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For my application I need a Treetable, where the child items are loaded lazyily when the user clicks on the expand icon.
For each "parent" entry in the Treetable, I know the exact number of child items in advance (if this information is relevant to you).
Is such a treetable lazy loading possible with vaadin?
Well, you can always listen for node expand and collapse events and add and remove the items in the listener. See this example.
Note that if the TreeTable is editable, you could have performance issues because of #17838, but if it's not editable, that's not a problem.