1 Lecture 11(chap 14) Exception Handling & Thread Instructors: Fu-Chiung Cheng ( 鄭福炯 ) Associate Professor Computer Science & Engineering Tatung Institute of Technology

1 Outline exception processing catching and handling exceptions creating new exceptions separate process threads synchronizing threads

2 Exceptions An exception is an object that describes an unusual or erroneous situation Exceptions are thrown by a program, and may be caught and handled by another part of the program A program can therefore be separated into a normal execution flow and an exception execution flow An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

3 Exception Handling A program can deal with an exception in one of three ways: –ignore it –handle it where it occurs –handle it an another place in the program The manner in which an exception is processed is an important design consideration

4 Exception Handling If an exception is ignored by the program, the program will terminate and produce an appropriate message The message includes a call stack trace that indicates on which line the exception occurred The call stack trace also shows the method call trail that lead to the execution of the offending line See Zero.java

public class Zero { public static void main (String[] args) { int numerator = 10; int denominator = 0; System.out.println (numerator / denominator); } // method main } // class Zero

5 The try Statement To process an exception when it occurs, the line that throws the exception is executed within a try block A try block is followed by one or more catch clauses, which contain code to process an exception Each catch clause has an associated exception type When an exception occurs, processing continues at the first catch clause that matches the exception type See Adding.java

import java.io.*; public class Adding { public static void main (String[] args) { int num1 = User_Reader.get_integer ("Enter a number: "); int num2 = User_Reader.get_integer ("Enter another number: "); System.out.println ("The sum is " + (num1+num2)); } // method main } // class Adding class User_Reader { public static int get_integer (String prompt) { BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); int number = 0; boolean valid = false;

while (! valid) { System.out.print (prompt); System.out.flush (); try { number = Integer.parseInt (stdin.readLine()); valid = true; } catch (NumberFormatException exception) { System.out.println ("Invalid input. Try again."); } catch (IOException exception) { System.out.println ("Input problem. Terminating."); System.exit(0); } return number; } // method get_integer } // class User_Reader

6 Exception Propagation If it is not appropriate to handle the exception where it occurs, it can be handled at a higher level Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost level A try block that contains a call to a method in which an exception is thrown can be used to catch that exception See Propagation_Demo.java

public class Propagation_Demo { static public void main (String[] args) { Exception_Scope demo = new Exception_Scope(); System.out.println("program beginning"); demo.level1(); System.out.println("program ending"); } // method main } // class Propagation_Demo class Exception_Scope { public void level3 (int adjustment) { int current = 1; System.out.println("level3 beginning"); current = current / adjustment; // divided by zero System.out.println("level3 ending"); } // method level3

public void level2() { System.out.println("level2 beginning"); level3 (0); System.out.println("level2 ending"); } // method level2 public void level1() { System.out.println("level1 beginning"); try { level2(); } catch (ArithmeticException problem) { // page 667 System.out.println (problem.getMessage()); problem.printStackTrace(); } System.out.println("level1 ending"); } // method level1 } // class Exception_Scope

7 Exceptions An exception is either checked or unchecked A checked exception can only be thrown within a try block or within a method that is designated to throw that exception The compiler will complain if a checked exception is not handled appropriately An unchecked exception does not require explicit handling, though it could be processed that way

8 The throw Statement A programmer can define an exception by extending the appropriate class Exceptions are thrown using the throw statement: throw exception-object ; See Throw_Demo.java Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown

import java.io.IOException; public class Throw_Demo { public static void main (String[] args) throws Ooops { Ooops problem = new Ooops ("Alert!"); throw problem; // execution never gets to this point } // method main } // class Throw_Demo class Ooops extends IOException { Ooops (String message) { super (message); } // constructor Ooops } // class Ooops

9 The finally Clause A try statement can have an optional clause designated by the reserved word finally If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete Also, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

9 The finally Clause Exception Handling: try { code that may generate exception; } catch (exception) { code to handle the exception; } finally { code will be executed; }

10 Threads Processing can be broken into several separate threads of control which execute at the same time "At the same time" could mean true parallelism or simply interlaced concurrent processing A thread is one sequential flow of execution that occurs at the same time another thread is executing the statement of the same program They are not necessarily executing the same statements at the same time

11 Threads A thread can be created by deriving a new thread from the Thread class The run method of the thread defines the concurrent activity, but the start method is used to begin the separate thread process See Simultaneous.java A thread can also be created by defining a class that implements the Runnable interface By implementing the interface, the thread class can be derived from a class other than Thread

public class Simultaneous { public static void main (String[] args) { Soda one = new Soda ("Coke"); Soda two = new Soda ("Pepsi"); Soda three = new Soda ("Diet Coke"); one.start(); two.start(); three.start(); } // method main } // class Simultaneous class Soda extends Thread { private String name; Soda (String str) { name = str; } // constructor Soda public void run() { for (int count = 0; count < 100; count++) System.out.println (name); } // method run } // class Soda

12 Shared Data Potential problems arise when multiple threads share data Specific code of a thread may execute at any point relative to the processing of another thread If that code updates or references the shared data, unintended processing sequences can occur that result in incorrect results

