[CC150][3.1] Describe how you could use a single array to implement three stacks

TripleStack.java

//* http://algs4.cs.princeton.edu/13stacks/
//* [statck1 element, statck2 element, statck3 element, statck1 element,statck2 element,...,statck3 element]
//* double the array size when one stack is full
//* shrink the array size to half when all three stacks are one-quarter size full
//* time complexity: average O(1) for push() and pop()
import java.lang.reflect.Array;
import java.util.Iterator;
import java.util.NoSuchElementException;

public class TripleStack<Item> {
	public stack stack1;
	public stack stack2;
	public stack stack3;
	private stack[] stacks;
	private Item[] data;
	private boolean[] oneQuarterFull;
	
	@SuppressWarnings("unchecked")
	public TripleStack() {
		data = (Item[]) new Object[6];
		//stacks = (stack[]) new Object[3]; // doesn't work!!
		stacks = (stack[]) Array.newInstance(stack.class, 3); // works
		stack1 = new stack(0);
		stack2 = new stack(1);
		stack3 = new stack(2);
		stacks[0] = stack1;
		stacks[1] = stack2;
		stacks[2] = stack3;
		oneQuarterFull = new boolean[3];
	}
	
	private class stack implements Iterable<Item> {
		private int N; //number of elements in stack1
		private int offset;
		
		public stack(int os) {
			this.offset = os;
			this.N = 0;
		}
		
		public void push(Item item) {
			data[this.N*3 + this.offset] = item;
			this.N++;
			if (this.N == data.length/3)
				resize(6*data.length);
		}
		
		public Item pop() {
			Item popOut = data[(this.N - 1)*3 + this.offset];
			this.N--;
			if (this.N == data.length/12) {
				oneQuarterFull[this.offset] = true;
				if (oneQuarterFull[0] && oneQuarterFull[1] && oneQuarterFull[2]) {
					resize(data.length/3);
					oneQuarterFull[0] = false;
					oneQuarterFull[1] = false;
					oneQuarterFull[2] = false;
				}
			}
			return popOut;
		}
		
		public Iterator<Item> iterator() {
			return new ArrayIterator();
		}
		
		private class ArrayIterator implements Iterator<Item> {
			private int i = 0;
			public boolean hasNext() { return i < N; }
			public void reomve() { throw new UnsupportedOperationException();}
			
			public Item next() {
				if (!hasNext()) throw new NoSuchElementException();
				return data[3*i++ + offset];
			}	
		}
		
	}
	
	@SuppressWarnings("unchecked")
	private void resize(int max) {
        Item[] temp = (Item[]) new Object[max];
        for (int i = 0; i < 3; i++) {
        	for (int j = 0; j < stacks[i].N; j++)
        		temp[j*3 + stacks[i].offset] = data[j*3 + stacks[i].offset];
        }
        data = temp;
	}
	
	public String toString() {
		StringBuilder s = new StringBuilder();
		for (int i = 0; i < 3; i++) {
			s.append("Stack " + (i+1) + ": ");
			for (Item it : stacks[i]) 
				s.append(it.toString() + " ");
			s.append("\n");
		}	
		return s.toString();
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		TripleStack<Integer> test = new TripleStack<Integer>();
		for (int i = 0; i < 5; i++) test.stack1.push(i);
		for (int i = 0; i < 5; i++) test.stack2.push(i);
		for (int i = 0; i < 5; i++) test.stack3.push(i);
		System.out.println(test.toString());
		
		System.out.println("************POP Test**************");
		System.out.print("Stack 1 - pop :");
		for (int i = 0; i < 5; i++)
			System.out.print(test.stack1.pop() + " ");
		System.out.print("\n");
		
		System.out.print("Stack 2 - pop :");
		for (int i = 0; i < 5; i++)
			System.out.print(test.stack2.pop() + " ");
		System.out.print("\n");
		
		System.out.print("Stack 3 - pop :");
		for (int i = 0; i < 5; i++)
			System.out.print(test.stack3.pop() + " ");
		System.out.print("\n");

		System.out.println("\n"+test.toString());
	}

}