java实现任意四则运算表达式求值算法

Map<String, Map<String, String>>

/** * 查表得到op1和op2的优先级 * @param op1 运算符1 * @param op2 运算符2 * @return ">", "<" 或 "=" */public String priority(String op1, String op2) { return priorityMap.get(op1).get(op2);}

/** * 算术表达式求值。 * 3 + 4 * 12 结果为51 * @author whf * */public class EvaluateExpression { // 运算符优先级关系表 private Map<String, Map<String, String>> priorityMap = new HashMap<String, Map<String, String>>(); private LinkedStack<String> optStack = new LinkedStack<String>(); // 运算符栈 private LinkedStack<Double> numStack = new LinkedStack<Double>(); // 操作数栈 /**  * 计算表达式  * @param exp 四则运算表达式, 每个符号必须以空格分隔  * @return  */ public double calcualte(String exp) {  StringTokenizer st = new StringTokenizer(exp);  while (st.hasMoreTokens()) {   String token = st.nextToken();   process(token);  }  return numStack.pop(); } /**  * 读入一个符号串。  * 如果是数字，则压入numStack  * 如果是运算符，则将optStack栈顶元素与该运算符进行优先级比较  * 如果栈顶元素优先级低，则将运算符压入optStack栈,如果相同，则弹出左括号，如果高，则取出２个数字，取出一个运算符执行计算，然后将结果压入numStack栈中  * @param token  */ private void process(String token) {  while (false == "#".equals(optStack.getTop()) || false == token.equals("#")) {   // token is numeric   if (true == isNumber(token)) {        numStack.push(Double.parseDouble(token));        break;        // token is operator   } else {        String priority = priority(optStack.getTop(), token);        if ("<".equals(priority)) {         optStack.push(token);         break;        } else if ("=".equals(priority)) {         optStack.pop();         break;        } else {          double res = calculate(optStack.pop(), numStack.pop(), numStack.pop());          numStack.push(res);        }   }  } } /**  * 执行四则运算  * @param opt  * @param n1  * @param n2  * @return  */ private double calculate(String opt, double n1, double n2) {  if ("+".equals(opt)) {   return n2 + n1;  } else if ("-".equals(opt)) {   return n2 - n1;  } else if ("*".equals(opt)) {   return n2 * n1;  } else if ("/".equals(opt)) {   return n2 / n1;  } else {   throw new RuntimeException("unsupported operator:" + opt);  } } /**  * 检查一个String是否为数字  * @param token  * @return  */ private boolean isNumber(String token) {  int LEN = token.length();  for (int ix = 0 ; ix < LEN ; ++ix) {   char ch = token.charAt(ix);   // 跳过小数点   if (ch == '.') {    continue;   }   if (false == isNumber(ch)) {    return false;   }  }  return true; } /**  * 检查一个字符是否为数字  * @param ch  * @return  */ private boolean isNumber(char ch) {  if (ch >= '0' && ch <= '9') {   return true;  }  return false; } /**  * 查表得到op1和op2的优先级  * @param op1 运算符1  * @param op2 运算符2  * @return ">", "<" 或 "="  */ public String priority(String op1, String op2) {  return priorityMap.get(op1).get(op2); } /**  * 构造方法，初始化优先级表  */ public EvaluateExpression() {  // initialize stack  optStack.push("#");  // initialize priority table  // +  Map<String, String> subMap = new HashMap<String, String>();  subMap.put("+", ">");  subMap.put("-", ">");  subMap.put("*", "<");  subMap.put("/", "<");  subMap.put("(", "<");  subMap.put(")", ">");  subMap.put("#", ">");  priorityMap.put("+", subMap);  // -  subMap = new HashMap<String, String>();  subMap.put("+", ">");  subMap.put("-", ">");  subMap.put("*", "<");  subMap.put("/", "<");  subMap.put("(", "<");  subMap.put(")", ">");  subMap.put("#", ">");  priorityMap.put("-", subMap);  // *  subMap = new HashMap<String, String>();  subMap.put("+", ">");  subMap.put("-", ">");  subMap.put("*", ">");  subMap.put("/", ">");  subMap.put("(", "<");  subMap.put(")", ">");  subMap.put("#", ">");  priorityMap.put("*", subMap);  // /  subMap = new HashMap<String, String>();  subMap.put("+", ">");  subMap.put("-", ">");  subMap.put("*", ">");  subMap.put("/", ">");  subMap.put("(", "<");  subMap.put(")", ">");  subMap.put("#", ">");  priorityMap.put("/", subMap);  // (  subMap = new HashMap<String, String>();  subMap.put("+", "<");  subMap.put("-", "<");  subMap.put("*", "<");  subMap.put("/", "<");  subMap.put("(", "<");  subMap.put(")", "=");  //subMap.put("#", ">");  priorityMap.put("(", subMap);  // )  subMap = new HashMap<String, String>();  subMap.put("+", ">");  subMap.put("-", ">");  subMap.put("*", ">");  subMap.put("/", ">");  //subMap.put("(", "<");  subMap.put(")", ">");  subMap.put("#", ">");  priorityMap.put(")", subMap);  // #  subMap = new HashMap<String, String>();  subMap.put("+", "<");  subMap.put("-", "<");  subMap.put("*", "<");  subMap.put("/", "<");  subMap.put("(", "<");  //subMap.put(")", ">");  subMap.put("#", "=");  priorityMap.put("#", subMap); }}

String exp = "4 * ( 10 + 2 ) + 1 #";EvaluateExpression ee = new EvaluateExpression();out.println(ee.calcualte(exp));