Java utils collections: Map

Difference between HashMap, LinkedHashMap and TreeMap

  • HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added.
  • TreeMap will iterate according to the “natural ordering” of the keys according to their compareTo() method (or an externally supplied Comparator). Additionally, it implements the SortedMap interface, which contains methods that depend on this sort order.
  • LinkedHashMap will iterate in the order in which the entries were put into the map


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Check the tomcat app.log in case of unexpected error

If you encounter some unexpected error with your web app, the best place to check what’s wrong is Tomcat’s app.log file.

1. Firstly, ssh to the server containing your app:

$ssh -A ( you can set shortcuts like in this file  ~/.ssh/config)

2. Get the root access

$sudo su –

3. Get the tail of the log. Tail command is useful to see the latest log entries and -f helps to keep refreshing it.

$tail -f app.log

Then replicate the bug, you should see the error message there. It can be null pointer exception for instance.