Understanding Java Exceptions

Outline

What exceptions are forWhat exceptions are NOT forCatching & Throwing exceptionsException SpecificationsStandard Java ExceptionsExceptions and PolymorphismThe finally clauseResource ManagementUncaught Exceptions

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)

What Exceptions are For~ continued ~

To force you to handle Bad Things because return codes can be tedious and sometimes you’re lazy

ExampleFile I/O

public FileReader(String fileName) throws FileNotFoundException

public void close() throws IOException

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); } } }}

What Exceptions are For~ continued ~

To signal errors from constructors because constructors have no return value

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!

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

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)

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)

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( )

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

Sample Program

FixedStack implements a stack with an array of Object various methods throw exceptions class StackException

StackTest must handle StackExceptions

class StackException extends Exception{ StackException() {}

StackException(String msg) { super(msg); }}

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; }

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; }}

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); }

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 = 0StackException: overflowTop: 3.0Popping...3.02one*/

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)

Standard Java Exceptions

Throwable

Exception Error

RuntimeException IOException . . .

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

java.lang.Exception Subclasses~ sample ~

AWTException

ClassNotFoundException

CloneNotSupportedException

IOException

NoSuchFieldException

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

java.lang.Error Subclasses

AWTError

LinkageError …

ThreadDeath

VirtualMachineError InternalError, OutOfMemoryError,

StackOverflowError, UnknownError

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”

java.lang.RuntimeException Subclasses (sample)

ArithmeticException (e.g., divide by 0)

ArrayStoreException

ClassCastException

IllegalArgumentException

IndexOutOfBoundsException

NullPointerException

UnsupportedOperationException

Principle

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

“Fixing” FixedStack

StackException should be a runtime exception

Then the “throws” specifications aren’t needed

class StackException extends RuntimeException{ StackException() {}

StackException(String msg) { super(msg); }}

• Then remove all “throws” specifications from FixedStack

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

// Relaxing the Exception Specificationclass 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(); }}

// Throwing a Subclass Exceptionclass 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(); }}

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)}

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

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"); }

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

Program Output0:

finally!last statement

1:finally!

2:(no output)

3a:same as 0:

3a + 3b:compiler error (last statement not reachable)

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. 644-645 (9th ed.)

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!!! } } }

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

Program Output

If no file name given (args.length == 0):java.lang.ArrayIndexOutOfBoundsException: 0

If file doesn’t exist:java.io.FileNotFoundException: <file name>

If file opened successfully:file openedfile closed

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

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!

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?

Uncaught Exceptions

What if there is no handler for an exception?

The thread dies! exceptions belong to a thread (stack-specific)