Chapter 20 Multithreading Big Java by Cay Horstmann

Chapter Goals • To understand how multiple threads can execute in parallel • To learn how to implement threads • To understand race conditions and deadlocks • To be able to avoid corruption of shared objects by using locks and conditions • To be able to use threads for programming animations Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Threads • Thread: a program unit that is executed independently of other parts of the program • The Java Virtual Machine executes each thread in the program for a short amount of time • This gives the impression of parallel execution Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Running a Thread • Implement a class that implements the Runnable interface: public interface Runnable { void run(); } • Place the code for your task into the run method of your class: public class My. Runnable implements Runnable { public void run() { Task statements . . . } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Running a Thread • Create an object of your subclass: Runnable r = new My. Runnable(); • Construct a Thread object from the runnable object: Thread t = new Thread(r); • Call the start method to start the thread: t. start(); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Example A program to print a time stamp and “Hello World” once a second for ten seconds: Mon Mon Mon Dec Dec Dec 28 28 28 23: 12: 03 23: 12: 04 23: 12: 05 23: 12: 06 23: 12: 07 23: 12: 08 23: 12: 09 23: 12: 10 23: 12: 11 23: 12 PST PST PST 2009 2009 2009 Hello, Hello, Hello, World! World! World! Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Greeting. Runnable Outline public class Greeting. Runnable implements Runnable { private String greeting; public Greeting. Runnable(String a. Greeting) { greeting = a. Greeting; } public void run() { Task statements . . . } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Thread Action for Greeting. Runnable • Print a time stamp • Print the greeting

Greeting. Runnable • We can get the date and time by constructing a Date object: Date now = new Date(); • To wait a second, use the sleep method of the Thread class: sleep(milliseconds) • A sleeping thread can generate an Interrupted. Exception • Catch the exception • Terminate thread Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Running Threads • sleep puts current thread to sleep for given number of milliseconds: Thread. sleep(milliseconds) • When a thread is interrupted, most common response is to terminate run Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Generic run method public void run() { try { Task statements } catch (Interrupted. Exception exception) { } Clean up, if necessary } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/greeting/Greeting. Runnable. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 import java. util. Date; /** A runnable that repeatedly prints a greeting. */ public class Greeting. Runnable implements Runnable { private static final int REPETITIONS = 10; private static final int DELAY = 1000; private String greeting; /** Constructs the runnable object. @param a. Greeting the greeting to display */ public Greeting. Runnable(String a. Greeting) { greeting = a. Greeting; } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/greeting/Greeting. Runnable. java (cont. ) 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 public void run() { try { for (int i = 1; i <= REPETITIONS; i++) { Date now = new Date(); System. out. println(now + " " + greeting); Thread. sleep(DELAY); } } catch (Interrupted. Exception exception) { } } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

To Start the Thread • Construct an object of your runnable class: Runnable t = new Greeting. Runnable("Hello World"); • Then construct a thread and call the start method: Thread t = new Thread(r); t. start(); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/greeting/Greeting. Thread. Runner. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 /** This program runs two greeting threads in parallel. */ public class Greeting. Thread. Runner { public static void main(String[] args) { Greeting. Runnable r 1 = new Greeting. Runnable("Hello, World!"); Greeting. Runnable r 2 = new Greeting. Runnable("Goodbye, World!"); Thread t 1 = new Thread(r 1); Thread t 2 = new Thread(r 2); t 1. start(); t 2. start(); } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/greeting/Greeting. Thread. Runner. java (cont. ) Program Run: Mon Mon Mon Mon Mon Dec Dec Dec Dec Dec 28 28 28 28 28 12: 04: 46 12: 04: 47 12: 04: 48 12: 04: 49 12: 04: 50 12: 04: 51 12: 04: 52 12: 04: 53 12: 04: 54 12: 04: 55 PST PST PST PST PST 2009 2009 2009 2009 2009 Hello, World! Goodbye, World! Hello, World! Goodbye, World! Hello, World! Goodbye, World! Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Thread Scheduler • Thread scheduler: runs each thread for a short amount of time (a time slice) • Then the scheduler activates another thread • There will always be slight variations in running times especially when calling operating system services (e. g. input and output) • There is no guarantee about the order in which threads are executed Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 1 What happens if you change the call to the sleep method in the run method to Thread. sleep(1)? Answer: The messages are printed about one millisecond apart. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 2 What would be the result of the program if the main method called r 1. run(); r 2. run(); instead of starting threads? Answer: The first call to run would print ten “Hello” messages, and then the second call to run would print ten “Goodbye” messages Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Terminating Threads • A thread terminates when its run method terminates • Do not terminate a thread using the deprecated stop method • Instead, notify a thread that it should terminate: t. interrupt(); • interrupt does not cause thread to terminate – it sets a boolean variable in the thread data structure Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Terminating Threads • The run method should check occasionally whether it has been interrupted • Use the interrupted method • An interrupted thread should release resources, clean up, and exit: public void run() { for (int i = 1; i <= REPETITIONS && !Thread. interrupted(); i++) { Do work } Clean up } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Terminating Threads • The sleep method throws an Interrupted. Exception when a sleeping thread is interrupted • Catch the exception • Terminate thread : public void run() { try { for (int i = 1; i <= REPETITIONS; i++) { Do work Sleep } } catch (Interrupted. Exception exception) { Clean up } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved. }

