java - 为什么这个 Java 代码在方法之外的大括号 ({}) 中？

Question

我正在为 Java 认证考试做准备，我在其中一个练习测试中看到了这样的代码:

class Foo {  
    int x = 1;  
    public static void main(String [] args) {  
        int x = 2;  
        Foo f = new Foo();  
        f.whatever();  
    }  
    { x += x; }  // <-- what's up with this?
    void whatever() {  
        ++x;  
        System.out.println(x);  
    }  
}

我的问题是......在方法之外用花括号编写代码是否有效？这些(如果有的话)有什么影响？

Answer 1

借自 here -

Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods. Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:

{
    // whatever code is needed for initialization goes here
} 

The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.

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