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