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Sorting HashMap by Values
Problem:
How can a HashMap be sorted according to the values stored in it, ensuring that the keys are automatically sorted as well?
Solution:
Generic Method (Pre-Java 8):
Implement a generic method to sort a map:
private static <K extends Comparable<K>, V extends Comparable<V>> Map<K, V> sort( final Map<K, V> unsorted, final boolean order) { final var list = new LinkedList<>(unsorted.entrySet()); list.sort((o1, o2) -> order ? o1.getValue().compareTo(o2.getValue()) == 0 ? o1.getKey().compareTo(o2.getKey()) : o1.getValue().compareTo(o2.getValue()) : o2.getValue().compareTo(o1.getValue()) == 0 ? o2.getKey().compareTo(o1.getKey()) : o2.getValue().compareTo(o1.getValue())); return list.stream().collect( Collectors.toMap( Entry::getKey, Entry::getValue, (a, b) -> b, LinkedHashMap::new ) ); }
Usage with Ascending and Descending Order:
import java.util.HashMap; import java.util.Map; public class SortMapByValue { public static final boolean ASC = true; public static final boolean DESC = false; public static void main(String[] args) { // Create an unsorted map Map<String, Integer> unsortMap = new HashMap<>(); unsortMap.put("B", 55); unsortMap.put("A", 80); unsortMap.put("D", 20); unsortMap.put("C", 70); // Sort in ascending order Map<String, Integer> sortedMapAsc = sort(unsortMap, ASC); // Sort in descending order Map<String, Integer> sortedMapDesc = sort(unsortMap, DESC); } }
Newer Java 8 and Above Features:
Alternatively, a more concise solution using Java 8 lambda expressions:
import java.util.*; import java.util.Map.Entry; import java.util.stream.Collectors; public class SortMapByValue { ... private static Map<String, Integer> sortByValue(Map<String, Integer> unsortMap, final boolean order) { List<Entry<String, Integer>> list = new LinkedList<>(unsortMap.entrySet()); list.sort((o1, o2) -> order ? o1.getValue().compareTo(o2.getValue()) == 0 ? o1.getKey().compareTo(o2.getKey()) : o1.getValue().compareTo(o2.getValue()) : o2.getValue().compareTo(o1.getValue()) == 0 ? o2.getKey().compareTo(o1.getKey()) : o2.getValue().compareTo(o1.getValue())); return list.stream().collect(Collectors.toMap(Entry::getKey, Entry::getValue, (a, b) -> b, LinkedHashMap::new)); } ... }
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