后缀表达式 Java实现

 1     public class PostfixEvaluator{
 2         private final static char ADD = ‘+‘;
 3         private final static char SUBTRACT = ‘-‘;
 4         private final static char MULTIPLY = ‘*‘;
 5         private final static char DIVIDE = ‘/‘;
 6         
 7         private Stack<Integer> stack;
 8         
 9         public PostfixEvaluator() {
10             stack = new Stack<Integer>();
11         }
12         
13         public int evaluate(String expr) {
14             int op1,op2,result=0;
15             String token;
16             Scanner parser = new Scanner(expr);
17             
18             while(parser.hasNext()) {
19                 token = parser.next();
20                 
21                 if(isOperator(token)) {
22                     op2 = (stack.pop()).intValue();
23                     op1 = (stack.pop()).intValue();
24                     result = evaluateSingleOperator(token.charAt(0),op1,op2);
25                     stack.push((Integer)result);
26                 }else {
27                     stack.push(Integer.parseInt(token));
28                 }
29             }
30             return result;
31         }
32         
33         private boolean isOperator(String token) {
34             return (token.equals("+") || token.equals("-") ||
35                     token.equals("*") || token.equals("/"));
36         }
37         
38         private int evaluateSingleOperator(char operation,int op1,int op2) {
39             int result = 0;
40             switch(operation) {
41             case ADD:
42                 result = op1+op2;
43                 break;
44             case SUBTRACT:
45                 result = op1-op2;
46                 break;
47             case MULTIPLY:
48                 result = op1*op2;
49                 break;
50             case DIVIDE:
51                 result = op1/op2;
52             }
53             return result;
54         }
55     }