Question

I am trying to write a Java function which has List object as output parameter.

boolean myFunction(int x, in y, List myList)
{
    /* ...Do things... */
    myList=anotherList.subList(fromIndex, toIndex);
    return true;
}

Before that I call the function I declare myList as follow:

List myList=null;

Then I call the function

myFunction(x,y,myList)

But when I try to manipulate myList, I find that myList is still null.

I am sure the variable anotherList in my function code is not null and I am sure that the subList function return a non-empty List.

What is the reason, and how can pass a List as output parameter in a Java function?

Answer 1

Java always uses pass by value. This means that manipulating a passed variable won’t affect the variable that was passed by the caller.

In order to solve your problem there are some possibilities:

• Return the sublist:

  List myFunction(int x, int y) { return anotherList.subList(....);}
  

  I know that this way gets rids of your boolean return value.

• Create a structure that holds the pointer to the List and.

  class Reference <T>
  {
      public T ref;
  }
  
  boolean myFunction(int x, int y, Reference<List> listRef)
  {
        listRef.ref = anotherList.subList(....);
        return true;
  }
  

• Create a structure that holds all the output you want the method to return:

  class MyFunctionOutput
  {
       List list;
       boolean b;
  }
  
  MyFunctionOutput myFunction(int x, int y)
  {
       MyFunctionOutput out = new MyFunctionOutput();
       out.list = anotherList.subList(....);
       out.b = true;
       return out;
  }
  

• Or the most easy way: pass an initialized List instead of null and let the function add the sublist, like Attila suggested.