新的LinkedList <>（）与新的LinkedList（）有何不同

Question

I just stumbled upon the compiler treating these two terms differently. when I type:

LinkedList<String> list = new LinkedList();

I get a compiler warning about a raw type. however:

LinkedList<String> list = new LinkedList<>();

removes the warning. It seems to me as though the two statements mean essentially the same thing (ie create a new LinkedList with no specified object type). Why then does the complier all ow the empty generics? What is the difference here?

Answer 1

It's the diamond operator in Java 7, that helps you save writing the type again. In Java 7 this is equivalent to the same generic type argument that is used on the left side of the declaration. So the initialization is type safe and no warning is issued.

Answer 2

The statements do not mean the same thing at all.

The first statement tries to fit an untyped LinkedList into a declared generic LinkedList<String> and appropriately throws a warning.

The second statement, valid in Java 1.7 onward, uses type inference to guess the type parameter by using the declaring type's type parameter. In addition, sometimes this can be used in method calls. It doesn't always work, however.

See this page for more info.

Answer 3

With LinkedList<>, you use the new Diamond Operator, from java 7.

The Diamod operator uses the generic value setted in the left side of the line.

In Java 6, this doesnt works!

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 of Collections!

Extracted from: https://stackoverflow.com/a/10093701/1281306

Answer 4

Backword compatibility (Inter-operating with legacy code) is the reason why java allows above signature. Generics are compile time syntax only. At runtime "all generic" syntax will be removed. You will just see if you de-compile any class file. Read this documentation .

LinkedList list = new LinkedList();