Java类的字段可以是多种类型而不使用泛型吗？

Question

I have a class, which's instances will be serialized. Thus the class implements Serializable . The class has a field of type List , which must be Serializable as well of course. Imagine the class looking something like this:

import java.io.Serializable;
import java.util.List;

public class Report implements Serializable {

    private static final long serialVersionUID = 12345L;

    private List<Record> records = new ArrayList<>();
}

Now I know that ArrayList is Serializable , but I'd rather declare my field to be of some interface type.

The issue comes when our SonarQube analyzes the code. It complains, that the field records should be of some Serializable type as well and SonarQube is right.

But due to the fact that the field is an implementation detail (a private member), I don't want to use generics on the class.

And here comes my question: With the self applied constraints to declare the field of some interface type and not use generics, is it possible to declare the field to be of multiple (interface) types?

FYI: I know one can cast to multiple types, but that's something entirely different.

Object o = (List<Serializable> & Serializable) new ArrayList<Serializable>();

The above is perfectly legal although unnecessary.

Answer 1

It seems like your field is private anyway, so whether it's a List or an ArrayList does not really matter. You could just make the field a serializable ArrayList and use List in the getter.

public class Report implements Serializable {

    private ArrayList<Record> records = new ArrayList<>();

    ...

    public List<Record> getRecords() {
        return this.records;
    }
}

(While this does not answer the question of whether a field can have multiple types, it might be a solution to your actual problem.)

Answer 2

The short and disillusioning answer is: No, a field cannot be of multiple types simultaneously without using generics or custom classes.

What I was looking for was a way to declare a field - actually any kind of variable - to be of an intersection type. Whilst intersection types exist in Java since the introduction of generics in Java 6, lambda support in Java 8 needed and brought us intersection casts. Still, intersection types for variables have yet to be implemented. Let's hope for the best.