Skip to main content

Guava: ClassToInstanceMap

If you have requirement of associating the class to an instance of it’s type then ClassToInstanceMap is the best choice for you. It comes with additional type safety.

It’s an extended interface of Map

public interface ClassToInstanceMap<B> extends Map<Class<? extends B>,B>


Here is a example where you can use this map
Some classes 

interface Writer{
class TemplateWriter implements Writer{
class PropertyWriter implements Writer{
class PropertySetWriter implements Writer{

And we have one Factory class

class WriterFactory{
 private static final ClassToInstanceMap<Writer> factoryMap =
   new ImmutableClassToInstanceMap.Builder<Writer>().
    put(TemplateWriter.class, new TemplateWriter()).
    put(PropertyWriter.class, new PropertyWriter()).
    put(PropertySetWriter.class, new PropertySetWriter()).

 public static Writer byType(Class<? extends Writer> clazz){
  return factoryMap.getInstance(clazz);

Now if you want to use the "PropertySetWriter" then


This kind of pattern is very easy to maintain and understand. It is very easy change as well. Suppose there is a change in “PropertySetWriter” then you just have to create a subclass of this and need to add in “WriterFactory” map and it’s done.

Found a nice example on stackoverflow and it showcases another very useful scenario

public class ActionHandler {

    private static final ClassToInstanceMap<Action> actionMap =
         new ImmutableClassToInstanceMap.Builder<Action>().
            put(DefaultEditorKit.CutAction.class, new DefaultEditorKit.CutAction()).
            put(DefaultEditorKit.CopyAction.class, new DefaultEditorKit.CopyAction()).
            put(DefaultEditorKit.PasteAction.class, new DefaultEditorKit.PasteAction()).
            put(RefreshAction.class, new RefreshAction()).
            put(MinimizeAction.class, new MinimizeAction()).
            put(ZoomAction.class, new ZoomAction()).

    public static Action getActionFor(Class<? extends Action> actionClasss) {
        return actionMap.getInstance(actionClasss);

Now if you want to do any operation on action then



Popular posts from this blog

ERROR: Ignored call to 'alert()'. The document is sandboxed, and the 'allow-modals' keyword is not set.

Recently I found this issue while writing code snippet in "JSFiddle". And after searching, found this was happening because of new feature added in "Chrome 46+". But at the same time Chrome doesn't have support for "allow-modals" property in "sandbox" attribute. Chromium issue for above behavior: To make it work you have to add "allow-scripts allow-modals" in "sandbox" attribute, and use "window.alert" instead of "alert". <!-- Sandbox frame will execute javascript and show modal dialogs --> <iframe sandbox="allow-scripts allow-modals" src="iframe.html"> </iframe> Feature added: Block modal dialog inside a sandboxed iframe. Link: Feature working Demo page:

Application Design Notes

Don’t be afraid to write your own code, but be absolutely sure you need to Don't reinvent the wheel Learn more about your libraries and take full advantage  Date time calculation is hard ( leap second ,  leap year ), use trusted library  js-joda ,  momentJs ,  joda (java) Simple is better than perfect (nearly) every time If you can deliver a sub-optimal solution (that solves the problem but has known limitation) in a week instead of a full featured one in a month DO, IT Simple system are Easy to reason about  Easy to debug Easy to refactor Easy to learn Simple doesn't mean you skip good engineering, but you can use duct tape. Build things the right way from the start, refactoring is hard and expensive Security Manage and store passwords securely Telemetry Common retrofitting "grunt work" Internationalization + localization Web Content Accessibility Factoring and styling HTML UI Adding unit test to an existing codebase LOG LOG LOG Log, but do it right We spend lot of t

How to store user password at server!!!

Trick is, you should never store user password… never ever. Now the real question is, then how to authenticate and authorize the user with password. And answer is when user enter the password, we should encrypt the password and store the hints. So next time when user enter the password we follow the same process and compare hints, if both hints are same then password is matched, else it is wrong password. Next question will be, what kind of hints, and how to generate these hints. In simple term hints are the obfuscated and fragmented form of user password. And very important part is hints generation process, which have to be collision resistant , means there will be very less possibility to find the data which generate same hints (like Cryptographic hashing functions ). Below is the simple checklist of password hashing and storing, which you should always keep in mind. PS You're Probably Storing Passwords Incorrectly Storing Passwords - done rig