使用初始化块有什么好处? [英] What is the advantage of using initialization blocks?

问题描述

我知道初始化块在首次加载类(静态初始化块)或创建实例(实例初始化块)时运行.

I know that Initialization blocks run when the class is first loaded (a static initialization block) or when an instance is created (an instance initialization block).

class SmallInit {
   static int x;
   int y;
   static { x = 7 ; } // static init block
   { y = 8; } // instance init block
}

但是，当我们可以这样做的时候，这样做有什么特殊的好处?

But what is the special benefit of this, when we can do it like this:

class SmallInit {
   static int x = 7;
   int y = 8;
}

推荐答案

关于实例初始化块的一件好事是，它们使

One nice thing about instance initialization blocks is that they make the Double Brace Initialization pattern possible.

代替此:

Set<String> names = new HashSet<String>();
names.add("Peter");
names.add("Paul");
names.add("Mary");

您可以执行以下操作:

Set<String> names = new HashSet<String>() {{
    add("Peter");
    add("Paul");
    add("Mary");
}};

第一个括号创建一个匿名内部类；第二个启动实例初始化程序块. (请注意，这意味着names是HashSet的匿名 subclass 的实例，但这通常不是问题.这也意味着该模式仅适用于非最终类.)

The first brace creates an anonymous inner class; the second starts the instance initializer block. (Note that this means names is an instance of an anonymous subclass of HashSet, but that's often not an issue. This also means that this pattern only works for non-final classes.)

这在一次性将对象初始化为表达式的情况下特别有用.例如:

This is especially useful in one-off situations where it would be convenient to initialize an object as an expression. For example:

doSomethingToSet(new HashSet<String>() {{
    add("Peter");
    add("Paul");
    add("Mary");
}});

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