解释器Interpreter

//Purpose.  Interpreter aside.  "The pattern doesn't address parsing."  "When
//the grammer is very complex, other techniques (such as a parser) are more
//appropriate." [GOF, p247]

public class InterpreterDemo {
	public static boolean precedence(char a, char b) {
		String high = "*/", low = "+-";
		if (a == '(')
			return false; // if (a == '(' && b == ')') return false;
		if (a == ')' && b == '(') {
			System.out.println(")-(");
			return false;
		}
		if (b == '(')
			return false;
		if (b == ')')
			return true;
		if (high.indexOf(a) > -1 && low.indexOf(b) > -1)
			return true;
		if (high.indexOf(a) > -1 && high.indexOf(b) > -1)
			return true;
		if (low.indexOf(a) > -1 && low.indexOf(b) > -1)
			return true;
		return false;
	}

	public static String convertToPostfix(String in) {
		StkChar opstk = new StkChar();
		StringBuffer out = new StringBuffer();
		String opers = "+-*/()";
		char topsym = '+';
		boolean empty;

		for (int i = 0; i < in.length(); i++)
			if (opers.indexOf(in.charAt(i)) == -1)
				out.append(in.charAt(i));
			else {
				while (!(empty = opstk.isEmpty())
						&& precedence(topsym = opstk.pop(), in.charAt(i)))
					out.append(topsym);
				if (!empty)
					opstk.push(topsym);
				if (empty || in.charAt(i) != ')')
					opstk.push(in.charAt(i));
				else
					topsym = opstk.pop();
			}
		while (!opstk.isEmpty())
			out.append(opstk.pop());
		return out.toString();
	}

	public static int evaluate(String in) {
		StkInt stack = new StkInt();
		String opers = "+-*/";
		for (int a, b, i = 0; i < in.length(); i++)
			if (opers.indexOf(in.charAt(i)) == -1)
				stack.push(in.charAt(i) - 48);
			else {
				b = stack.pop();
				a = stack.pop();
				if (in.charAt(i) == '+')
					a = a + b;
				else if (in.charAt(i) == '-')
					a = a - b;
				else if (in.charAt(i) == '*')
					a = a * b;
				else if (in.charAt(i) == '/')
					a = a / b;
				stack.push(a);
			}
		return stack.pop();
	}

	public static void main(String[] args) {
		System.out.print(args[0]);
		String postfix = convertToPostfix(args[0]);
		System.out.print(" -- " + postfix);
		System.out.println(" -- " + evaluate(postfix));
	}
}

// 2+3*4-5+6 -- 234*+5-6+ -- 15
// (2+3)*4-5+6 -- 23+4*5-6+ -- 21
// 2+3*(4-5)+6 -- 2345-*+6+ -- 5
// 2+3*((4-5)+6) -- 2345-6+*+ -- 17
// (3-(4*(5+6))/(7-8))*9/4 -- 3456+*78-/-9*4/ -- 105

class StkChar {
	private char[] arr = new char[9];
	private int sp = -1;

	void push(char ch) {
		if (!isFull())
			arr[++sp] = ch;
	}

	char pop() {
		if (isEmpty())
			return '\0';
		return arr[sp--];
	}

	boolean isFull() {
		return sp == arr.length - 1;
	}

	boolean isEmpty() {
		return sp == -1;
	}
}

class StkInt {
	private int[] arr = new int[9];
	private int sp = -1;

	void push(int ch) {
		if (!isFull())
			arr[++sp] = ch;
	}

	int pop() {
		if (isEmpty())
			return 0;
		return arr[sp--];
	}

	boolean isFull() {
		return sp == arr.length - 1;
	}

	boolean isEmpty() {
		return sp == -1;
	}
}