Terminating Threads • Java does not force a thread to terminate when it is interrupted • It is entirely up to the thread what it does when it is interrupted • Interrupting is a general mechanism for getting the thread’s attention Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 3 Suppose a web browser uses multiple threads to load the images on a web page. Why should these threads be terminated when the user hits the “Back” button? Answer: If the user hits the “Back” button, the current web page is no longer displayed, and it makes no sense to expend network resources for fetching additional image data. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 4 Consider the following Runnable: public class My. Runnable implements Runnable { public void run() { try { System. out. println(1); Thread. sleep(1000); System. out. println(2); } catch (Interrupted. Exception exception) { System. out. println(3); } System. out. println(4); } } Big. Continued Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 4 (cont. ) Suppose a thread with this Runnable is started and immediately interrupted. Thread t = new Thread(new My. Runnable()); t. start(); t. interrupt(); What output is produced? Answer: The run method prints the values 1, 3, and 4. The call to interrupt merely sets the interruption flag, but the sleep method immediately throws an Interrupted. Exception. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Race Conditions • When threads share a common object, they can conflict with each other • Sample program: multiple threads manipulate a bank account Here is the run method of Deposit. Runnable: public void run() { try { for (int i = 1; i <= count; i++) { account. deposit(amount); Thread. sleep(DELAY); } } catch (Interrupted. Exception exception) { } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Race Conditions • The Withdraw. Runnable class is similar Big Java by Cay Horstmann

Sample Application • Create a Bank. Account object • Create two sets of threads: • Each thread in the first set repeatedly deposits $100 • Each thread in the second set repeatedly withdraws $100 • deposit and withdraw have been modified to print messages: public void deposit(double amount) { System. out. print("Depositing " + amount); double new. Balance = balance + amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Sample Application • The result should be zero, but sometimes it is not • Normally, the program output looks somewhat like this: Depositing 100. 0, new balance is 100. 0 Withdrawing 100. 0, new balance is 0. 0 Depositing 100. 0, new balance is 100. 0 Depositing 100. 0, new balance is 200. 0 Withdrawing 100. 0, new balance is 100. 0. . . Withdrawing 100. 0, new balance is 0. 0 • But sometimes you may notice messed-up output, like this: Depositing 100. 0 Withdrawing 100. 0, new balance is -100. 0 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Scenario to Explain Non-zero Result: Race Condition 1. A deposit thread executes the lines: System. out. print("Depositing " + amount); double new. Balance = balance + amount; The balance variable is still 0, and the new. Balance local variable is 100 2. The deposit thread reaches the end of its time slice and a withdraw thread gains control 3. The withdraw thread calls the withdraw method which withdraws $100 from the balance variable; it is now -100 4. The withdraw thread goes to sleep Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Scenario to Explain Non-zero Result: Race Condition 5. The deposit thread regains control and picks up where it left off; it executes: System. out. println(", new balance is " + new. Balance); balance = new. Balance; The balance is now 100 instead of 0 because the deposit method used the OLD balance Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Corrupting the Contents of the balance Variable Big Java by Cay Horstmann Copyright ©

