Why were generics introduced for the Collection classes?

Question

Was type safety the only reason to introduce generics for the java.util.Collection classes, so the addition of heterogeneous elements in a Collection is prevented? Or have there been other reasons too?

Why are you asking? Is it just of curiosity or there is some other reason too? — Andrew Logvinov, Mar 23 '14 at 08:00
Because it's redundant to write `List` for `Integer`, then `List` for `String`, then for `Double`, then for...... — Maroun, Mar 23 '14 at 08:01
@MarounMaroun no I don't think so,they wrote it for object,so redundancy is not the reason — user3380123, Mar 23 '14 at 08:04
possible duplicate of [What are Generics in Java?](http://stackoverflow.com/questions/7815528/what-are-generics-in-java) — Joe, Mar 23 '14 at 08:41

Stephen C · Answer 1 · 2014-03-23T08:28:40.317

I'd say that the reason is that most use-cases require a collection of just one class, or one class and its subclasses. For those use-cases, it is convenient to have a type-safe collection where you don't need to typecast the keys / values / entries extracted from the respective collections.

Use-cases where you genuinely need a "collection of any object" are unusual. But these use-cases are supported by using a ? as the collection type parameter; e.g.

    List<Object> listOfAnything = new ArrayList<>();

Clearly, generics do not prevent heterogeneous collections. But if you want homogeneous collections, generics allow you to do this conveniently and with type-safety.

True .......................... – Stephen C Mar 23 '14 at 08:28 — Stephen C, Mar 23 '14 at 08:28

score 1 · Answer 2 · answered Mar 23 '14 at 08:18

Generics were introduced to the Java language to increase the type safety.

Say you have a list of items, to which you are only adding instances of class A:

List list = new LinkedList();
list.add(new A());
list.add(new A());
...

When you want to use some of the objects you entered the list, you do something like:

A a1 = (A) list.get(0);
A a2 = (A) list.get(1);
...

However, nothing prevents anyone from doing the following:

list.add(new NonA());

which will result in you getting a ClassCastException in your code. This is because the types are being checked in run time not in compilation time.

Enter generics, which allow you to do the following:

List<A> list = new LinkedList<A>();
list.add(new A());
list.add(new A());
...

And now, if anyone tries the following:

list.add(new NonA());

They will get a compilation error, thus preventing this error.

Why were generics introduced for the Collection classes?

2 Answers2