强制不变性并在接口声明中创建实例

Question

I have an interface that its API are only getter methods thereby ensuring that the class that implements the interface is in that respect immutable.
But if I need to update something that changes the behavior of the interface how would I declare my interface so that it creates a new instance of the concrete class?
Right now I have something like:

    public interface Generator {  
      public int[] values();  
      public float[] indications();  
      //etc  
      public MyEnum type;  
    }  

    public class ConcreteClass implements Generator {  
      private ConcreteClass() {  
      }  
      public static ConcreteClass createInstance() {  

       }  
   }    

I have omitted arguments for clarity.
How would I move the createInstance to the interface?

Answer 1

An interface is not designed to have implementation. With Java 8 you could have default method but it would be a bad idea that interface knows the implementation classes. An interface should stay abstract and not know its subclasses.

If ConcreteClass is immutable, you should document it and ensure that each method with side-effect doesn't apply any modification on the current object but instead create a new instance of ConcreteClass .

For example suppose you want provide method to update the values field, you could provide this method in ConcreteClass :

public ConcreteClass withValues(int[] values){
   // copy all existing properties from the original to the copy but values
   ConcreteClass copy = new ConcreteClass(values, this.indications, this.type);    
   return copy;
}

If you want really move out the instance creation from ConcreteClass , you could create a factory that provides this service.