Eclipse插件-使用适配器而不是强制转换有什么好处？

Question

If you get a selection of projects as IStructuredSelection, use .toArray() to get their Object[]-content, then why does casting each Object to IAdaptable and then simply casting again to IJavaProject work fine for me and I get all the Classes/Interfaces with their methods....

But when using (IJavaProject) adaptable.getAdapter(IJavaProject.class) instead, some interface methods get lost?

What exactly does the Adapter do (other than "Returns an object which is an instance of the given class parameter...") and why does everyone recommend to use it instead of a cast? Here is the relevant code

    IAdaptable adaptable = (IAdaptable) selectedObject;
    IJavaProject javaProject = (IJavaProject) adaptable.getAdapter(IJavaProject.class);
    final IProject iProject = javaProject.getProject();
    iProject.refreshLocal(IResource.DEPTH_INFINITE, new NullProgressMonitor());
    final IJavaProject javaRefreshedProject = JavaCore.create(iProject);
    //processing further using ASTParser and CompilationUnit

Answer 1

Many objects you get from a selection do not implement things like IProject or IJavaProject instead they are some internal class that is more suited to the way the code wants to present a view or editor.

In this case you cannot use a cast because the selection object does not implement the interface you want.

The user interface objects may implement IAdaptable which allows you to retrieve the underlying class using object.getAdapter(...) .

However not all objects implement IAdaptable themselves, instead they use the registerAdapters method of the IAdapterManager to declare a completely different class that converts from the UI object to the underlying object. In this case you use

IProject project = (IProject)Platform.getAdapterManager().getAdapter(object, IProject.class);

You can use a utility method like the following to deal with all possibilities:

public static <AdapterType> AdapterType adapt(Object adaptableObject, Class<AdapterType> adapterType)
{
  if (adaptableObject == null)
    return null;

  // Is the object the desired type?

  if (adapterType.isInstance(adaptableObject))
    return adapterType.cast(adaptableObject);

  // Does object adapt to the type?

  if (adaptableObject instanceof IAdaptable)
   {
     AdapterType result = adapterType.cast(((IAdaptable)adaptableObject).getAdapter(adapterType));
     if (result != null)
       return result;
   }

  // Try the platform adapter manager

  return adapterType.cast(Platform.getAdapterManager().getAdapter(adaptableObject, adapterType));
}