Race Condition • Occurs if the effect of multiple threads on shared data depends on the order in which they are scheduled • It is possible for a thread to reach the end of its time slice in the middle of a statement • It may evaluate the right-hand side of an equation but not be able to store the result until its next turn: public void deposit(double amount) { balance = balance + amount; System. out. print("Depositing " + amount + ", new balance is " + balance); } • Race condition can still occur: balance = the right-hand-side value Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. Thread. Runner. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 /** This program runs threads that deposit and withdraw money from the same bank account. */ public class Bank. Account. Thread. Runner { public static void main(String[] args) { Bank. Account account = new Bank. Account(); final double AMOUNT = 100; final int REPETITIONS = 100; final int THREADS = 100; for (int i = 1; i <= THREADS; i++) { Deposit. Runnable d = new Deposit. Runnable( account, AMOUNT, REPETITIONS); Withdraw. Runnable w = new Withdraw. Runnable( account, AMOUNT, REPETITIONS); Thread dt = new Thread(d); Thread wt = new Thread(w); Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. Thread. Runner. java (cont. ) 24 25 26 27 28 dt.

ch 20/unsynch/Deposit. Runnable. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** A deposit runnable makes periodic deposits to a bank account. */ public class Deposit. Runnable implements Runnable { private static final int DELAY = 1; private Bank. Account account; private double amount; private int count; /** Constructs a deposit runnable. @param an. Account the account into which to deposit money @param an. Amount the amount to deposit in each repetition @param a. Count the number of repetitions */ public Deposit. Runnable(Bank. Account an. Account, double an. Amount, int a. Count) { account = an. Account; amount = an. Amount; Continued count = a. Count; } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Deposit. Runnable. java (cont. ) 25 26 27 28 29 30 31 32 33 34 35 36 37 public void run() { try { for (int i = 1; i <= count; i++) { account. deposit(amount); Thread. sleep(DELAY); } } catch (Interrupted. Exception exception) {} } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Withdraw. Runnable. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** A withdraw runnable makes periodic withdrawals from a bank account. */ public class Withdraw. Runnable implements Runnable { private static final int DELAY = 1; private Bank. Account account; private double amount; private int count; /** Constructs a withdraw runnable. @param an. Account the account from which to withdraw money @param an. Amount the amount to withdraw in each repetition @param a. Count the number of repetitions */ public Withdraw. Runnable(Bank. Account an. Account, double an. Amount, int a. Count) { account = an. Account; amount = an. Amount; Continued count = a. Count; } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Withdraw. Runnable. java (cont. ) 25 26 27 28 29 30 31 32 33 34 35 36 37 public void run() { try { for (int i = 1; i <= count; i++) { account. withdraw(amount); Thread. sleep(DELAY); } } catch (Interrupted. Exception exception) {} } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 /** A bank account has a balance that can be changed by deposits and withdrawals. */ public class Bank. Account { private double balance; /** Constructs a bank account with a zero balance. */ public Bank. Account() { balance = 0; } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. java (cont. ) 17 18 19 20 21 22 23 24 25 26 27 28 /** Deposits money into the bank account. @param amount the amount to deposit */ public void deposit(double amount) { System. out. print("Depositing " + amount); double new. Balance = balance + amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. java (cont. ) 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 /** Withdraws money from the bank account. @param amount the amount to withdraw */ public void withdraw(double amount) { System. out. print("Withdrawing " + amount); double new. Balance = balance - amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; } /** Gets the current balance of the bank account. @return the current balance */ public double get. Balance() { return balance; } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/unsynch/Bank. Account. java (cont. ) Program Run: Depositing 100. 0, new balance is 100. 0 Withdrawing 100. 0, new balance is 0. 0. . . Withdrawing 100. 0, new balance is 400. 0 Depositing 100. 0, new balance is 500. 0 Withdrawing 100. 0, new balance is 400. 0 Withdrawing 100. 0, new balance is 300. 0 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 5 Give a scenario in which a race condition causes the bank balance to be -100 after one iteration of a deposit thread and a withdraw thread. Answer: There are many possible scenarios. Here is one: • The first thread loses control after the first print statement. • The second thread loses control just before the assignment balance = new. Balance. • The first thread completes the deposit method. • The second thread completes the withdraw method. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 6 Suppose two threads simultaneously insert objects into a linked list. Using the implementation in Chapter 15, explain how the list can be damaged in the process. Answer: One thread calls add. First and is preempted just before executing the assignment first = new. Link. Then the next thread calls add. First, using the old value of first. Then the first thread completes the process, setting first to its new link. As a result, the links are not in sequence. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Synchronizing Object Access • To solve problems such as the one just seen, use a lock object • Lock object: used to control threads that manipulate shared resources • In Java: Lock interface and several classes that implement it • Reentrant. Lock: most commonly used lock class • Locks are a feature of Java version 5. 0 • Earlier versions of Java have a lower-level facility for thread synchronization Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Synchronizing Object Access • Typically, a lock object is added to a class whose methods access shared resources, like this: public class Bank. Account { private Lock balance. Change. Lock; public Bank. Account() { balance. Change. Lock = new Reentrant. Lock(); . . . } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Synchronizing Object Access • Code that manipulates shared resource is surrounded by calls to lock and unlock: balance. Change. Lock. lock(); Manipulate the shared resource balance. Change. Lock. unlock(); • If code between calls to lock and unlock throws an exception, call to unlock never happens Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Synchronizing Object Access • To overcome this problem, place call to unlock into a finally clause: public void deposit(double amount) { balance. Change. Lock. lock(); try { System. out. print("Depositing " + amount); double new. Balance = balance + amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; } finally { balance. Change. Lock. unlock(); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Synchronizing Object Access • When a thread calls lock, it owns the lock until it calls unlock • A thread that calls lock while another thread owns the lock is temporarily deactivated • Thread scheduler periodically reactivates thread so it can try to acquire the lock • Eventually, waiting thread can acquire the lock Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Visualizing Object Locks Big Java by Cay Horstmann Copyright © 2009 by John Wiley

