entity/Article.java

(생략) 
public Article(Long id, String title, String content) { --- ➊
    this.id = id;
    this.title = title;
    this.content = content;
}
(생략)

dto/ArticleForm.java

public class ArticleForm {
(중략)
    public Article toEntity() {

        return new Article(null, title, content);--- ➊

                           ➊ 생성자 입력 양식에 맞게 작성
    }
}