In this post, we will see how to convert HashMap to TreeMap in Java. The resultant TreeMap should contain all mappings of the HashMap, sorted by their natural ordering of keys.
We know that HashMap is implemented using a Hash Table while TreeMap is implemented by Red-Black tree.
HashMap is much faster than TreeMap (O(1) time versus O(log(n)) time for inserting and searching but offers no ordering guarantees like TreeMap. In a TreeMap, map is ordered according to the natural ordering of its keys or a specified Comparator in the TreeMap’s constructor.
If we want near-HashMap performance and insertion-order iteration, we can use LinkedHashMap.
Below are few ways to convert HashMap to TreeMap in Java –
1. Java 8
The idea is to convert HashMap to a Stream and collect elements of a stream in a TreeMap using Stream.collect() method which accepts a collector.
We can use collector returned by Collectors.toMap() method that accepts TreeMap constructor reference TreeMap::new.
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import java.util.HashMap; import java.util.Map; import java.util.TreeMap; import java.util.stream.Collectors; class MapUtils { // Generic function to construct a new TreeMap from HashMap public static <K,V> Map<K,V> getTreeMap(Map<K,V> hashMap) { Map<K, V> treeMap = hashMap.entrySet() .stream() .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> newValue, TreeMap::new)); return treeMap; } // Program to convert HashMap to TreeMap in Java 8 public static void main(String args[]) { Map<String, String> hashMap = new HashMap<>(); hashMap.put("RED", "#FF0000"); hashMap.put("BLUE", "#0000FF"); hashMap.put("GREEN", "#008000"); // construct a new TreeMap from HashMap Map<String, String> treeMap = getTreeMap(hashMap); System.out.println(treeMap); } } |
Output:
{BLUE=#0000FF, GREEN=#008000, RED=#FF0000}
2. Plain Java
To construct a new TreeMap from HashMap, we can either pass HashMap instance to the TreeMap constructor or to putAll() method.
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// Generic function to construct a new TreeMap from HashMap public static <K,V> Map<K,V> getTreeMap(Map<K,V> hashMap) { Map<K, V> treeMap = new TreeMap<>(); treeMap.putAll(hashMap); return treeMap; } |
3. Google’s Guava library
Guava also provides TreeMap implementation which can be used to create an empty TreeMap instance.
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// Generic function to construct a new TreeMap from HashMap public static <K extends Comparable,V> Map<K,V> getTreeMap(Map<K,V> hashMap) { Map<K, V> treeMap = Maps.newTreeMap(); treeMap.putAll(hashMap); return treeMap; } |
4. Conversion between incompatible types
If HashMap and TreeMap is expected to have incompatible types of keys or values, we’ll need to convert them manually:
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import java.util.HashMap; import java.util.Map; import java.util.TreeMap; class MapUtils { // Program to convert HashMap to TreeMap in Java 8 public static void main(String args[]) { Map<String, String> hashMap = new HashMap<>(); hashMap.put("1", "ONE"); hashMap.put("2", "TWO"); hashMap.put("3", "THREE"); // construct a new TreeMap<Integer, String> from HashMap<String, String> Map<Integer, String> treeMap = new TreeMap<>(); for (Map.Entry<String, String> e : hashMap.entrySet()) treeMap.put(Integer.parseInt(e.getKey()), e.getValue()); System.out.println(treeMap); } } |
Output:
{1=ONE, 2=TWO, 3=THREE}
We can do better in Java 8 as illustrated below:
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// construct a new TreeMap<Integer, String> from HashMap<String, String> Map<Integer, String> treeMap = hashMap.entrySet().stream() .collect(Collectors.toMap(x -> Integer.parseInt(x.getKey()), x -> x.getValue(), (oldValue, newValue) -> newValue, TreeMap::new)); |
or even better,
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// construct a new TreeMap<Integer, String> from HashMap<String, String> Map<Integer, String> treeMap = new TreeMap<>(); hashMap.entrySet() .stream() .forEach(x -> treeMap.put(Integer.parseInt(x.getKey()), x.getValue())); |
Exercise: Convert HashMap to LinkedHashMap in Java
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