Java putting Hashmap into Treemap

Question

I am currently reading 2 million lines from a textfile as asked in the previous question Java Fastest way to read through text file with 2 million lines

Now I store these information into HashMap and I want to sort it via TreeMap because I want to use ceilingkey. Is the following method correct?

private HashMap<Integer, String> hMap = new HashMap();

private TreeMap<Integer, String> tMap = new TreeMap<Integer, String>(hMap);

Collections.sort(hMap) ? ,Collections.sort(hMap,WITH_MY_OWN_COMPARATOR) ? — Suresh Atta
– Suresh Atta, Commented Oct 22, 2013 at 8:35
Why not just put it directly into a TreeMap ? Why the extra step? — Stewart
– Stewart, Commented Oct 22, 2013 at 8:36
hrm...i still prefer to sort it with treemap but as of my code, the treemap is empty — BeyondProgrammer
– BeyondProgrammer, Commented Oct 22, 2013 at 8:36
@suresh atta : Collections.sort(hMap) won't work, sort() work with List only — Kushal
– Kushal, Commented Feb 12, 2018 at 7:17

Akkusativobjekt · Accepted Answer · 2013-10-22 08:37:04Z

36

HashMap<Integer, String> hashMap = new HashMap<Integer, String>();
TreeMap<Integer, String> treeMap = new TreeMap<Integer, String>();
treeMap.putAll(hashMap);

Should work anyway.

answered Oct 22, 2013 at 8:37

Akkusativobjekt

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8 Comments

Suresh Atta Over a year ago

Are you just kidding ??? Passing constructor also calls putAll() :). Checkout the source code link, I added

Suresh Atta Over a year ago

@user2822351 I'm not telling that this answer is wrong, What I'm telling is this answer is equals to what you are doing right now.

Akkusativobjekt Over a year ago

It's correct, but the usage of the constructor is more elegant.

codeMan Over a year ago

@user2822351The question is... is it any different from what you have posted in ur question above !?

BeyondProgrammer Over a year ago

@ Akkusativobjekt, i am using constructor but it doesnt work. the treemap is empty when i try to print out

|

Community · Accepted Answer · 2017-05-23 12:32:08Z

8

This would work just fine:

HashMap<Integer, String> hashMap = new HashMap<>();
TreeMap<Integer, String> treeMap = new TreeMap<>(hashMap);

But I wouldn't advise using HashMap to store the input. You end up with two Maps holding the same huge data. Either do it on the fly and add directly into TreeMap or use List to TreeMap conversion.

Also, for even more efficiency consider primitive collections.

edited May 23, 2017 at 12:32

CommunityBot

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answered Jun 7, 2016 at 9:55

husayt

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Comments

Nagmani Verma · Accepted Answer · 2016-09-06 07:42:53Z

2

HashMap<Integer, String> hashMap = new HashMap<Integer, String>();
TreeMap<Integer, String> treeMap = new TreeMap<Integer, String>();
hashMap.remove(null);
treeMap.putAll(hashMap);

HashMap will allow null but TreeMap not so before adding into Treemap, remove null from keyset

answered Sep 6, 2016 at 7:42

Nagmani Verma

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