack(3); doTest(s); } public static void doTest(FixedStack s) { try { System.out.println("Initial size = " + s.size()); s.push("one"); s.push(new Integer(2)); s.push(new Float(3.0)); s.push("one too many"); // error! } catch(StackException x) { System.out.println(x); }

19 try { System.out.println("Top: " + s.top()); System.out.println("Popping..."); while (s.size() > 0) System.out.println(s.pop()); } catch(StackException x) { throw new InternalError(x.toString()); } /* Output: Initial size = 0 StackException: overflow Top: 3.0 Popping one */

20 Using printStackTrace( ) catch(StackException x) { x.printStackTrace(System.out); } … StackException: overflow at FixedStack.push(FixedStack.java:18) at StackTest.doTest(StackTest.java, Compiled Code) at StackTest.main(StackTest.java:6)

21 Standard Java Exceptions Throwable ExceptionError RuntimeException IOException...

22 Class java.lang.Exception The one you usually derive from “Checked Exceptions” specifications checked at compile time you must either catch or advertise these Used for recoverable errors Not programmer errors

23 java.lang.Exception Subclasses ~ sample ~ AWTException ClassNotFoundException CloneNotSupportedException IOException NoSuchFieldException

24 Class java.lang.Error For JVM Failures and other Weird Things let program terminate InternalError is one of these Don’t catch them you don’t know what to do! These are “unchecked exceptions” not required to advertise

25 java.lang.Error Subclasses AWTError LinkageError … ThreadDeath VirtualMachineError InternalError, OutOfMemoryError, StackOverflowError, UnknownError

26 Class java.lang.RuntimeException Stupid Name! Same as logic_error in C++ Program logic errors e.g., bad cast, using a null handle, array index violation, etc. Shouldn’t happen! fixed during testing Similar in spirit to C’s assert( ) macro mainly for debugging These are called “unchecked exceptions”

27 java.lang.RuntimeException Subclasses (sample) ArithmeticException (e.g., divide by 0) ArrayStoreException ClassCastException IllegalArgumentException IndexOutOfBoundsException NullPointerException UnsupportedOperationException

28 Principle “Use checked exceptions for recoverable conditions and run-time exceptions for programming errors” (Bloch, Effective Java)

29 “Fixing” FixedStack StackException should be a runtime exception Then the “throws” specifications aren’t needed

30 class StackException extends RuntimeException { StackException() {} StackException(String msg) { super(msg); } Then remove all “throws” specifications from FixedStack

31 Exceptions and Inheritance ~ Subclass Overrides ~ Methods overridden in subclasses must maintain the parent method’s contract substitutability cannot add exceptions to specification can omit, however can throw subclasses of parent’s exceptions

32 // Relaxing the Exception Specification class Parent { public void f() throws Exception {} } class Child extends Parent { public void f()// OK! {} } class Override { public static void main(String[] args) { Child c = new Child(); c.f(); }

33 // Throwing a Subclass Exception class MyException extends Exception {} class AnotherException extends MyException {} class Parent { public void f() throws MyException {} } class Child extends Parent { public void f() throws AnotherException {} } class Override { public static void main(String[] args) throws AnotherException { Child c = new Child(); c.f(); }

34 Exception-handling Syntax ~ The Whole Picture ~ try { // normal code (conditionally executes) } catch (ExceptionType1 x) { // handle ExceptionType1 error } … catch (ExceptionTypeN x) { // handle ExceptionTypeN error } finally { // invariant code ("always" executes) }

35 The finally Clause For code that must ALWAYS run No matter what! Even if a return or break occurs first Exception: System.exit( ) Placed after handlers (if they exist) try-block must either have a handler or a finally-block

36 class FinallyTest { public static void f() throws Exception { try { // 0 // return; // 1 // System.exit(0); // 2 // throw new Exception(); // 3a } catch (Exception x) { // throw new Exception(); // 3b } finally { System.out.println("finally!"); } System.out.println("last statement"); }

37 public static void main(String[] args) { try { f(); } catch(Exception x) {} }

38 Program Output 0: finally! last statement 1: finally! 2: (no output) 3a: same as 0: 3a + 3b: compiler error (last statement not reachable)

39 Managing Resources Other than memory files, connections, etc. Need to deallocate, even if exceptions occur Use finally UPDATE: As of Java 7, you can use the try-with-resources statement. See Core Java Ch. 11, pp (9 th ed.)

40 import java.io.*; class Manage { public static void f(String fname) throws IOException { FileReader f = null; // must define outside try try { f = new FileReader(fname); System.out.println("File opened"); int c = f.read(); // read a byte //... } finally { if (f != null) { System.out.println("File closed"); f.close(); // beware lost exception!!! }

41 public static void main(String[] args) { try { f(args[0]); } catch (Exception x) { System.out.println(x); }

42 Program Output If no file name given (args.length == 0): java.lang.ArrayIndexOutOfBoundsException: 0 If file doesn’t exist: java.io.FileNotFoundException: If file opened successfully: file opened file closed

43 When to Handle Exceptions Note: Manage.f( ) didn’t catch anything wouldn’t know what to do if it did! You often let exceptions pass up the call stack Or you can re-throw in a catch throw x; // in a handler where x was caught or re-throw a new type of exception

44 Exception Etiquette Don’t catch what you can’t (at least partially) handle re-throw if only partially handled (“catch & release”: if you’re not going to eat it, throw it back!) Don’t catch & ignore catch (Exception x){} // disables exceptions!

45 How Exceptions Work When an exception is thrown execution backtracks up the runtime stack (list of active function invocations) Each stack frame contains information regarding local handlers, if any Otherwise, execution returns up to the next caller, looking for a suitable catch What happens if there isn’t a matching catch?

46 Uncaught Exceptions What if there is no handler for an exception? The thread dies! exceptions belong to a thread (stack-specific)


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