What is Double Brace initialization in Java?

Question

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

Answer 1

Double brace initialization generates an unknown class received from the designated class (the outside braces) and gives an initializer segment inside that class (the internal braces). e.g.

new ArrayList<Integer>() {{

   add(1);

   add(2);

}};

Note that the impact of practicing this double brace initialization is that you're planning anonymous inner classes. The generated class has an absolute this pointer to the enclosing outer class. 

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Whilst not usually a difficulty, it can create anxiety in some situations e.g. when serializing or garbage collecting, and it's deserving being informed of this.