Java Trick: Double brace initialization

This is a trick I found from the Core Java™: Volume I—Fundamentals, Ninth Edition, 6.4.6. Anonymous Inner Classes.

import java.util.ArrayList;
import java.util.List;

public class TestArrayList {
 private String s1;
 private static String s2;
 public static void main(String[] args) {
  TestArrayList tl = new TestArrayList();
  tl.printList(new ArrayList(){
   {add("a"); add("b"); add(s2);}
   });

 }

 public void printList(/*only final variables are accessible to local inner classes*/final List sList){
  printDynamicList(new ArrayList(){{add(s1); addAll(sList);}});
 }
 public void printDynamicList(List dList){
  
 }
}

As shown in the above code:

• As shown in the line# 10, you can add elements to the anonymous array: this is called Double brace initialization.
• Again in the line# 16, in the double brace initialization, s1 is added to the array which is a property fo the outer class.
• To add the parameter in line# 16, it should be final, this is the rule of local inner classes.

If you have more info please add to this blog.