Question

Should I replace a simple setter

public void setCategories(Set<String> categories)
{
    this.categories = categories;
}

with something like this:

public void setCategories(Collection<String> categories)
{
    this.categories.clear();
    if (categories != null)
    {
        this.categories.addAll(categories);
    }
}

so object’s attribute categories won’t be further modifiable through passed parameter reference as it could be:

Set<String> categories = new TreeSet<String>();
categories.add("cityguide");
categories.add("other");
Document document = new Document("http://unique-address.com");
document.setCategories(categories);
System.out.println(categories); //outputs [cityguide, other]
System.out.println(document.getCategories()); //outputs [cityguide, other]
document.setCategories(categories);
categories.add("traveling");
System.out.println(categories); //outputs [cityguide, other, traveling]
System.out.println(document.getCategories()); //outputs [cityguide, other, traveling]

Answer 1

yes, yes you should

another option is also making a new set for the field in the setter but your solution is better as it avoid the (unnecessary) allocation

or remove and add each category in a loop

public void setCategories(Collection<String> categories)
{
    for(String cat:this.category.toArray(new String[0])){//using toArray to avoid ConcurrentModificationException
        removeCategory(cat);
    }
    if (categories != null)
    {
        for(String cat:categories){
            addCategory(cat);
        }
    }
}

so you won’t have to duplicate any triggers for removing and/or or adding a category