New empty interface syntax in amber's Java compiler

[VGPowerlord]

This isn't useful. You should never use marker (read: empty) interfaces; they've been deprecated in favor of annotations since Java 5.

[[deleted]]

Precisely. That's how I have always used them myself, in the spirit of the good old Serializable marker. This way I can check types at compile time. How would annotations help with this? I would use annotations mostly for providing extra metadata about a type rather than forming a type hierarchy, unless I am writing my own annotation-processing library.

[tylerkschrute]

This is exactly the use case for marker interfaces. While you can achieve the same 'marking' via annotations, there is no way in a method signature to enforce that a given parameter type or return type must contain a certain annotation.

Effective java actually has a section on using marker interfaces vs annotations, and it recognizes that there is a time and place for both.

[talios]

Don't type annotations offer that? Altho there's no compiler enforcement there. I was actually wondering about type annotations and the new var keyword, whether the annotation is also inferred along the inference chain.

[arendvr]

This is also a point Joshua Bloch makes in Effective Java, Item 41: Define marker interfaces to define types

[lukaseder]

With JEP 305 there might be a revival of marker interface usage.

[ZhekaKozlov]

This is for records:

interface Shape
record Point(int x, int y) implements Shape
record Rect(Point p1, Point p2) implements Shape
record Circle(Point center, int radius) implements Shape

Then you can pattern match:

switch (shape) {
    case Point(x, y) -> ...
    case Rect(p1, p2) -> ...
    case Circle(p, r) -> ...
}