Simple Calculator

StackMachine.java

import java.util.Deque;

/**
 * @author taichi
 */
public class StackMachine {
	interface Op {
		void execute(Deque<String> stack);
	}

	static abstract class UniOp implements Op {
		@Override
		public void execute(Deque<String> stack) {
			stack.push(this.execute(stack.pop()));
		}

		abstract String execute(String value);
	}

	static abstract class BiOp implements Op {
		@Override
		public void execute(Deque<String> stack) {
			stack.push(this.execute(stack.pop(), stack.pop()));
		}

		String execute(String l, String r) {
			return String.valueOf(this.execute(Integer.parseInt(l),
					Integer.parseInt(r)));
		}

		abstract int execute(int l, int r);
	}

	public Op to(final String v) {
		switch (v) {
		case "!":
			return new UniOp() {
				@Override
				String execute(String value) {
					long result = 1L;
					for (int i = 1, v = Integer.parseInt(value); i <= v; i++) {
						result *= i;
					}
					return String.valueOf(result);
				}
			};
		case "+":
			return new BiOp() {
				@Override
				int execute(int l, int r) {
					return r + l;
				}
			};
		case "-":
			return new BiOp() {
				@Override
				int execute(int l, int r) {
					return r - l;
				}
			};
		case "*":
			return new BiOp() {
				@Override
				int execute(int l, int r) {
					return r * l;
				}
			};
		case "/":
			return new BiOp() {
				@Override
				int execute(int l, int r) {
					return r / l;
				}
			};
		default: // do nothing.
			break;
		}
		return new Op() {
			@Override
			public void execute(Deque<String> stack) {
				stack.push(v);
			}
		};
	}

	public String execute(String input) {
		Deque<String> stack = new java.util.ArrayDeque<>();
		for (String s : input.split("\\s")) {
			this.to(s).execute(stack);
		}
		return stack.pop();
	}
}

StackMachineTest.java

import static org.junit.Assert.assertEquals;

import java.util.Arrays;
import java.util.Collection;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/**
 * Junit4.11 is not released. use <a
 * href="https://github.com/KentBeck/junit/downloads">snapshot</a>.
 * 
 * @author taichi
 * @see
 * @see <a
 *      href="https://github.com/KentBeck/junit/commit/e0cddcdfa8ec823da136af500cce0910a1dd2264">Add
 *      names for parameterized tests. Fixes #24 and #44.</a>
 */
@RunWith(value = Parameterized.class)
public class StackMachineTest {

	String input;

	String expected;

	StackMachine target;

	public StackMachineTest(String input, String expected) {
		this.input = input;
		this.expected = expected;
	}

	// if you use Junit4.10 or less, remove name attribute.
	@Parameters(name = "{0} -> {1}")
	public static Collection<Object[]> data() {
		Object[][] data = new Object[][] { //
		{ "1", "1" }, //
				{ "1 2 +", "3" }, //
				{ "3 1 -", "2" }, //
				{ "10 2 /", "5" }, //
				{ "2 5 8 + +", "15" }, //
				{ "2 5 8 + *", "26" }, //
				{ "2 5 8 * +", "42" }, //
				{ "1 2 + 3 4 + *", "21" }, //
				{ "2 3 * 4 5 * +", "26" }, //
				{ "3 ! !", "720" }, //
				{ "4 1 - ! 1 + ! 56 /", "90" }, //
				{ "0 !", "1" } };
		return Arrays.asList(data);
	}

	@Before
	public void setUp() {
		this.target = new StackMachine();
	}

	@Test
	public void test() {
		assertEquals(this.expected, this.target.execute(this.input));
	}

}