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javaimmutabilityencapsulationmutable

Encapsulation for mutable objects in Java

I was studying the "Java SE 7 Programmer I & II Study Guide" and I did not understand the explanation below.

class Fortress{
  private String name;
  private ArrayList<Integer> list;

  Fortress（） {list=new ArrayList<Integer>;

  String getName{return name;}
  void addToList(int x){list.add(x);}
  ArrayList getList(){return list;} // line 1
}

Which lines of code break encapsulation? Answer: line 9. "When encapsulating a mutable object like an ArrayList, your getter must return a reference to a copy of the object, not just the reference to the original object".

I did not either understand the explanation or how to modifiy the original code.

So in the getList() instead of 

return list;

Should we do this?

ArrayList<Integer> list2=list;
return list2;
Solution

  • You would have replace:

    return list;

    with:

    return new ArrayList<Integer>(list);

    Otherwise the client can do...

    foo.getList().add(5);

    breaking encapsulation.