Generic Collections

[Martin Allison]

Hi

What is the difference between

Set<myClass> s1 = new TreeSet<myClass>();
and
Set<myClass> s2 = new TreeSet();

s2 seems to have the same restricions as s1, so why is there a warning on s2?

Marty

[Henry Wong]

For backward compatability, you are allowed to declare references to classes with generics, and instantiate classes with generics, without specifying the generic type.... However, it will generate a warning error.

Henry

[Rob Spoor]

Consider the following code:

TreeSet<Integer> intSet = new TreeSet<Integer>();
TreeSet rawSet = intSet; // just fine, not even a warning
TreeSet<String> stringSet = rawSet; // warning but no runtime exceptions

intSet.add(1); // autoboxing 1 to Integer
// intSet == stringSet, so stringSet contains an Integer!
for (String string: stringSet)
{
    ...
}

The above code will throw a ClassCastException when iterating over stringSet, because it will cast the Integer object to a String.

By warning you the compiler tells you that the code may cause problems at some stage. Therefore, in an ideal situation, you make sure there are no warnings. Sometimes you can't however; in those cases it is vital that you document why the warning is safe to ignore. If you can't then it isn't safe and you should prevent the warning.

An example of a warning you may ignore:

class MyClass<E>
{
    private E[] array; // will be used internally only

    public MyClass()
    {
        // you can't create an E[]
        array = (E[])new Object[10]; // warning, but you won't use arrayoutside of this class 
                                     // so you will make sure it only contains E elements
    }

    public E get(int index)
    {
        return array[index];
    }

    public void set(E element, int index)
    {
        array[index] = element;
    }
}