Java 8 How to sort a Map
Java 8 Stream examples to sort a Map, by keys or by values.
1. Quick Explanation
Steps to sort a Map in Java 8.
- Convert a Map into a Stream
- Sort it
- Collect and return a new LinkedHashMap (keep the order)
Map result = map.entrySet().stream() .sorted(Map.Entry.comparingByKey()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new));
P.S By default, Collectors.toMap will returns a HashMap
2. Sort by Keys
package com.mkyong.test; import java.util.HashMap; import java.util.LinkedHashMap; import java.util.Map; import java.util.stream.Collectors; public class SortByKeyExample { public static void main(String[] argv) { Map<String, Integer> unsortMap = new HashMap<>(); unsortMap.put("z", 10); unsortMap.put("b", 5); unsortMap.put("a", 6); unsortMap.put("c", 20); unsortMap.put("d", 1); unsortMap.put("e", 7); unsortMap.put("y", 8); unsortMap.put("n", 99); unsortMap.put("g", 50); unsortMap.put("m", 2); unsortMap.put("f", 9); System.out.println("Original..."); System.out.println(unsortMap); // sort by keys, a,b,c..., and return a new LinkedHashMap // toMap() will returns HashMap by default, we need LinkedHashMap to keep the order. Map<String, Integer> result = unsortMap.entrySet().stream() .sorted(Map.Entry.comparingByKey()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); // Not Recommend, but it works. //Alternative way to sort a Map by keys, and put it into the "result" map Map<String, Integer> result2 = new LinkedHashMap<>(); unsortMap.entrySet().stream() .sorted(Map.Entry.comparingByKey()) .forEachOrdered(x -> result2.put(x.getKey(), x.getValue())); System.out.println("Sorted..."); System.out.println(result); System.out.println(result2);
Output
Original... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99} Sorted... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10} {a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10}
3. Sort by Values
package com.mkyong.test; package com.mkyong; import java.util.Comparator; import java.util.HashMap; import java.util.LinkedHashMap; import java.util.Map; import java.util.stream.Collectors; public class SortByValueExample { public static void main(String[] argv) { Map<String, Integer> unsortMap = new HashMap<>(); unsortMap.put("z", 10); unsortMap.put("b", 5); unsortMap.put("a", 6); unsortMap.put("c", 20); unsortMap.put("d", 1); unsortMap.put("e", 7); unsortMap.put("y", 8); unsortMap.put("n", 99); unsortMap.put("g", 50); unsortMap.put("m", 2); unsortMap.put("f", 9); System.out.println("Original..."); System.out.println(unsortMap); //sort by values, and reserve it, 10,9,8,7,6... Map<String, Integer> result = unsortMap.entrySet().stream() .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); //Alternative way Map<String, Integer> result2 = new LinkedHashMap<>(); unsortMap.entrySet().stream() .sorted(Map.Entry.<String, Integer>comparingByValue().reversed()) .forEachOrdered(x -> result2.put(x.getKey(), x.getValue())); System.out.println("Sorted..."); System.out.println(result); System.out.println(result2);
Output
Original... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99} Sorted... {n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1} {n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1}
4. Map<Object,Object>
The Stream can’t sort the Map<Object,Object> directly. To solve it, convert it into Map<String,String>, review below example.
If you have better idea to sort it, do let me know.
package com.mkyong; import java.util.LinkedHashMap; import java.util.Map; import java.util.Properties; import java.util.Set; import java.util.stream.Collectors; public class DisplayApp { public static void main(String[] args) { Properties properties = System.getProperties(); // not easy to sort this Set<Map.Entry<Object, Object>> entries = properties.entrySet(); LinkedHashMap<String, String> collect = entries.stream() //Map<String, String> .collect(Collectors.toMap(k -> (String) k.getKey(), e -> (String) e.getValue())) .entrySet() .stream() .sorted(Map.Entry.comparingByKey()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); collect.forEach((k, v) -> System.out.println(k + ":" + v));
Not using Java 8? Try this classic way to sort a Map in Java.
References
- How to sort a Map in Java
- LinkedHashMap JavaDoc
- Collectors.toMap() JavaDoc
- More Fun with Wildcards
- Collections.sort() signature
- Difference between and in Java