java7新特性之Diamond syntax

java7新特性之Diamond syntax

Java 7 also introduces a change that means less typing for you when dealing with
generics. One of the problems with generics is that the definitions and setup of
instances can be really verbose. Let’s suppose that you have some users, whom you
identify by userid (which is an integer), and each user has one or more lookup tables
specific to that user. What would that look like in code?

Map<Integer, Map<String, String>> usersLists =
new HashMap<Integer, Map<String, String>>();

That’s quite a mouthful, and almost half of it is duplicated characters. Wouldn’t it be
better if you could write something like this,

Map<Integer, Map<String, String>> usersLists = new HashMap<>();

and have the compiler work out the type information on the right side? Thanks to the
magic of Project Coin, you can. In Java 7, the shortened form for declarations like that
is entirely legal. It’s backwards compatible as well, so when you find yourself revisiting
old code, you can cut the older, more verbose declaration and start using the new
type-inferred syntax to save a few pixels.
We should point out that the compiler is using a new form of type inference for
this feature. It’s working out the correct type for the expression on the right side, and
isn’t just substituting in the text that defines the full type.

The “diamond syntax” name
This form is called “diamond syntax” because, well, the shortened type information
looks like a diamond. The proper name in the proposal is “Improved Type Inference
for Generic Instance Creation,” which is a real mouthful and has ITIGIC as an acronym, which sounds stupid, so diamond syntax it is.

读书笔记：The Well-Grounded Java Develope