The java Manual says “By convention, classes that implement this interface should override Object.clone (which is protected) with a public method.”

public class Foo implements Cloneable {
    @Override
    protected Object clone() throws CloneNotSupportedException { // Violation, must be public
    }
}

public class Foo implements Cloneable {
    @Override
    protected Foo clone() { // Violation, must be public
    }
}

public class Foo implements Cloneable {
    @Override
    public Object clone() // Ok
}