Modify the ArrayList implementation so that all methods can be safely accessed from multiple threads using the given code.

ArrayList.java

public class ArrayList

{

private Object[] buffer;

private int currentSize;

private boolean lock;

/**

Constructs an empty array list.

*/

public ArrayList()

{

final int INITIAL_SIZE = 10;

buffer = new Object[INITIAL_SIZE];

currentSize = 0;

lock = false;

}

/**

Gets the size of this array list.

@return the size

*/

public int size()

{

//complete this method

}

/**

Throws an IndexOutOfBoundsException if the checked index is out of bounds

@param n the index to check

*/

private void checkBounds(int n)

{

if (n < 0 || n >= currentSize)

{

throw new IndexOutOfBoundsException();

}

}

/**

Gets the element at a given position.

@param pos the position

@return the element at pos

*/

public Object get(int pos)

{

//complete this method

}

/**

Sets the element at a given position.

@param pos the position

@param element the new value

*/

public void set(int pos, Object element)

{

//complete this method

}

/**

Removes the element at a given position.

@param pos the position

@return the removed element

*/

public Object remove(int pos)

{

//complete this method

}

/**

Adds an element after a given position.

@param pos the position

@param newElement the element to add

@return true

*/

public boolean add(int pos, Object newElement)

{

//complete this method

}

/**

Adds an element after the end of the array list

@param newElement the element to add

@return true

*/

public boolean addLast(Object newElement)

{

//complete this method

}

/**

Grows the buffer if the current size equals the buffer's capacity.

*/

private void growBufferIfNecessary()

{

if (currentSize == buffer.length)

{

Object[] newBuffer = new Object[2 * buffer.length];

for (int i = 0; i < buffer.length; i++)

{

newBuffer[i] = buffer[i];

}

buffer = newBuffer;

}

}

}

ArrayListDemo

import java.util.Random;

/** This demo for demonstrating a thread safe array list. */ public class ArrayListThreadDemo implements Runnable { private static ArrayList list; private static Random randomizer;

public void run() { int choice = list.size(); if (list.size() > 0) { choice = randomizer.nextInt(list.size() + 1); } if (choice == list.size()) { String token = randomizer.nextInt(1000) + ""; list.addLast(token); System.out.println("Added: " + token); } else { String token = (String) list.remove(choice); System.out.println("Removed: " + token); } if (list.size() > 0) { run(); } }

public static void main(String[] args) { randomizer = new Random(); list = new ArrayList(); Thread thread1 = new Thread(new ArrayListThreadDemo()); thread1.start(); Thread thread2 = new Thread(new ArrayListThreadDemo()); thread2.start(); } }