File file = new File(filename);
		IWorkbenchWindow window = PlatformUI.getWorkbench().getActiveWorkbenchWindow();
		IWorkbenchPage page = window.getActivePage();
		IEditorDescriptor desc = PlatformUI.getWorkbench().
		        getEditorRegistry().getDefaultEditor(file.getName());
		try {
			page.openEditor(new FileEditorInput(file), desc.getId()); //if I replaced "FileEditorInput" instance with "NullEditorInput", it would work correctly: an editor opened saying there is an exception because of the lack of an input file
		} catch (PartInitException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

//precondition is org.eclipse.ui.ide plugin should be in dependencies
import java.io.File;
import org.eclipse.core.filesystem.EFS;
import org.eclipse.core.filesystem.IFileStore;
import org.eclipse.ui.PartInitException;
import org.eclipse.ui.IWorkbenchPage;
import org.eclipse.ui.PlatformUI;
import org.eclipse.ui.ide.IDE;
... ...
... ...
private void openExternalFile(String filename) {
		
		File fileToOpen = new File(filename);
		 
		if (fileToOpen.exists() && fileToOpen.isFile()) {
		    IFileStore fileStore = EFS.getLocalFileSystem().getStore(fileToOpen.toURI());
		    IWorkbenchPage page = PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage();
		 
		    try {
		        IDE.openEditorOnFileStore( page, fileStore );
		    } catch ( PartInitException e ) {
		    	e.printStackTrace();
		    }
		} else {
			System.err.println("File " + filename + " does not exist.");
		}
	}