Understanding Generics in Object Initialization

Question

I have a basic question about generics in Java: what is difference between the following two initializations of a map?

        Map<String, String> maplet1 = new HashMap<String, String>();

        Map<String, String> maplet2 = new HashMap();

I understand the the first initialization is specifying the generics in the object construction, but I don't understand the underlying ramifications of doing this, rather than the latter object construction (maplet2). In practice, I've always seen code use the maplet1 construction, but I don't understand where it would be beneficial to do that over the other.

There is no functional difference. the benefit is it does not cause a compiler warning so you don't have to sort "real" raw type warnings from "sloppy syntax" raw type warnings. — Affe, Feb 27 '13 at 23:48
But note that @Affe's comment is only true for a no-arg constructor. Consider that this compiles: `Map maplet2 = new HashMap(mapOfIntsToLongs);` — Paul Bellora, Feb 28 '13 at 02:58

score 5 · Accepted Answer · edited May 23 '17 at 12:21

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The second Map is assigned to a raw type and will cause a compiler warning. You can simply use the first version to eliminate the warning.

For more see: What is a raw type and why shouldn't we use it?

edited May 23 '17 at 12:21

Community

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answered Feb 27 '13 at 23:45

Reimeus

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score 2 · Answer 2 · answered Feb 27 '13 at 23:47

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The first one is type-safe.

You can shorthand the right side by using the diamond operator <>. This operator infers the type parameters from the left side of the assignment.

Map<String, String> maplet2 = new HashMap<>();

answered Feb 27 '13 at 23:47

Mr. Polywhirl

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user1428716 · Answer 3 · 2013-02-27T23:53:15.833

Lets understand the concept of Erasure. At RUNTIME HashMap<String, String>() and HashMap() are the same represented by HashMap.

The process of converting HashMap<String,String> to HashMap (Raw Type) is called Erasure.

Without the use of Generics , you have to cast , say the value in the Map , to String Explicitly every time.

The use of Generics forces you to Eliminate cast.

If you don't use Generics , there will be high probability that a Future Developer might insert another type of Object which will cause ClassCastException

Understanding Generics in Object Initialization

3 Answers3