java - errors/exceptions accumulating design pattern -

a method returns result, making number of "attempts" build it. first attempt succeeds should return. if none of them succeed exception should thrown:

class calculator {   public string calculate() throws exception {     // how design it?   }   private string attempt1() throws exception {     // try calculate , throw if fails   }   private string attempt2() throws exception {     // try calculate , throw if fails   }   private string attempt3() throws exception {     // try calculate , throw if fails   } } 

it's important mention exception thrown calculate should preserve stack traces of other exceptions thrown private methods. how recommend design calculate() method, extendability , maintainability in mind?

i use composite , command.

interface calculatecommand {      public void calculate(calculatecontext context); } 

now create implementation each attempt want.

next create compositecommand -- here outline (you need fill in blanks)

public class compositecalculatecommand implements calculatecommand {      compositecalculatecommand(list<compositecommand> commands) {         this.commands = commands; // define field     }      public void calculate(commandcontext context) {          (calculatecommand command : commands) {                try {                    command.calculate(context);                } catch(runtimeexception e) {                    this.exceptions.add(e) // initialize list hold exceptions                }                if (context.hasresult) return; // break          }          // throw here. didn't success since never saw success in context.  have list of exceptions.     }  } 

finally use like

calculatecommand allcommands = new compositecalculatecommand(somelistofcommands); allcommands.calculate(somecontextthatyoudefine); // results on context. 

note each command implementation testable on own, maintainable. if need add calculations, define new type of calculatecommand, extensible. play dependency injection. note define commandcontext object different commands can take different types of stuff (put in context).