What is the difference between Collection.add() method and Collection.addAll() method in Collections in Java

Question

I have two ArrayLists (a1 and a2) as below:

ArrayList a1 = new ArrayList();
a1.add(8);
a1.add("a1");

ArrayList a2 = new ArrayList();
a2.add(a1); //output : [[8, a1]]
a2.addAll(a1); //output : [[8, a1], 8, a1]

My Questions:

1. Why does a2.addAll() method prints the data twice?
2. What is the exact difference between add and addAll? Both the methods return boolean.

Answer 1

Why a2.addAll() method prints the data twice?

Because the first copy is already there after you called add(a1) on the previous line.

What is the exact difference between add() and addAll()? Both the methods return boolean.

add adds a single item, while addAll adds each item from the collection one by one. In the end, both methods return true if the collection has been modified. In case of ArrayList this is trivial, because the collection is always modified, but other collections, such as Set, may return false if items being added are already there.

Note: Part of the confusion is that your lists are untyped, so they contain a mixture of objects: a1 mixes strings and numbers, while a2 mixes strings, numbers, and lists. Using a specific generic type for your collection would prevent this confusion by letting you do either add or addAll, but not both:

List<String> a1 = new ArrayList<>();
a1.add("8");
a1.add("a1");
List<List<String>> a2 = new ArrayList<>();
a2.add(a1);
a2.addAll(a1); // <<== Does not compile
List<String> a3 = new ArrayList<>();
a3.add(a1);    // <<== Does not compile
a3.addAll(a1);