Implement inner-class-like reference behaviour? [closed]

First of all, allow me to explain a “behaviour” that I observed, then I will ask in “Example” section.

Inner-class-reference behaviour

In Java we can have (non-static) inner-classes, which seem to behave like having a strong-reference to their container-class and/or owner-class, but without causing memory-leak.

I mean, I observed that even if both owner-class and inner-class keep strong-reference to each other, the classes are garbage-collected anyway (once no external class references them, although having reference-recursion).

In other words, or in other programming-languages, where “reference-counters” are often used (unlike Java?);

We could achieve such behaviour if owner-class and all inner-classes share the same reference-counter. Where just like above, even if we only keep a single reference to inner-class, owner-class is kept as well.

Example (background and question)

My previous logic, which used above said behaviour, was something like:

public class WebApi {
    public UserApi user = new UserApi();
    public PostApi post = new PostApi();

    protected String post(String url, String json) {
        // ...
    }

    public class UserApi {
        public void login() {
            WebApi.this.post(...);
        }

        public void logout() {
            WebApi.this.post(...);
        }

        // ...
    }

    public class PostApi {
        // ...
    }
}

Then day by day the project did grow, till each of UserApi and PostApi classes deserved their own separate files (instead of being inner-classes of one huge file).

How can we implement above described “Inner-class-reference” behaviour for external classes?

Answer

In Java we can have (non-static) inner-classes, which seem to “behave” like having a strong-reference to their container-class and/or owner-class, but without causing memory-leak.

I mean, I observed that even if both owner-class and inner-class keep strong-reference to each other, the classes are garbage-collected anyway (once no external class references them, although having reference-recursion).

Yes, that’s how references work in Java: if an object is reachable, then it’s retained, and if not, then it’s (eventually) garbage-collected.

But how can we implement above described “Inner-class-reference” behaviour for external classes?

You don’t need to do anything special: just give UserApi a field of type WebApi, and initialize it in the constructor.

Java’s reference-tracing and garbage collection will ensure that, as long as any reachable UserApi instance holds a reference to a given WebApi instance, the WebApi instance is also considered reachable, and hence retained.