Collection question

[henri henri]

I just learned that it is better programming practice if I change TreeMap at the beginning of the second line of code to Map.

Map<String, FilmBean>filmBeans = new TreeMap<String, FilmBean>();

after I do this, I get a compiler error for the return statement at the end of the method.

 

     public TreeMap getFilmBeansByLetter(String letter) {
        TreeMap<String, FilmBean>filmBeans = new TreeMap<String, FilmBean>();
                 try {
        FileInputStream file = new FileInputStream("FilmContainer.ser");
        ObjectInputStream input = new ObjectInputStream(file);
        container = (FilmContainer)input.readObject();  
        input.close();
    } catch(Exception ex) {} 
           Collection values = container.values();
             Iterator it = values.iterator();
               while(it.hasNext()) {
                  bean = (FilmBean)it.next();
                   String name = (String)bean.getName();
                   for(int i = 0; i < 10; i++) {
                       String s = String.valueOf(i);
                       if((name.startsWith(s)) && (letter.equals("A")))filmBeans.put(name, bean);
                   }  if(name.startsWith(letter))filmBeans.put(name, bean); 
             }
       return filmBeans;
 } 

[Henry Wong]

Originally posted by henri henri:
I just learned that it is better programming practice if I change TreeMap at the beginning of the second line of code to Map.

Map<String, FilmBean>filmBeans = new TreeMap<String, FilmBean>();

after I do this, I get a compiler error for the return statement at the end of the method.

Well, you can't make the change and everything magically still works. You have to change your method to return a Map instead of a TreeMap. And you have to change all the places in your program to use a Map instead of a TreeMap.

Henry

[Stuart Ash]

It's all about object typing.

Please read about this in a good Java textbook, I recommend Bruce Eckel's thinking in Java.

Keywords: Object type, class type, object references, handle, casting.

[marc weber]

Originally posted by Stuart Ash:
... I recommend Bruce Eckel's thinking in Java...

I second that recommendation. And here's a link to an online version...

http://www.faqs.org/docs/think_java/TIJ3.htm

[Jeff Albertson]

To sum up some previous responses, yes it's good programming practice to
"program to the interface" when possible. In your example, it's more important
for your getFilmBeansByLetter method to have return type Map (or Map<String FilmBean>
in Java 1.5) that for its local value filmBeans to have type Map<String FilmBean>
versus TreeMap<String FilmBean>, although the two can go hand-in-hand.