java - How to select the best .class file from a folder? -

i have 1 application handling math operator . app has interface operator so:

public interface operator {     double calculate(double firstnumber,double secondnumber);     string getsign(); } 

i have 4 class implement operator so:

public class plus implements operator {        public double calculate(double firstnumber,double secondnumber)     {         return firstnumber + secondnumber;     }      public string getoperator()      {         return "+";     } } public class minus implements operator {        public double calculate(double firstnumber,double secondnumber)     {         return firstnumber - secondnumber;     }      public string getoperator()      {         return "-";     } } 

and on...

i compiling classes , put .class file in folder("myfolder") . in main program(main app folder) type of operator user , according operator want choose .class file correct 1 don't know how . can me ?

don't. why? using reflection cost down performance.

suggest:

final string op_plus = "+"; final string op_minus = "-";  list<operator> listoperator = new list<>(){ new plus(), new minus() };  operator getoperator(string inputoperator) throws notsupportedexception {     for(operator op : listoperator)         if(op.getoperator().equals(inputoperator))             return op;     throw new notsupportedexception(); } 

using:

operator op = getoperator(op_plus); double result = op.calculate(d1, d2); 

---

in case of have way (teacher tell you, etc....), can use java relection.
read java reflection first or thread http://tutorials.jenkov.com/java-reflection/methods.html
following answer you:

string requiredoperator =... // input  file file = new file("c:\\myclasses\\"); url url = file.tourl();          // file:/c:/myclasses/ url[] urls = new url[]{url};  // create new class loader directory classloader cl = new urlclassloader(urls);  // load in class; myclass.class should located in // directory file:/c:/myclasses/com/mycompany // class cls = cl.loadclass("com.mycompany.myclass"); <= how load class class[] classes; // load classes: plus.class, minus.class array classes = new class[]{ cl.loadclass("com.mycompany.plus"), cl.loadclass("com.mycompany.minus") }; method method; object instance; string identifier; for(class cls : classes) {     instance = cls.newinstance();     method = cls.getmethod("getoperator", null); // method getoperator of operator interface not have parameter     identifier = (string)method.invoke(instance, null);     if(identifier.equals(requiredoperator))     {         method = cls.getmethod("calculate", new class[] { double.class, double.class } ); // 2 params double         return (double)method.invoke(instance, double1, double2);     } } 

i did not test code that's way you