Self Check 20. 7 If you construct two Bank. Account objects, how many lock objects are created? Answer: Two, one for each bank account object. Each lock protects a separate balance variable. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 8 What happens if we omit the call unlock at the end of the deposit method? Answer: When a thread calls deposit, it continues to own the lock, and any other thread trying to deposit or withdraw money in the same bank account is blocked forever. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Avoiding Deadlocks • A deadlock occurs if no thread can proceed because each thread is waiting for another to do some work first • Bank. Account example: public void withdraw(double amount) { balance. Change. Lock. lock(); try { while (balance < amount) Wait for the balance to grow. . . } finally { balance. Change. Lock. unlock(); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Avoiding Deadlocks • How can we wait for the balance to grow? • We can’t simply call sleep inside withdraw method; thread will block all other threads that want to use balance. Change. Lock • In particular, no other thread can successfully execute deposit • Other threads will call deposit, but will be blocked until withdraw exits • But withdraw doesn’t exit until it has funds available • DEADLOCK Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Condition Objects • To overcome problem, use a condition object • Condition objects allow a thread to temporarily release a lock, and to regain the lock at a later time • Each condition object belongs to a specific lock object Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Condition Objects (cont. ) • You obtain a condition object with new. Condition method of Lock interface: public class Bank. Account { public Bank. Account() { balance. Change. Lock = new Reentrant. Lock(); sufficient. Funds. Condition = balance. Change. Lock. new. Condition(); . . . }. . . private Lock balance. Change. Lock; private Condition sufficient. Funds. Condition; } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Condition Objects • It is customary to give the condition object a name that describes condition to test • You need to implement an appropriate test Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Condition Objects (cont. ) • As long as test is not fulfilled, call await on the condition object: public void withdraw(double amount) { balance. Change. Lock. lock(); try { while (balance < amount) sufficient. Funds. Condition. await(); . . . } finally { balance. Change. Lock. unlock(); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Condition Objects • Calling await • Makes current thread wait • Allows another thread to acquire the lock object • To unblock, another thread must execute signal. All on the same condition object : sufficient. Funds. Condition. signal. All(); • signal. All unblocks all threads waiting on the condition • signal: randomly picks just one thread waiting on the object and unblocks it • signal can be more efficient, but you need to know that every waiting thread can proceed • Recommendation: always call signal. All Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. Thread. Runner. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 /** This program runs threads that deposit and withdraw money from the same bank account. */ public class Bank. Account. Thread. Runner { public static void main(String[] args) { Bank. Account account = new Bank. Account(); final double AMOUNT = 100; final int REPETITIONS = 100; final int THREADS = 100; for (int i = 1; i <= THREADS; i++) { Deposit. Runnable d = new Deposit. Runnable( account, AMOUNT, REPETITIONS); Withdraw. Runnable w = new Withdraw. Runnable( account, AMOUNT, REPETITIONS); Thread dt = new Thread(d); Thread wt = new Thread(w); Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. Thread. Runner. java (cont. ) 24 25 26 27 28 dt.

ch 20/synch/Bank. Account. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 import java. util. concurrent. locks. Condition; import java. util. concurrent. locks. Lock; import java. util. concurrent. locks. Reentrant. Lock; /** A bank account has a balance that can be changed by deposits and withdrawals. */ public class Bank. Account { private double balance; private Lock balance. Change. Lock; private Condition sufficient. Funds. Condition; /** Constructs a bank account with a zero balance. */ public Bank. Account() { balance = 0; balance. Change. Lock = new Reentrant. Lock(); sufficient. Funds. Condition = balance. Change. Lock. new. Condition(); Continued } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. java (cont. ) 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 /** Deposits money into the bank account. @param amount the amount to deposit */ public void deposit(double amount) { balance. Change. Lock. lock(); try { System. out. print("Depositing " + amount); double new. Balance = balance + amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; sufficient. Funds. Condition. signal. All(); } finally { balance. Change. Lock. unlock(); } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. java (cont. ) 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 /** Withdraws money from the bank account. @param amount the amount to withdraw */ public void withdraw(double amount) throws Interrupted. Exception { balance. Change. Lock. lock(); try { while (balance < amount) sufficient. Funds. Condition. await(); System. out. print("Withdrawing " + amount); double new. Balance = balance - amount; System. out. println(", new balance is " + new. Balance); balance = new. Balance; } finally { balance. Change. Lock. unlock(); } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. java (cont. ) 69 70 71 72 73 74 75 76 77 /** Gets the current balance of the bank account. @return the current balance */ public double get. Balance() { return balance; } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/synch/Bank. Account. java (cont. ) Program Run: Depositing 100. 0, new balance is 100. 0 Withdrawing 100. 0, new balance is 0. 0 Depositing 100. 0, new balance is 100. 0 Depositing 100. 0, new balance is 200. 0. . . Withdrawing 100. 0, new balance is 100. 0 Depositing 100. 0, new balance is 200. 0 Withdrawing 100. 0, new balance is 100. 0 Withdrawing 100. 0, new balance is 0. 0 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 9 What is the essential difference between calling sleep and await? Answer: A sleeping thread is reactivated when the sleep delay has passed. A waiting thread is only reactivated if another thread has called signal. All or signal. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 10 Why is the sufficient. Funds. Condition object an instance variable of the Bank. Account class and not a local variable of the withdraw and deposit methods? Answer: The calls to await and signal/signal. All must be made to the same object. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

An Application of Threads: Animation • Shows different objects moving or changing as time progresses • Is often achieved by launching one or more threads that compute how parts of the animation change • Can use Swing Timer class for simple animations • More advanced animations are best implemented with threads • An algorithm animation helps visualize the steps in the algorithm Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Algorithm Animation • Runs in a separate thread that periodically updates an image of the current state of the algorithm • It then pauses so the user can see the change • After a short time the algorithm thread wakes up and runs to the next point of interest • It updates the image again and pauses again Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation • Items in the algorithm’s state • The array of values • The size of the already sorted area • The currently marked element • This state is accessed by two threads: 1. One that sorts the array, and 2. One that repaints the frame • To visualize the algorithm • Show the sorted part of the array in a different color • Mark the currently visited array element in red Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

