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it will have no compile time problem, there will be only one entry with that set element, s when you retrieve it, only one value will get printed

eg:

HashSet<Integer> a = new HashSet<Integer>();
		 a.add(4);
		 a.add(4);
		 Iterator<Integer> b = a.iterator();
		 while(b.hasNext()){
			 System.out.println(b.next());
		 }

i think this question is about hashCode / equals method in Object class.

user defined class should override those methods to work properly with collection framework.

A set cannot have duplicate elements, so nothing will happen, the call to add() will do nothing.

A set cannot have duplicate elements, so nothing will happen, the call to add() will do nothing.

A set cannot have duplicate elements, so nothing will happen, the call to add() will do nothing.

indeed, the question is about hashCode / equals method overriding, as if we dont override these methods, the duplicate objects(except String/Integer/etc objects, which already have equals/hashCode methods implemented in there classes) can be added to the set

A set cannot have duplicate elements, if you try add a duplicate element to set , nothing will be happen and the set.add method will return false.