Class with a collection of mutable objects but the elements in the class are not mutable

[Angus Ferguson]

Hi

I am a newbie and I have a question which I am trying to understand,

what would you do if if you have a class with a collection of mutable objects but the elements in the class are not mutable

also what about the effect of using web services on client designs

Thansk if somebody can help me in order to clear my ideas

Cheers!

[Steve Luke]

Hi Angus,

Welcome to the Ranch!

Can you be more specific about your question? Especially about the second one, I am not sure what you want with that.

What would I do if I had a class [class A] with a collection [collection C] of mutable objects (the objects can be changed) but the elements in the class are not mutable (the members of Class A are not mutable)? Such as:

public final class ClassA
{
  private final String aString;
  private final Number aNumber;
  private final List<ClassB> aList;

  public ClassA(String a, Number b, List<ClassB> c) 
  {
    this.aString = a;  this.aNumber = b;
    this.aList = Collections.unmodifiableList(new ArrayList<ClassB>(c));  //**
  }

  public String getString() { return aString; }
  public Number getNumber() { return aNumber; }
  public List<ClassB> getList() { return aList; }
}

public class ClassB
{
  public int aChangeableValue = 3;
}

If this were the case, I would not consider ClassA immutable, because its data can be changed by modifying the values in the mutable ClassB. If I wanted to make it immutable I would have to make local copies that I only have access to inside this class. I would also make sure that if I were to pass the List or any view of the List out, that I would again make copies of the mutable objects and pass out the copies.

public final class ClassA
{
  private final String aString;
  private final Number aNumber;
  private final List<ClassB> aList;

  private static final List<ClassB> copyListOfMutableData (List<ClassB> sourceList)
  {
    List<ClassB> newList = new ArrayList<ClassB>();
    for (ClassB data : sourceList)
    {
      ClassB newData = new ClassB();
      newData.aChangeableValue = data.aChangeableValue;
      newList.append(newData);
    }

    return Collections.unmodifiableList(newList);
  }

  public ClassA(String a, Number b, List<ClassB> c) 
  {
    this.aString = a;  this.aNumber = b;
    this.aList = copyListOfMutableData(c);
  }

  public String getString() { return aString; }
  public Number getNumber() { return aNumber; }
  public List<ClassB> getList() { return copyListOfMutableData(this.aList); }
}

As for your second question, I don't know the context that you mean.


** This is required to ensure the List help by ClassA instances is immutable. First, we don't know about the incoming List. Someone could change it. So we will copy from the incoming List to our own ArrayList. Then, just to make sure we can't change the List we will only hold an unmodifiable view of the new List.

[Angus Ferguson]

Thanks Steve, it has helped me a lot. You have explained it very well

Regards, Isaac