Singleton.java

final class DiskPersistence {
  
  private final static DiskPersistence instance = new DiskPersistence();
  
  public static DiskPersistence getInstance() {
    return instance;
  }
  
  private DiskPersistence() {
    // prevents to create a new instance
  }
  
  void add(String value){
    // save persistent
  } 
  
}