Recently I had a problem in converting java.io.File to IFile for external files. The external files sind the files which are not located in the eclipse workspace. So they are not eclipse "resources" and can not be converted into IFile instances. With the following code you can solve this problem:

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                 IEditorInput input= createEditorInput(selectedFile);
                 String editorId= getEditorId(selectedFile);
                 IWorkbenchPage page= fWindow.getActivePage();
                 try {
                     page.openEditor(input, editorId);
                 } catch (PartInitException e) {
                     //EditorsPlugin.log(e.getStatus());
                 }

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This idea comes from org.eclipse.ui.internal.editors.text.OpenExternalFileAction. In this class you can also find the implementation for the methods "createEditorInput" and "getEditorId".