Java Object Oriented Design: Returning multiple ob

问题:

The below code in Java throws Null pointer exception.

public class New{
  int i;

  New(int i)
  {
    this.i = i;
  }

  public void func(New temp)
  {
     temp.i = 10;
     temp = new New(20);
  } 

  public static void main(String[] args)
  {
     New n = null;
     n.func(n);
     System.out.println("value "+ n.i);
  }
}

The reason being, java passes objects references by value. If I wanted to return one object, then I can return it from the function.

But, If I have multiple objects, the only way I could return the object references is, by keeping them into another object, like having some container which has references to all the objects.

Is there a better way to do it?

In C++, I normally just pass the address of pointer to handle this scenario. If I wanted to just return two objects of a single type, creating a container and passing it is a over kill.

What is the problem with returning multiple objects from a function? Why cannot the semantics of the function in all these languages be changed?

回答1:

Most often you create an object to hold the combination of objects you want to return.

For a more general-purpose solution, you can either return a collection, and array or some sort of tuple, such as Pair, Triple, etc (the latter you will need to create).

Note, you don't generally pass a mutable object as a parameter, but return an immutable one:

public Pair<Integer,Integer> getLowHighTemp() {
    int                        low,hgh;

    // do stuff... 
    return new Pair(low,hgh);
    }

回答2:

This is more of 2 questions than one.

Firstly the problem with your code is that you are not declaring n before you use it. That is throwing the exception.

Secondly if you would like to return 2 objects, you need to have a container object that will hold 2 objects.

回答3:

You can return some kind of Collection. Returning a Map or List is pretty common.