DoubleBraceInit.java

import java.util.HashMap;

public class DoubleBraceInit {
  public static void main(String[] args) {
    var m1 = new HashMap<>() {{
      put("k1", 1);
      put("k2", 2);
      put("k3", 3);
    }};

    var m2 = new HashMap<>() {{
      put("k1", new HashMap<>() {{
        put("k1.1", 11);
        put("k1.2", 12);
      }});
      put("k2", new HashMap<>() {{
        put("k2.1", 21);
        put("k2.2", 22);
      }});
    }};

    System.out.println("m1: " + m1);
    System.out.println("m2: " + m2);
  }
}