The diamond operator in java 7 allows code like the following:
List<String> list = new LinkedList<>();
However in Java 5/6, I can simply write:
List<String> list = new LinkedList();
My understanding of type erasure is that these are exactly the same. (The generic gets removed at runtime anyway).
Why bother with the diamond at all? What new functionality / type safety does it allow? If it doesn't yield any new functionality why do they mention it as a feature? Is my understanding of this concept flawed?
Best Answer
The issue with
is that on the left hand side, you are using the generic type
List<String>
where on the right side you are using the raw typeLinkedList
. Raw types in Java effectively only exist for compatibility with pre-generics code and should never be used in new code unless you absolutely have to.Now, if Java had generics from the beginning and didn't have types, such as
LinkedList
, that were originally created before it had generics, it probably could have made it so that the constructor for a generic type automatically infers its type parameters from the left-hand side of the assignment if possible. But it didn't, and it must treat raw types and generic types differently for backwards compatibility. That leaves them needing to make a slightly different, but equally convenient, way of declaring a new instance of a generic object without having to repeat its type parameters... the diamond operator.As far as your original example of
List<String> list = new LinkedList()
, the compiler generates a warning for that assignment because it must. Consider this:Generics exist to provide compile-time protection against doing the wrong thing. In the above example, using the raw type means you don't get this protection and will get an error at runtime. This is why you should not use raw types.
The diamond operator, however, allows the right hand side of the assignment to be defined as a true generic instance with the same type parameters as the left side... without having to type those parameters again. It allows you to keep the safety of generics with almost the same effort as using the raw type.
I think the key thing to understand is that raw types (with no
<>
) cannot be treated the same as generic types. When you declare a raw type, you get none of the benefits and type checking of generics. You also have to keep in mind that generics are a general purpose part of the Java language... they don't just apply to the no-arg constructors ofCollection
s!