四則演算をするスタックマシン

StackMachine.java

import java.util.ArrayDeque;
import java.util.Deque;

interface Token {
	Deque<Integer> execute(Deque<Integer> stack);
}

class Operand implements Token {
	Integer value;

	Operand(Integer val) {
		value = val;
	}

	@Override
	public Deque<Integer> execute(Deque<Integer> stack) {
		stack.push(value);
		System.out.println("[スタック] <= " + value);
		return stack;
	}
}

interface Operator extends Token {
	// Deque<Integer> execute(Deque<Integer> stack);
}

class Add implements Operator {
	@Override
	public Deque<Integer> execute(Deque<Integer> stack) {
		if (stack != null) {
			Integer num1 = stack.pop();
			System.out.println("[スタック] => " + num1);
			Integer num0 = stack.pop();
			System.out.println("[スタック] => " + num0);

			stack.push(num0 + num1);
			System.out.println("[スタック] <= " + (num0 + num1));
		}
		return stack;
	}
}

class Sub implements Operator {
	@Override
	public Deque<Integer> execute(Deque<Integer> stack) {
		if (stack != null) {
			Integer num1 = stack.pop();
			System.out.println("[スタック] => " + num1);
			Integer num0 = stack.pop();
			System.out.println("[スタック] => " + num0);

			stack.push(num0 - num1);
			System.out.println("[スタック] <= " + (num0 - num1));
		}
		return stack;
	}
}

class Mul implements Operator {
	@Override
	public Deque<Integer> execute(Deque<Integer> stack) {
		if (stack != null) {
			Integer num1 = stack.pop();
			System.out.println("[スタック] => " + num1);
			Integer num0 = stack.pop();
			System.out.println("[スタック] => " + num0);

			stack.push(num0 * num1);
			System.out.println("[スタック] <= " + (num0 * num1));
		}
		return stack;
	}
}

class Div implements Operator {
	@Override
	public Deque<Integer> execute(Deque<Integer> stack) {
		if (stack != null) {
			Integer num1 = stack.pop();
			System.out.println("[スタック] => " + num1);
			Integer num0 = stack.pop();
			System.out.println("[スタック] => " + num0);

			if (num1 != 0) {
				stack.push(num0 / num1);
				System.out.println("[スタック] <= " + (num0 / num1));
			}
		}
		return stack;
	}
}

public class StackMachine {

	protected Token createToken(String s) {
		Token result = null;
		if (s.equals("+")) {
			result = new Add();
		} else if (s.equals("-")) {
			result = new Sub();
		} else if (s.equals("*")) {
			result = new Mul();
		} else if (s.equals("/")) {
			result = new Div();
		} else {
			result = new Operand(Integer.valueOf(s));
		}
		return result;
	}

	protected int execute(String input) {
		Deque<Integer> stack = new ArrayDeque<>();

		String[] splited = input.split("\\s");
		if (splited != null && 0 < splited.length) {
			for (String s : splited) {
				Token token = createToken(s);
				token.execute(stack);
			}
		}
		return stack.pop();
	}

	public static void main(String[] args) {
		StackMachine stackMachine = new StackMachine();

		// (1 + 2) * 4 + (4 - 2) == 14
		System.out.println(stackMachine.execute("1 2 + 4 * 4 2 - +"));

		// (1 + 4) * (3 + 7) / 5 == 10
		System.out.println(stackMachine.execute("1 4 + 3 7 + * 5 /"));

		// 10 + 2 == 12
		System.out.println(stackMachine.execute("10 2 +"));

		// 10 / 2 == 5
		System.out.println(stackMachine.execute("10 2 /"));
	}
}