NullObject.java

interface Persistence {
  void delete();
  void add(String value);
  String get();
}

class FooBar {
  private Persistence persistence;
  
  void persistence(Persistence persistence) {
    this.persistence = persistence;
  }
  
  private void delete() {
    // some different things to handle
    if (persistence != null) {
      persistence.delete();
    }
  }
  
  private String get() {
    if (persistence != null) {
      return persistence.get();
    }
    return null;
  }
  
  private void save(String value) {
    if (persistence != null) {
      persistence.save(value);
    }
  }
  
}