A Step in the Animation of the Selection Sort Algorithm Big Java by Cay

Selection Sort Algorithm Animation: Implementation • Use a lock to synchronize access to the shared state • Add a component instance variable to the algorithm class and augment the constructor to set it • That instance variable is needed for • Repainting the component, and • Finding out the dimensions of the component when drawing the algorithm state Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: Implementation public class Selection. Sorter { private JComponent component; public Selection. Sorter(int[] an. Array, Jcomponent a. Component) { a = an. Array; sort. State. Lock = new Reentrant. Lock(); component = a. Component; }. . . } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: Implementation • At each point of interest, algorithm needs to pause so user can observe the graphical output • We need a pause method that repaints component and sleeps for a small delay: public void pause(int steps) throws Interrupted. Exception { component. repaint(); Thread. sleep(steps * DELAY); } • Delay is proportional to the number of steps involved • pause should be called at various places in the algorithm Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: Implementation • We add a draw method to the algorithm class • draws the current state of the data structure, highlighting items of special interest • draw is specific to the particular algorithm • In this case, draws the array elements as a sequence of sticks in different colors • The already sorted portion is blue • The marked position is red • The remainder is black Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: draw public void draw(Graphics 2 D g 2) { sort. State. Lock. lock(); try { int delta. X = component. get. Width() / a. length; for (int i = 0; i < a. length; i++) { if (i == marked. Position) g 2. set. Color(Color. RED); else if (i <= already. Sorted) g 2. set. Color(Color. BLUE); else g 2. set. Color(Color. BLACK); g 2. draw(new Line 2 D. Double(i * delta. X, 0, i * delta. X, a[i])); } Continued } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: draw (cont. ) finally { sort. State. Lock. unlock(); }

