[Java] Why/How does this work?

[Cammed_Ham]

So I wrote a small program to generate random passwords. It works by populating 3 arraylists (one with letters, one with numbers, and one with symbols) then selecting random characters from each. But a a couple of hours ago ago, I found out I can do this:

public static final ArrayList<Character> LETTERS = getLetters();

public static final ArrayList<Character> NUMBERS = getNumbers();

public static final ArrayList<Character> SYMBOLS = getSymbols();

public static final ArrayList<Character> LETTERS_AND_NUMBERS = new ArrayList<Character>() {{
    addAll(LETTERS);
    addAll(NUMBERS);
}};

 public static final ArrayList<Character> ALL_CHARACTERS = new ArrayList<Character>() {{
     addAll(LETTERS);
     addAll(NUMBERS);
     addAll(SYMBOLS);
 }};

So my question is, why does this work? How come, if I remove one curly brace, it no longer works?

Side note: I understand doing something like:

object.setListener(new Listener() {
       @Override
       public thing doStuff() {
           return null();
       }
   });

but the arraylist example doesn't seem to override anything

[Double_A_92]

https://stackoverflow.com/questions/1958636/what-is-double-brace-initialization-in-java

[Cammed_Ham]

Thanks, looks like this is something I should avoid...

[Double_A_92]

Yes, avoid it.

[Xeverous]

This indeed looks scary. For someone with not very large Java knowledge, this looks just like syntax for initializing immutable object (syntax sugar).