Double Checked Locking in Singleton

Double Checked Locking in Singleton

here is my custom class for singleton pattern. in this code, I use
double-checked locking as below. As I read many posts on some source, they
say that double check is useful because it prevents two concurrent threads
run at same times make two different objects.
public class DoubleCheckLocking {
public static class SearchBox {
private static volatile SearchBox searchBox;
// private attribute of this class
private String searchWord = "";
private String[] list = new String[]{"Stack", "Overflow"};
// private constructor
private SearchBox() {}
// static method to get instance
public static SearchBox getInstance() {
if (searchBox == null) { // first time lock
synchronized (SearchBox.class) {
if (searchBox == null) { // second time lock
searchBox = new SearchBox();
}
}
}
return searchBox;
}
}
I still don't understand above code so much. What is the problem, if two
threads together run same line of code when instance is null ?
if (searchBox == null) {
synchronized (SearchBox.class) {
if (searchBox == null) {
searchBox = new SearchBox();
}
}
}
When that appear. both two threads will see object is null. then both
synchronize. and then, they check again, and still see it null. and create
object. OOOPS.
Please explain for me. What have I understand wrong ?
Thanks :)