How to make linked hash map navigable?

Question

I have a LinkedHashMap with the UUID keys and instance of my custom class as the value. Order of these entries must be the same as the inserting order. I need to process entries, and during processing I want to know, is I process the last entry, or not, and I want to move to previous or next entity. Looks like I need a hybrid between NavigableMap (navigation function) and LinkedHashMap (save inserting order).

How can I resolve this, what is the best way? If the LinkedHashMap elements are linked, why can't I move beetwen it?

• David Wilson: Hi and welcome to Stack Overflow. Its really important to ask questions properly here. Have a look at How to Ask and minimal reproducible example. Thanks
• Thiyagu: Can you show an example to demonstrate the use case and some code too
• ctomek: Maybe this will help: thebiasplanet.blogspot.com/2017/11/… You can google more solutions probably
• McMill: This is a Use Case - I have a list of task, in order user was wrote. This task present a process. I need to execute this process. In normal case it knows I must execute every task in initial order. But after task executing I need to handle it, and every task can transfer control to the other task - next, previous, or any other. And I need to know what is task number in order, maybe it is the last task, and I need to finish process handling?

Answer 1

ListOrderedMap from Apache commons offers the functionality you describe. E.g.

ListOrderedMap<String, String> data = new ListOrderedMap<>();
data.put("2", "b");
data.put("1", "a");
data.put("5", "c");
System.out.println(data.firstKey());
System.out.println(data.lastKey());
System.out.println(data.get(1));
System.out.println(data.get("2"));

Note that there are get methods by int (insertion order) and by key and both are O(1) as it's backed by a HashMap and an ArrayList

• McMill: Wow, thanks, I'll try this. But is the code example correct? You talking about of ListOrderedMap, but there is an LinkedMap in example.
• Manos Nikolaidis: It wasn't but it is now ;-) LinkedMap uses a LinkedList instead of an ArrayList which makes searching slower but inserting/deleting faster.
• McMill: I think it will really help me. Thank you!