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public final class Helper {
  private final int n;
 
  public Helper(int n) {
    this.n = n;
  }
 
  // Other fields and methods, all fields are final
}
 
final class Foo {
  private Helper helper = null;
 
  public Helper getHelper() {
    private Helper h = helper;       // only one racy read of helper
    if (h == null) {
      synchronized (this) {
        h = helper;
        if (h == null) {
          h = new Helper(42);
          helper = h;
        }
      }
    }
    return h;
  }
}