public class Response implements Serializable {

    @SerializedName("message")
    @Expose
    private List<Message> messages;

    public List<Message> getMessages() {
        return messages;
    }

    public void setMessages(List<Message> messages) {
        this.messages = messages;
    }

}

public class Message implements Serializable {

        @SerializedName("type")
        @Expose
        @MessageType
        private int type;

        @SerializedName("position")
        @Expose
        @MessagePosition
        private String position;

        public int getType() {
            return type;
        }

        public String getPosition() {
            return position;
        }

        public void setType(@MessageType int type) {
            this.type = type;
        }

        public void setPosition(@MessagePosition String position) {
            this.position = position;
        }

}

public class TextMessage extends Message {

@SerializedName("text")
@Expose
private String text;

public String getText() {
    return text;
}

public void setText(String text) {
    this.text = text;
}

public class ImageMessage extends Message {

    @SerializedName("attachment")
    @Expose
    private Attachment attachment;

    public Attachment getAttachment() {
        return attachment;
    }

    public void setAttachment(Attachment attachment) {
        this.attachment = attachment;
    }

}

---- Response
   |
    - TextResponse -> List<TextMessage>
   |
    - ImageResponse -> List<ImageMessage>