Java空块范围 [英] Java empty block scope

问题描述

我想知道使用空块的目的是什么。例如，

I am wondering what's the purposes of using empty block. For example,

    static{
        int x = 5;
    }

    public static void main (String [] args){

          int i = 10;

          {
               int j = 0 ;
               System.out.println(x);  // compiler error : can't find x ?? why ??
               System.out.println(i);  // this is fine
          }
          System.out.println(j); //compiler error :  can't find j

    }

有人可以解释

1. 在什么情况下我们想要使用空块。


2. 是所有变量在那个空块里面还是 stack ？


3. 为什么无法访问静态变量x ？

1. In what situation that we would want to use empty block.
2. Are all the variables inside that empty block still goes on stack ?
3. Why couldn't it access the static variable x ?

推荐答案

1. 您在帖子中显示的块是它不是空块，而是静态初始化器。它用于为类的静态变量提供非平凡的初始化


2. 在初始化期间使用的局部变量进入堆栈，但从堆中分配的对象除外


3. 您无法访问静态变量 x ，因为您没有声明它。相反，您在静态初始化程序中声明了一个局部变量 x 。

1. The block that you are showing in your post is not an empty block, it is a static initializer. It is used to provide non-trivial initialization for static variables of your class
2. Local variables that you use during initialization go on stack, except for objects that you allocate from the heap
3. You cannot access static variable x because you did not declare it. Instead, you declared a local variable x inside a static initializer.

如果你想让 x 一个静态变量，然后在静态初始化块中初始化它，执行以下操作：

If you would like to make x a static variable and then initialize it in a static initialization block, do this:

private static int x;
static {
    x = 5;
}

在这样的琐碎案例中，一个简单的初始化语法最有效：

In trivial cases like this, a straightforward initialization syntax works best:

private static int x = 5;

初始化程序块保留用于更复杂的工作，例如，当您需要使用循环初始化结构时：

Initializer blocks are reserved for more complex work, for example, when you need to initialize structures using a loop:

private static List<List<String>> x = new ArrayList<List<String>>();
static {
    for (int i = 0 ; i != 10 ; i++) {
        x.add(new ArrayList<String>(20));
    }
}

这篇关于Java空块范围的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！