Selection Sort Algorithm Animation: Pausing • Update the special positions as the algorithm progresses • Pause the animation whenever something interesting happens • Pause should be proportional to the number of steps that are being executed • In this case, pause one unit for each visited array element • Augment minimum. Position and sort accordingly Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: Pausing public int minimum. Position(int from) throws Interrupted. Exception { int min. Pos = from; for (int i = from + 1; i < a. length; i++) { sort. State. Lock. lock(); try { if (a[i] < a[min. Pos]) min. Pos = i; marked. Position = i; } finally { sort. State. Lock. unlock(); Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: Pausing (cont. ) } pause(2); // two array elements were inspected } return min. Pos; } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: paint. Component • paint. Component calls the draw method of the algorithm object: public class Selection. Sort. Component extends JComponent { private Selection. Sorter sorter; public void paint. Component(Graphics g) { if (sorter == null) return; Graphics 2 D g 2 = (Graphics 2 D) g; sorter. draw(g 2); }. . . } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: start. Animation public void start. Animation() { int[] values = Array. Util. random. Int. Array(30, 300); sorter = new Selection. Sorter(values, this); class Animation. Runnable implements Runnable { public void run() { try { sorter. sort(); } catch (Interrupted. Exception exception) { } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Selection Sort Algorithm Animation: start. Animation (cont. ) } Runnable r = new Animation. Runnable(); Thread t = new Thread(r); t. start(); } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sort. Viewer. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 import java. awt. Border. Layout; import javax. swing. JButton; import javax. swing. JFrame; public class Selection. Sort. Viewer { public static void main(String[] args) { JFrame frame = new JFrame(); final int FRAME_WIDTH = 300; final int FRAME_HEIGHT = 400; frame. set. Size(FRAME_WIDTH, FRAME_HEIGHT); frame. set. Default. Close. Operation(JFrame. EXIT_ON_CLOSE); final Selection. Sort. Component component = new Selection. Sort. Component(); frame. add(component, Border. Layout. CENTER); frame. set. Visible(true); component. start. Animation(); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sort. Component. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 import java. awt. Graphics; import java. awt. Graphics 2 D; import javax. swing. JComponent; /** A component that displays the current state of the selection sort algorithm. */ public class Selection. Sort. Component extends JComponent { private Selection. Sorter sorter; /** Constructs the component. */ public Selection. Sort. Component() { int[] values = Array. Util. random. Int. Array(30, 300); sorter = new Selection. Sorter(values, this); } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sort. Component. java (cont. ) 21 22 23 24 25 26 public void paint. Component(Graphics g) { Graphics 2 D g 2 = (Graphics 2 D)g; sorter. draw(g 2); } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sort. Component. java (cont. ) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 /** Starts a new animation thread. */ public void start. Animation() { class Animation. Runnable implements Runnable { public void run() { try { sorter. sort(); } catch (Interrupted. Exception exception) { } } } Runnable r = new Animation. Runnable(); Thread t = new Thread(r); t. start(); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 import import java. awt. Color; java. awt. Graphics 2 D; java. awt. geom. Line 2 D; java. util. concurrent. locks. Lock; java. util. concurrent. locks. Reentrant. Lock; javax. swing. JComponent; /** This class sorts an array, using the selection sort algorithm. */ public class Selection. Sorter { private static final int DELAY = 100; private int[] a; private Lock sort. State. Lock; // The component is repainted when the animation is paused private JComponent component; // These instance variables are needed for drawing private int marked. Position = -1; private int already. Sorted = -1; Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 25 26 27 28 29 30 31 32 33 34 35 36 37 /** Constructs a selection sorter. @param an. Array the array to sort @param a. Component the component to be repainted when the animation pauses */ public Selection. Sorter(int[] an. Array, JComponent a. Component) { a = an. Array; sort. State. Lock = new Reentrant. Lock(); component = a. Component; } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 /** Sorts the array managed by this selection sorter. */ public void sort() throws Interrupted. Exception { for (int i = 0; i < a. length - 1; i++) { int min. Pos = minimum. Position(i); sort. State. Lock. lock(); try { swap(min. Pos, i); // For animation already. Sorted = i; } finally { sort. State. Lock. unlock(); } pause(2); } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 /** Finds the smallest element in a tail range of the array @param from the first position in a to compare @return the position of the smallest element in the range a[from]. . . a[a. length - 1] */ private int minimum. Position(int from) throws Interrupted. Exception { int min. Pos = from; for (int i = from + 1; i < a. length; i++) { sort. State. Lock. lock(); try { if (a[i] < a[min. Pos]) min. Pos = i; // For animation marked. Position = i; } finally { sort. State. Lock. unlock(); } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 pause(2); } return min. Pos; } /** Swaps two entries of the array. @param i the first position to swap @param j the second position to swap */ private void swap(int i, int j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 /** Draws the current state of the sorting algorithm. @param g 2 the graphics context */ public void draw(Graphics 2 D g 2) { sort. State. Lock. lock(); try { int delta. X = component. get. Width() / a. length; for (int i = 0; i < a. length; i++) { if (i == marked. Position) g 2. set. Color(Color. RED); else if (i <= already. Sorted) g 2. set. Color(Color. BLUE); else g 2. set. Color(Color. BLACK); g 2. draw(new Line 2 D. Double(i * delta. X, 0, i * delta. X, a[i])); } } Continued Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

ch 20/animation/Selection. Sorter. java (cont. ) 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 finally { sort. State. Lock. unlock(); } } /** Pauses the animation. @param steps the number of steps to pause */ public void pause(int steps) throws Interrupted. Exception { component. repaint(); Thread. sleep(steps * DELAY); } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 11 Why is the draw method added to the Selection. Sorter class and not the Selection. Sort. Component class? Answer: The draw method uses the array values and the values that keep track of the algorithm’s progress. These values are available only in the Selection. Sorter class. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 20. 12 Would the animation still work if the start. Animation method simply called sorter. sort() instead of spawning a thread that calls that method? Answer: Yes, provided you only show a single frame. If you modify the Selection. Sort. Viewer program to show two frames, you want the sorters to run in parallel. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.