Can attributes be declared inside an interface?

In Java, interfaces cannot contain regular attributes (instance variables) because they do not have state. However, interfaces can declare:

• Constants — these are public static final fields. By default, all fields in an interface are public static final.

For example:

public interface MyInterface {
    int CONSTANT_VALUE = 10; // public static final by default
}

Thus, declaring non-static fields (instance attributes) inside an interface is not allowed. An interface only describes behavior (methods) and constants.