Question

请帮我如何将字符串更改为一组命令行。请考虑以下代码：

public class TestStringCommand{

   public static void main(String[] args) {
       System.out.println("Change String to Command");

       int a , b, c;
       String S = "a =1 ; b =2; c = a + b;";
       //How to change S into a set of command lines
     // c will be 3

   }
}

Answer 1

一般来说，如果它只是那些变量而只是添加，你可以摆脱所有的空格并打开字符。

int a, b, c; Pattern integer = Pattern.compile("-?\\d+"); //fields

主：

String input = ... ;
String statements = input.replaceAll("\\s+","");  //remove the white space
int next_statement_start, current_stmnt_start = 0;
while ( ! statements.isEmpty() ) {
      //^ while we have more statements to process
      next_statement_start = statements.indexOf(';') + 1;
      String statement = statments.substr(current_stmnt_start, next_statement_start);

      processStatement(statement);

      current_stmnt_start = next_statement_start;
      next_statement_start = statements.indexOf(';', next_statement_start) + 1
}

processStatement：

String[] parts = stmnt.split("=");
if ( parts.length != 2) throw new IllegalArgumentException(...);
int val = evaluateExpression(parts[1].substr(0, parts[1].length()-1)); //remove the ';' 
setVar(parts[0], val);

SETVAR：

switch ( var ) {
   case "a": a = val; break;
   case "b": b = val; break;
   ...
   default:
      throw new IllegalArgumentException(...);
}

getVar：

switch ( var ) {
    case "a": return a;
    case "b": return b;
    ...
    default:
      throw new IllegalArgumentException (...)
}

evaluateExpression：

if (expr.isEmpty()) throw new ...;
int sum = 0; Matcher m = integer.matcher("");
for (String part :  expr.split("+")) {
    if (m.reset(part).matches()) {
        sum += Integer.parseInt(part);
    } else {
        sum += getVar(part);
    }
}
return sum;

我甚至没有尝试编译上面的代码，但这是个主意。您也可以使用reflection虽然功能强大，但通常不是正确的方法。

或者这就是你想要的？ https://en.wikipedia.org/wiki/Recursive_descent_parser

或者这是你真正需要的吗？https://docs.oracle.com/javase/tutorial/essential/environment/properties.html

如何将S更改为一组命令行

1 个答案: