Java putting Hashmap into Treemap

问题

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);

回答1:

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

Should work anyway.

回答2:

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.

回答3:

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

来源：https://stackoverflow.com/questions/19512850/java-putting-hashmap-into-treemap