Why we use static block in Java? and why can't we replace it with simple statements?

Question

I recently fumbled into this code:

  public static ArrayList<HelloGreeting> greetings = new ArrayList<HelloGreeting>();

  static {
    greetings.add(new HelloGreeting("hello world!"));
    greetings.add(new HelloGreeting("goodbye world!"));
  }

The question is why we use static here, can't we do just this:

  public static ArrayList<HelloGreeting> greetings = new ArrayList<HelloGreeting>();

    greetings.add(new HelloGreeting("hello world!"));
    greetings.add(new HelloGreeting("goodbye world!"));

What benefit we get, space or time?

That wouldn't compile, or where exactly would you put that code instead? — morgano, Sep 10 '15 at 06:10
Put it inside some method and call that method or constructor .If you dont want to use static block. But then you have to create object — Subodh Joshi, Sep 10 '15 at 06:11
A good read: http://www.jusfortechies.com/java/core-java/static-blocks.php — Rahul Tripathi, Sep 10 '15 at 06:12
When you need something initialization before class loading you can do in `static block` — Rustam, Sep 10 '15 at 06:14
It's all right here: https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html — Stultuske, Sep 10 '15 at 06:16
@Rustam - Its actually ==> *after class loading, during class initialization* — TheLostMind, Sep 10 '15 at 06:24
@morgano, my simple question was that what if I remove `static {}` around that code block? — CodeYogi, Sep 10 '15 at 06:25
@user996142 - Yes, because we can't have *statements* without a block — TheLostMind, Sep 10 '15 at 06:28
@user996142, why? actually I am learning java so don't know much details about it. — CodeYogi, Sep 10 '15 at 06:28
@TheLostMind, I think initializing ArrayList is a statement too and that too without a block. — CodeYogi, Sep 10 '15 at 06:46

score 2 · Accepted Answer · answered Sep 10 '15 at 06:28

If you want to initialize some data after the class is loaded, when it is being initialized, then you can use static blocks. static blocks are run once per class initialization (multiple times if a class is loaded and initialized by different class loaders multiple times).

Difference between static blocks and non-static blocks (your second case should be nclosed within {..}) is that non-static blocks will execute for each instance of YourClass i.e, when new instance is being constructed. Next, static blocks are thread-safe (although a class can be loaded / initialized by multiple threads)

Why we use static block in Java? and why can't we replace it with simple statements?

1 Answers1