try {
   InputStream inputstream = new StringBufferInputStream(mystring);
   IEditorInput edInput = new MemoryEditorInput("MemFile", inputstream);
   IWorkbench wb = PlatformUI.getWorkbench();
   IWorkbenchWindow win = wb.getActiveWorkbenchWindow();
   IWorkbenchPage page = win.getActivePage();
   page.openEditor(edInput, "org.eclipse.ui.DefaultTextEditor");
} catch (Exception e) {
   ...
}

public class MemoryEditorInput implements IStorageEditorInput {

   private InputStream inputStream;
   private String dataName;
	
   public MemoryEditorInput(String dataName, InputStream inputStream) {
      this.inputStream = inputStream;
      this.dataName = dataName;
   }

   public boolean exists() { return false; }
   public ImageDescriptor getImageDescriptor() { return null; }
   public String getName() { return dataName; }
   public IPersistableElement getPersistable() { return null; }
   public String getToolTipText() { return dataName + " - " + inputStream; }
   public Object getAdapter(Class adapter) { return null; }
   public IStorage getStorage() throws CoreException {
      return new IStorage() {
         public Object getAdapter(Class adapter) { return null; }
         public InputStream getContents() throws CoreException { return inputStream; }
         public IPath getFullPath() { return null; }
         public String getName() { return null; }
         public boolean isReadOnly() { return false; }
     };
   }
}