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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>

Implementations:

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

interface Writer{
 //SOME CODE
}
class TemplateWriter implements Writer{
 //SOME CODE
}
class PropertyWriter implements Writer{
 //SOME CODE
}
class PropertySetWriter implements Writer{
 //SOME CODE
}

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()).
    build();

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

Now if you want to use the "PropertySetWriter" then

WriterFactory.byType(PropertySetWriter.class);

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()).
            build();

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

Now if you want to do any operation on action then

ActionHandler.getActionFor(ZoomAction.class).setEnabled(false);


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