Is it possible to declare attributes inside an interface in object-oriented programming?

In Java, interfaces cannot contain regular attributes (instance variables). However, within an interface, you can only declare constants — that is, variables that are by default public static final.

Example:

public interface MyInterface {
    int CONSTANT_VALUE = 10; // public static final by default
    void doSomething();
}

Thus, in interfaces, you cannot declare mutable fields, only constants. All non-static fields must be in classes that implement the interface.