How to split an HashMap in Java

Question

I was wondering if it is possible to split a HashMap into smaller sub-maps.

In my case I have a HashMap of 100 elements and I would like to create 2 (or more) smaller HashMaps from the original one, the first containing the Entries from 0 to 49, the second containing the Entries from 50 to 99.

Map <Integer, Integer> bigMap = new HashMap <Integer, Integer>();

//should contains entries from 0 to 49 of 'bigMap'
Map <Integer, Integer> smallMap1 = new HashMap <Integer, Integer>(); 


//should contains entries from 50 to 99 of 'bigMap'
Map <Integer, Integer> smallMap2 = new HashMap <Integer, Integer>();

Any suggestions? Many thanks!

Answer 1

Do you have to use HashMap?

TreeMap is really good for this kind of things. Here's an example (note that 0, 50, and 99 are map keys, not indices):

TreeMap<Integer, Integer> sorted = new TreeMap<Integer, Integer>(bigMap);

SortedMap<Integer, Integer> zeroToFortyNine = sorted.subMap(0, 50); // toKey inclusive, fromKey exclusive
SortedMap<Integer, Integer> fiftyToNinetyNine = sorted.subMap(50, true, 99, true);

Answer 2

You can use Guava Iterables partition method and Java stream interface to solve it.

import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public static <K, V> List<Map<K, V>> split(Map<K, V> map, int size) {
    List<List<Map.Entry<K, V>>> list = Lists.newArrayList(Iterables.partition(map.entrySet(), size));

    return list.stream()
            .map(entries ->
                    entries.stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))
            )
            .collect(Collectors.toList());
}