MathematicalExpression.java
 
import java.io.IOException;
import java.io.StreamTokenizer;
import java.io.StringReader;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
 
/**
* MathematicalExpression 基于逆波兰式，即数学表达式的后缀表示，
* 对指定表达式进行求
* 值。暂时支持简单的二元运算
* @author Terry
* @version 2010-8-30
*/
public class MathematicalExpression {
     private String expression = "";
     private List postExp = new ArrayList();
/**
*
* @param expression
*/
public MathematicalExpression(String expression) {
      this.expression = expression;
}
public MathematicalExpression(){};public String getExpression() {
      return expression;
}
public void setExpression(String expression) {
     this.expression = expression;
}
/**
* 得到表达式的值
* @return
*/
public double getValue() {
    toPostExpression();
   Stack stack = new Stack();
 
   double n1, n2, result;
for(String element : postExp) {
    if (isOperator(element)) {
    n1 = Double.parseDouble(stack.pop());
    n2 = Double.parseDouble(stack.pop());
    result = doOperate(n2, n1, element);
     stack.push(new Double(result).toString());
   } else {
     stack.push(element);
  }
}
return Double.parseDouble(stack.pop());
}
private void toPostExpression(){
    expression += "#";
    char ch = '#', ch1, op;
    Stack s = new Stack();
    StreamTokenizer tf = new StreamTokenizer(new StringReader(expression));
    tf.ordinaryChar('/');
    tf.ordinaryChar('(');
    tf.ordinaryChar(')');
    tf.ordinaryChar('-');
    s.push(ch);
    int i;
    try {
        i = tf.nextToken();
        while (i != StreamTokenizer.TT_EOF && !s.isEmpty()) {
        switch (i) {
            case StreamTokenizer.TT_EOF:
            case StreamTokenizer.TT_EOL:
                break;
            case StreamTokenizer.TT_NUMBER:
                postExp.add(String.valueOf(tf.nval));
                i = tf.nextToken();
                break;
            case StreamTokenizer.TT_WORD:
                break;
            default:
                ch = (char) i;
                ch1 = s.peek();
                if (isp(ch1) < icp(ch)) {
                    s.push(ch);
                    i = tf.nextToken();
                } else if (isp(ch1) > icp(ch)) {
                    op = s.pop();
                    postExp.add(String.valueOf(op));
                } else {
                    op = s.pop();
                    if (op == '(')
                    i = tf.nextToken();
                }
        }
    }
    } catch (IOException e) {
    // TODO Auto-generated catch block
    //e.printStackTrace();
    }
}
 
//实际运算
private  double doOperate(double n1, double n2, String operator) {
	if (operator.equals("+"))
		return n1 + n2;
	else if (operator.equals("-"))
		return n1 - n2;
	else if (operator.equals("*"))
		return n1 * n2;
	else
		return n1 / n2;
}
//判断是否是操作符
private static boolean isOperator(String str){
    return str.equals("+") || str.equals("-") || str.equals("*")
             || str.equals("/");
}
 
// 栈内优先级
private  short isp(char c) {
	short priority = 0;
	switch (c) {
	case '#':
		priority = 0;
		break;
	case '(':
		priority = 1;
		break;
	case '*':
	case '/':
		priority = 5;
		break;
	case '+':
	case '-':
		priority = 3;
		break;
	case ')':
		priority = 6;
		break;
	default:
		break;
	}
	return priority;
}
 
// 栈外优先级
private  short icp(char c) {
	short priority = 0;
	switch (c) {
	case '#':
		priority = 0;
		break;
	case '(':
		priority = 6;
		break;
	case '*':
	case '/':
		priority = 4;
		break;
	case '+':
	case '-':
		priority = 2;
		break;
	case ')':
		priority = 1;
		break;
	default:
		break;
	}
	return priority;
}

public static void main(String[] args) {
     Scanner scanner = new Scanner(System.in);
     DecimalFormat df=new DecimalFormat("0.00");
     MathematicalExpression me = new MathematicalExpression();
     while(scanner.hasNext())
     {
         me.setExpression(scanner.next());
          System.out.println(df.format(me.getValue()));
     }
 }