Good day everybody.
I have some confusion about abstract superclass and subclasses in servlets:
I have abstract servlet superclass:
public abstract class CatalogPage extends HttpServlet {
public CatalogPage() {
super();
}
private CatalogItem[] items;
private String[] itemsID;
private String title;
public void setItems(String[] itemsID) {
this.itemsID = itemsID;
items = new CatalogItem[itemsID.length];
for(int i=0; i<items.length; i++) {
items[i] = Catalog.getItem(itemsID[i]);
}
}
public void setTitle(String title) {
this.title = title;
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
if (items == null) {
response.sendError(response.SC_NOT_FOUND, "Missing Items");
return;
}
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String docType =
"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
"Transitional//EN\">\n";
out.println(docType +
"<HTML>\n" +
"<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<H1 ALIGN=\"CENTER\">" + title + "</H1>");
CatalogItem item;
for(int i=0; i<items.length; i++) {
item = items[i];
if(item == null) {
out.println("<FONT COLOR=\"RED\">" +
"Unknown item ID " + itemsID[i] +
"</FONT>");
} else {
out.println();
String formURL = "OrderPage";
formURL = response.encodeURL(formURL);
out.println
("<FORM ACTION=\"" + formURL + "\">\n" +
"<INPUT TYPE=\"HIDDEN\" NAME=\"itemID\" " +
" VALUE=\"" + item.getItemID() + "\">\n" +
"<H2>" + item.getShortDescription() +
" ($" + item.getCost() + ")</H2>\n" +
item.getLongDescription() + "\n" +
"<P>\n<CENTER>\n" +
"<INPUT TYPE=\"SUBMIT\" " +
"VALUE=\"Add to Shopping Cart\">\n" +
"</CENTER>\n<P>\n</FORM>");
out.println("<HR>\n</BODY></HTML>");
}
}
}
}
And of abstract superclass, subclass:
@WebServlet("/KidsCatalogPage")
public class KidsBooksPage extends CatalogPage {
public void init() {
String[] kbp = {"lewis001", "alexander001", "rowling001"};
setItems(kbp);
setTitle("All-Time Best Children's Fantasy Books");
}
}
If I invoke for subclass KidsBooksPage I know it do initialisation init() method first, but the question is:
What make my subclass KidsBooksPage invoke abstract super class CatalogPage . How it works?? Can Understand it. Please explain to me.
Thank you.
As Jon Skeet said in comments, this is about a core concept (Inheritance) in Object-Oriented programming (OOP), has nothing to do with Servlets specifically (except maybe your homework 😉), and is not really a question for StackOverflow.com.
If you understand conventional programming and how conventional compiling and linking predetermine at compile-time exactly what bits will be executed at run-time, and therefore wonder how execution can seem to "jump around" between classes, then you need to learn about 'late binding' & 'dynamic dispatch', the special sauce that makes OOP so powerful.
'abstract' is a bit of distraction regarding your question. Simply means that a concrete subclass is required, but without abstract your question remains the same.
Try reading up on Inheritance and Late Binding.