Question

    public static void main(String[] args) {
       Set vals = new TreeSet();
       vals.add("one");
       vals.add(1);
       vals.add("two");
       System.out.println(vals);
}

(i)-What does it mean to define a collection without giving it a type?For what purpose
does it made for?

(ii)-Can I add different type to the collection any way?

Here’s an example- There’s no compilation error, though it’s warning me.

But, as excepted, there’s a run time error.

Answer 1

1. Defining a collection (or any typed class) without specifying a type argument is called using a raw type. It exists only for backwards compatibility and should not be used in new code. If effectively removes all effects of generics on the type.
2. Generally, you could define your set to accept Object: it would then accept all values: Set<Object> vals = new HashSet<Object>(). However this won’t work for TreeSet, because it needs its values to be Comparable with each other. If you want to add arbitrary types to the same TreeSet (which is usually a sign of some architecture problems, i.e. a design smell), then you’ll need to implement your own Comparator that can compare arbitrary elements and pass that into the appropriate TreeSet constructor.