What characteristics should a class have to be used correctly as an entity in Hibernate?

To correctly use a class as an entity in Hibernate, it must:

• Have a unique identifier (usually a field annotated with @Id).
• Be serializable (implement Serializable — not always necessary, but recommended).
• Have a public or protected no-argument constructor.
• Use getters and setters for fields (Hibernate works through properties).
• Not contain complex logic in the constructor.

Example:

@Entity
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    public User() {} // mandatory constructor

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}