What is Double Brace initialization in Java?

Question

What is Double Brace initialization syntax ({{ ... }}) in Java?

Answer 1

You can put some Java statements as loop to initialize collection:

List characters = new ArrayList() {
    {
        for (char c = 'A'; c <= 'E'; c++) add(c);
    }
};

---

Random rnd = new Random();

List integers = new ArrayList() {
    {
         while (size() < 10) add(rnd.nextInt(1_000_000));
    }
};

But this case affect to performance, check this discussion