import java.util.*;
var d1 = new HashMap<Integer, String>() {{
put(1, "one");
}};
var d2 = new HashMap<Integer, String>() {{
put(2, "two");
put(3, "three");
}};
var dAll = new HashMap<>(d1);
d2.forEach((key, value) -> dAll.put(key, value));
//dAll is {1=one, 2=two, 3=three}
System.out.println(dAll);