Question

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

    }

Can someone explains

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 ?

Answer 1

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.

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;
}

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

private static int x = 5;

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));
    }
}