RPN Evaluator

RPNDemo.java

package michalwa.rpn;

import java.util.HashMap;
import java.util.Scanner;
import java.util.Stack;

public class RPNDemo
{
	@FunctionalInterface
	private static interface Operator
	{
		public double eval(double a, double b);
	}
	
	public static void main(String[] args)
	{
		HashMap<Character, Operator> ops = new HashMap<Character, Operator>();
		ops.put('+', (a, b) -> a + b);
		ops.put('-', (a, b) -> a - b);
		ops.put('*', (a, b) -> a * b);
		ops.put('/', (a, b) -> a / b);
		
		System.out.print("Type in your RPN expression: ");
		Scanner scanner = new Scanner(System.in);
		
		Stack<Double> stack = new Stack<Double>();
		boolean noOps = true;
		while(scanner.hasNext()) {
			if(scanner.hasNextDouble()) {
				stack.push(scanner.nextDouble());
			} else {
				char op = scanner.next().charAt(0);
				if(ops.containsKey(op)) {
					if(stack.size() >= 2) {
						double b = stack.pop();
						double a = stack.pop();
						stack.push(ops.get(op).eval(a, b));
						noOps = false;
					}
				}
			}
		}
		
		if(noOps || stack.size() != 1) {
			System.out.println("This is not a valid RPN expression.");
			System.exit(0);
		} else {
			scanner.close();
			System.out.println(" = " + stack.peek());
		}
	}
}