Stack Assignment

Motivation

We will build an initial not-thread-safe implementation of a Stack. This exercise will allow us to gain experience building a data structure and see how it performs when used in a parallel for which it was not designed. Later in the semester we will build two thread-safe implementations: one concurrent and one atomic. Your work on this exercise will pay dividends on these later exercises.

Background

Video: HackerRank: Stacks and Queues

interface Stack

public interface Stack<E> {
	void push(E value);
	E peek();
	E pop();
}

Example

empty

Stack<String> stack = new NotThreadSafeStack<>();

In this state, stack.peek() will return null() and stack.pop() will also return null.

push A, B, C, D, E

Stack<String> stack = new NotThreadSafeStack<>();
stack.push("A");
stack.push("B");
stack.push("C");
stack.push("D");
stack.push("E");

In this state, stack.peek() will return "E".

push A, B, C, D, E, pop

Stack<String> stack = new NotThreadSafeStack<>();
stack.push("A");
stack.push("B");
stack.push("C");
stack.push("D");
stack.push("E");
String e = stack.pop();

In this state, e will hold "E" and stack.peek() will return "D".

Code To Implement

Node

The mighty cons cell goes back to the early days of computing. People have been building amazing systems with this data structure since the 1950s.

We will build an @Immutable class Node.

class:	Node.java
methods:	value next
package:	stack.node.exercise
source folder:	student-pre-aspect/src/main/java

Warning:class Node is found in the stack.node.exercise package in ***student-pre-aspect***

constructor and instance variables

The constructor

public Node(E value, Node<E> next)

is passed a value and a next. Hang on to this data in instance variables. As instances of Node are to be immutable, the instance variables should be marked as final which prevents them from being re-assigned.

value

return the value.

next

return the next node.

NotThreadSafeStack

@NotThreadSafe
public class NotThreadSafeStack<E> implements Stack<E>

@NotThreadSafe

class:	NotThreadSafeStack.java
methods:	push peek pop
package:	stack.notthreadsafe.exercise
source folder:	student/src/main/java

Note: The constructor has been intentionally omitted. Simply initialize the instance variable you need when you declare it.

instance variables

What state do you need to keep track of to support a mutable non-thread-safe Stack?

push

public void push(E value)

peek

public E peek()

If the current instance is empty, null should be returned. Otherwise, it should return the value on top of the stack. The current instance should NOT be mutated.

pop

public E pop()

If the current instance is empty, null should be returned. Otherwise, it should remove the top of the stack and return it. NOTE: if not empty, the current instance should be mutated.

Where should the top of the stack now be pointing?

Distasters To Investigate

NOTE: you only need to investigate these disaster clients. You need not fix them. Our NotThreadSafeStack is designed to be used in a single sequential thread. Not surprisingly, it fails when it is mutated in parallel.

ParallelPushStackDisasterClient

class:	ParallelPushStackDisasterClient.java	Broken
methods:	main
package:	stack.notthreadsafe.disaster
source folder:	src/main/java

ParallelPushAndPopStackDisasterClient

class:	ParallelPushAndPopStackDisasterClient.java	Broken
methods:	main
package:	stack.notthreadsafe.disaster
source folder:	src/main/java

Testing

class:	_NotThreadSafeStackTestSuite.java
package:	stack.notthreadsafe.exercise
source folder:	testing/src/test/java

Pledge, Acknowledgments, Citations

file:

stack-pledge-acknowledgments-citations.txt

More info about the Honor Pledge