Outline. Example stack Version 1 -- int stack with fixed array Version 2 -- int stack with flexible array Version 3 -- with interface

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2 Outline Example stack Version 1 -- int stack with fixed array Version 2 -- int stack with flexible array Version 3 -- with interface Visibility issues 6 :: Spring 2010

3 Stacks push(value) -- store in stack, put on top and increment top_of_stack Pop -- return top element, decrement top_of_stack TOS: top_of_stack Containers 6 :: Spring 2010

4 Stacks push (3) push (2) pop() TOS 2 TOS TOS 6 :: Spring 2010

5 A start You want to design a stack class The stack should hold int values The services are Pop() Push(int) Top() abstract class Stack { public abstract int pop(); public abstract void push(int i); public abstract int top(); 6 :: Spring 2010

6 Comment Other interfaces are possible as well Predicate isempty() to check if stack is empty might allow client to anticipate error situations Maybe introduce a way to duplicate the top element of the stack 6 :: Spring 2010

7 Thread-safety Stack instance should be thread-safe Responsibility rests with the designer of stack Want to handle unexpected behavior pop() with empty stack top() with empty stack 6 :: Spring 2010

8 Record that we expect throws abstract class Stack { public abstract void push (int i); public abstract int pop() throws StackException; public abstract int top() throws StackException; Start of concrete class: Need a place to store data -- int array class SimpleStack extends Stack { private int stack[]; private int tos; 6 :: Spring 2010

9 Finite data array A push operation can fail -- not enough data Expect the problem and tell client to deal with it Find class that can grow (or has no space limitations), e.g. linked list Ignore abstract class Stack { public abstract void push (int i) throws StackException; public abstract int pop() throws StackException; public abstract int top() throws StackException; 6 :: Spring 2010

10 Implementation class SimpleStack extends Stack { private int stack[]; private int tos; private void initstack() { for(int i = 0; i < stack.length; i++) stack[i] = -1; this.tos = 0; public SimpleStack(int size) { this.stack = new int[size]; initstack(); 6 :: Spring 2010

11 Implementation // SimpleStack continued public synchronized void push(int val) throws StackException { if (tos >= stack.length) { throw(new StackException("PUSH: StackOverflowException")); else { stack[tos++] = val; public synchronized int pop() throws StackException { if (tos <= 0) { throw(new StackException("POP: EmptyStackException")); else { return stack[--tos]; 6 :: Spring 2010

12 Thread-safety Only one thread can push or pop at a time Each push/pop operation is executed atomically What about top()? 6 :: Spring 2010

13 Implementation // SimpleStack continued public synchronized int top() throws StackException { if (tos <= 0) { throw(new StackException("TOP: EmptyStackException")); else { return stack[tos-1]; 6 :: Spring 2010

14 Client import java.util.random; public class Worker implements Runnable{ private int id; private Stack stack; public Worker(Stack stack, int id) { this.id = id; this.stack = stack; 6 :: Spring 2010

15 Comment import java.util.random; Import -- make class available Random - useful to get random numbers nextint(range) -- next random number in a range 0.. range 6 :: Spring 2010

16 Client, continued public void run() { Random rand_value = new Random(); int tmp; for (int i=0; i< 10; i++) { try { tmp = rand_value.nextint(10); stack.push(tmp); catch (StackException e) { System.out.println(e.getMessage()); System.out.println("fatal error - bailing out"); System.exit(-1); break; ; try { Thread.sleep(rand_value.nextInt(1000)); catch (InterruptedException e){ System.out.println(e.getMessage()); 6 :: Spring 2010 ;

17 Client, continued // run() continued for (int i=0; i< 10; i++) { try { tmp = stack.pop(); catch (StackException e) { ; try { System.out.println(e.getMessage()); System.out.println("fatal error - bailing out"); System.exit(-1); break; Thread.sleep(rand_value.nextInt(1000)); catch (InterruptedException e){ System.out.println(e.getMessage()); ; 6 :: Spring 2010

18 Threads Threads can be managed Need reference to thread object Threads can be put to sleep for x ms At least for x ms No guarantee about wakeup May throw an exception try { Thread.sleep(rand_value.nextInt(1000)); catch (InterruptedException e){ System.out.println(e.getMessage()); ; 6 :: Spring 2010

19 Discussion In the recitation group you ll see a version that performs push/pop at random In a real setting, asking the client to handle stack overflow is debatable. tos not handled exactly as shown on first slides Let s adjust the size of the internal storage as needed 6 :: Spring 2010

20 Implementation, version 2 abstract class Stack { public abstract void push (int i) ; public abstract int pop() throws StackException; public abstract int top() throws StackException; 6 :: Spring 2010

21 Implementation, version 2 class SimpleStack extends Stack { private int stack[]; private int tos; private int objectat(int index) { return stack[index]; private void setobjectat(int index, int value) { stack[index] = value; private void growstack() { int newstack [] = new int [stack.length * 2]; for (int i=0; i< stack.length; i++) { newstack[i] = stack[i]; stack = newstack; 6 :: Spring 2010

22 Implementation, version 2 public SimpleStack(int size) { this.stack = new int[size]; this.tos = 0; // next entry to be written by push public synchronized void push(int val) { if (tos >= stack.length) { growstack(); setobjectat(tos, val); ++tos; 6 :: Spring 2010

23 Implementation, version 2 public synchronized int pop() throws StackException { if (tos <= 0) { throw(new StackException("POP: EmptyStackException")); else { tos--; return objectat(tos); public synchronized int top() throws StackException { if (tos <= 0) { throw(new StackException("TOP: EmptyStackException")); else { return objectat(tos-1); 6 :: Spring 2010

24 Interfaces Instead of an abstract class, we could use an interface No change except in Worker client Now that push will never throw a StackException, we cannot put it into a try block. 6 :: Spring 2010

25 Implementation, version 3 interface Stack { void push (int i) ; int pop() throws StackException; int top() throws StackException; class SimpleStack implements Stack { 6 :: Spring 2010

26 Example You want to design a stack class for use by multiple threads for objects (not just ints) abstract class Stack { public abstract Object pop(); public abstract void push(object obj); public abstract Object top(); 6 :: Spring 2010

27 Visibility 6 :: Spring 2010

28 Aside: Java restrictions on access 6 :: Spring 2010

29 When are updates visible 6 :: Spring 2010

30 Volatile data 6 :: Spring 2010

31 Why bother? 6 :: Spring 2010

32 This code may not work 6 :: Spring 2010