13 Shared Data Consider two withdrawals from the same bank account at the same time task: withdraw 300 Is amount <= balance YES balance -= 300 balance task: withdraw 300 Is amount <= balance YES balance -= 300

14 Synchronization Multiple threads of control can be made safe if areas of code that use shared data are synchronized When a set of code is synchronized, then only one thread can be using that code at a time The other threads must wait until the first thread is complete This is an implementation of a synchronization mechanism called a monitor See ATM_Accounts.java

public class ATM_Accounts { public static void main (String[] args) { Savings_Account savings = new Savings_Account(4321, 531); ATM west_branch = new ATM (savings); ATM east_branch = new ATM (savings); west_branch.start(); east_branch.start(); } } // class ATM_Accounts class Savings_Account { protected int account; protected int balance; public Savings_Account (int account_num, int initial) { account = account_num; balance = initial; } // constructor Savings_Account

public synchronized boolean withdrawal (int amount) { boolean result = false; System.out.println ("Withdrawal from account " + account); System.out.println ("Amount: " + amount); if (amount <= balance) { balance -= amount; System.out.println ("New balance: " + balance); result = true; } else System.out.println ("Insufficient funds."); System.out.println(); return result; } // method withdrawal } // class Savings_Account

class ATM extends Thread { Savings_Account account; ATM (Savings_Account savings) { account = savings; } // constructor ATM public void run () { account.withdrawal (300); } // method run } // class ATM

Thread Synchronization synchronized statement: synchronized (expr) statement;

15 Controlling Threads Thread processing can be temporarily suspended, then later resumed, using methods from the Thread class A thread can also be put to sleep for a specific amount of time These mechanisms can be quite helpful in certain situations, like controlling animations See Bouncing_Ball2.java

import java.awt.event.*; import java.applet.Applet; import java.awt.*; public class Bouncing_Ball2 extends Applet { private final int SIZE = 300; private Ball ball = new Ball(150, 10, 250, 200); private Graphics page; private Control_Panel controls; public void init() { setVisible(true); setSize(SIZE, SIZE); page=getGraphics(); page.setXORMode (getBackground()); } // method init public void start() { controls = new Control_Panel (Thread.currentThread()); controls.start(); ball.pause(); while (ball.moving()) ball.bounce (page); } // method start } // class Bouncing_Ball2

class Control_Panel extends Thread { private Button suspend = new Button ("suspend"); private Button resume = new Button ("resume"); private Frame frame = new Frame ("B B Control Panel"); private Thread applet_thread; Control_Panel (Thread applet_thread) { this.applet_thread = applet_thread; } // constructor Control_Panel public void run() { Resume_Action resume_action = new Resume_Action (applet_thread); Suspend_Action suspend_action = new Suspend_Action (applet_thread); suspend.addActionListener (suspend_action); resume.addActionListener (resume_action); frame.setLayout (new FlowLayout()); frame.add (suspend); frame.add (resume); frame.pack(); frame.setLocation (250, 200); frame.setVisible (true); } } // class Control_Panel

class Suspend_Action implements ActionListener { Thread applet_thread; Suspend_Action (Thread applet_thread) { this.applet_thread = applet_thread; } // constructor Suspend_Action public void actionPerformed(ActionEvent action) { applet_thread.suspend(); } // method actionPerformed } // class Suspend_Action

class Resume_Action implements ActionListener { Thread applet_thread; Resume_Action (Thread applet_thread) { this.applet_thread = applet_thread; } // constructor Resume_Action public void actionPerformed(ActionEvent action) { applet_thread.resume(); } // method actionPerformed } // class Resume_Action

class Ball { private final int MOVE = 2; private final float DISTANCE = 0.97f; private final int SIZE = 20; private final int PAUSE = 5; private int x; private int start_y; private int end_y; private int length; private boolean moving_up = true; Ball (int new_x, int new_start_y, int new_end_y, int new_length) { x = new_x; start_y = new_start_y; end_y = new_end_y; length = new_length; } // constructor Ball public void pause() { try { Thread.currentThread().sleep (PAUSE); } catch (InterruptedException exception) { System.out.println ("have an exception"); } } // method pause

void move() { if (moving_up) end_y = end_y - MOVE; else end_y = end_y + MOVE; } // method move void draw_ball (Graphics page) { page.drawOval (x-(SIZE/2), end_y, SIZE, SIZE); page.drawLine (x, start_y, x, end_y); } // draw_ball public boolean moving () { return length != 0; } // method moving

public void bounce (Graphics page) { for (int count = 1; count < length; count += MOVE) { draw_ball(page); pause(); // for loop in previous example draw_ball(page); move(); } moving_up = !moving_up; length = (int) (DISTANCE * length); } // method bounce } // class Ball

Conclusion Exception handling make a program to separate to a normal execution flow and an exception execution flow Programmers must carefully consider how exception should be handled. Threads: created by A. inheriting from Thread class B. implementing Runnable interface Synchronization may be used to prevent concurrent access shared data.