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;
        }
    }