后缀表达式 Java实现

基本原理：

　　从左到右扫描字符串：1、是操作数：压栈。　　2、是操作符：出栈两个操作数，将运算结果压栈。

扫描字符串通过java.util.Scanner类实现，其next方法可以读取以空格（默认）或指定符号分割的元素。

算法代码：

     public class PostfixEvaluator{
         private final static char ADD = '+';
         private final static char SUBTRACT = '-';
         private final static char MULTIPLY = '*';
         private final static char DIVIDE = '/';

         private Stack<Integer> stack;

         public PostfixEvaluator() {
             stack = new Stack<Integer>();
         }

         public int evaluate(String expr) {
             int op1,op2,result=0;
             String token;
             Scanner parser = new Scanner(expr);

             while(parser.hasNext()) {
                 token = parser.next();

                 if(isOperator(token)) {
                     op2 = (stack.pop()).intValue();
                     op1 = (stack.pop()).intValue();
                     result = evaluateSingleOperator(token.charAt(0),op1,op2);
                     stack.push((Integer)result);
                 }else {
                     stack.push(Integer.parseInt(token));
                 }
             }
             return result;
         }

         private boolean isOperator(String token) {
             return (token.equals("+") || token.equals("-") ||
                     token.equals("*") || token.equals("/"));
         }

         private int evaluateSingleOperator(char operation,int op1,int op2) {
             int result = 0;
             switch(operation) {
             case ADD:
                 result = op1+op2;
                 break;
             case SUBTRACT:
                 result = op1-op2;
                 break;
             case MULTIPLY:
                 result = op1*op2;
                 break;
             case DIVIDE:
                 result = op1/op2;
             }
             return result;
         }
     }