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public class Helper {
  private final int n;

  public Helper(int n) {
    this.n = n;
  }

  // other fields & methods, all fields are final
}


class Foo {
  private Helper helper = null;
  
  public Helper getHelper() { 
    if (helper == null) {
      synchronized(this) {
        if (helper == null) {
          helper = new Helper(); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}