What does the {{ syntax on ArrayList initializer really do

Question

I have recently found what appears to me to be a new syntax for statically initializing an ArrayList:

new ArrayList<String>() {{
   add("first");
   add("second");
}};

My question is, what is really happening there? Is that a shortcut for defining a static block (I thought it would need the static keyword)? Or just a way to define a default constructor? Something else? What version of Java did this become valid?

An explanation plus a link to further reading would be greatly appreciated.

edit: My test class for showing whether initializer block executes before or after the constructor is below. Results show that initializer blocks execute before the other constructor code:

import org.junit.Test;

public class InitializerBlockTest {
    class InitializerTest {
        {
        System.out.println("Running initalizer block");
        }

        public InitializerTest() {
            System.out.println("Running default constructor");
        }
    }
    
    class SubClass extends InitializerTest {
      {
        System.out.println("Running subclass Initializer block");
      }

      public SubClass()  {
        System.out.println("Running subclass constructor");
      }
    }

    @Test
    public void testIt() {
        new SubClass();
    }
}

Output:

Running initalizer block
Running default constructor
Running subclass Initializer block
Running subclass constructor

Answer 1

You are creating a new anonymous subclass of ArrayList, with an instance initializer which calls add() twice.

It's the same as:

class MyList extends ArrayList
{

{ // This is an instance initializer; the code is invoked before the constructor.
add("first"); 
add("second");
}

public MyList() {
    super();
    // I believe initializers run here, but I have never specifically tested this
    }
}

...

List list=new MyList();

Note that, personally, I do not advise it as an idiom, since it will lead to class-file explosion.