问题描述
下面代码中的{{ ... }}块是什么意思?
What does {{ ... }} block mean in the following code?
class X { private Y var1; private X() { Z context = new Z(new SystemThreadPool()) {{ var1 = new Y(); }}; } }
推荐答案
叫做 双花括号初始化.(链接已删除,存档在这里)
It's called double curly brace initialization. ( Link removed, archived here)
这意味着您正在创建一个匿名子类,而双括号内的代码基本上是一个构造函数.它通常用于向集合添加内容,因为 Java 用于创建本质上是集合常量的语法有些笨拙.
It means you're creating an anonymous subclass and the code within the double braces is basically a constructor. It's often used to add contents to collections because Java's syntax for creating what are essentially collection constants is somewhat awkward.
所以你可以这样做:
List<String> list = new ArrayList<String>() {{ add("one"); add("two"); add("three"); }};
代替:
List<String> list = new ArrayList<String>(); list.add("one"); list.add("two"); list.add("three");
我其实不喜欢这样,我更喜欢这样做:
I actually don't like that and prefer to do this:
List<String> list = Arrays.asList("one", "two", "three");
所以在这种情况下它没有多大意义,而对于没有方便的助手的地图来说,它没有多大意义.
So it doesn't make much sense in that case whereas it does for, say, Maps, which don't have a convenient helper.