Parsing in Android of WordPress JSON API

I have no error message but the parsing simply doesn’t seem to work.
Here’s the compact JSON response :

My “WPPost” has a problem :

    public class WPPost {

    int id;
    String title;
    String content;
    ArrayList<WPPostAttachment> attachments;
    WPPostCustomFields custom_fields;

    public String getContent() {
        return content;
    }

    public String getTitle() {
        return title;
    }

    public int getId() {
        return id;
    }

    public ArrayList<WPPostAttachment> getAttachments() {
        return attachments;
    }

    public WPPostCustomFields getCustomFields() {
        return custom_fields; //Problem here
    }

}

The WPPostAttachement getter works well but its structure is different to the custom field structure.

    public class WPPostAttachment {

    String id;
    String url;
    String title;

    public String getId() {
        return id;
    }

    public String getUrl() {
        return url;
    }

    public String getTitle() {
        return title;
    }

}

    public class WPPostCustomFields {

    String address;

    public String getAddress() {
        return address;
    }

}

How should I change the WPPost class to make the parsing work ?