NullObject2.java

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

static class EmptyPersistence implements Persistence {
  @Override void delete(){
  }
  
  @Override void add(String value) {
  }
  
  @Override String get() {
    return null;
  }
}

class FooBar {
  private Persistence persistence = new EmptyPersistence();
  
  void persistence(Persistence persistence) {
    this.persistence = persistence;
  }
  
  private void delete() {
    ...
    persistence.delete();
  }
  
  private String get() {
    return persistence.get();
  }
  
  private void save(String value) {
    persistence.save(value);
  }
  
}