Basic RPN.java

import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		String  inputString;

		Stack   stack = new Stack();

		while(true) {
			System.out.print("> ");
			inputString = input.nextLine();

			if(inputString.contentEquals("q"))
				break;

			for(String s : inputString.split(" ")) {
				if(s.length() == 0)
					break;

				// First try to see if s is just a number and can be pushed...
				try {
					stack.push(Double.parseDouble(s));

				// Otherwise move on with life.
				} catch (NumberFormatException e) {

					if (s.contentEquals("clear"))
						stack.clear();

					else {
						try {
							if (s.contentEquals("drop"))
								stack.pop();
							else {
								double n1 = stack.pop();
								double n2 = stack.pop();

								if (s.contentEquals("+"))
									stack.push(n1 + n2);

								else if (s.contentEquals("-"))
									stack.push(n2 - n1);

								else if (s.contentEquals("/"))
									stack.push(n2 / n1);

								else if (s.contentEquals("*"))
									stack.push(n1 * n2);
							}

						} catch (EmptyStackException f) {
							System.out.println("Stack is empty!");
						}
					}
				}
			}

			stack.printStack();
		}

		System.out.println("Bye!");
	}
}

class Stack {
	List<Double> stackList = new ArrayList<Double>();

	public void push(double n) {
		stackList.add(n);
	}

	public double pop() {
		if(stackList.size() == 0)
			throw new EmptyStackException();
		return stackList.remove(stackList.size() - 1);
	}

	public void printStack() {
		System.out.println("Size: " + stackList.size());
		for(double n : stackList)
			System.out.println(n);
	}

	public void clear() {
		stackList.clear();
	}
}