Difference Betwixt Hashmap, Linkedhashmap Together With Treemap Inwards Java
The java.util.Map is ane of the near of import interfaces from Java Collection Framework. It provides hash tabular array information construction functionality past times its implementations similar HashMap, Hashtable, LinkedHashMap together with a trivial flake of sorting amongst the TreeMap. So if you lot are looking to shop key-value pairs inwards Java program, you convey a broad arrive at of choices available depending upon your requirement. The primary divergence betwixt LinkedHashMap, TreeMap together with HashMap comes inwards their internal implementation together with specific features, which makes them useful inwards sure scenarios. For example, the HashMap is a full general purpose Map (hash tabular array information structure), which should move used whenever you lot necessitate a hashing-based information construction for storing your mappings (key-value pairs).
TreeMap is a Red-Black tree based NavigableMap implementation provides you lot sorting, on top of hashing offered past times Map interface. This agency you lot tin non solely retrieve elements inwards guaranteed log(n) fourth dimension (Algorithms are adaptations of those inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms), but also iterate through those mapping inwards a predefined sorted order, but you lot necessitate to pay a heavy toll to give-up the ghost along mappings inwards sorted order.
On the other hand, LinkedHashMap is a compromise betwixt these two, it doesn't render sorting but dissimilar HashMap, it provides ordering e.g. maintaining mappings inwards an gild they are inserted into Map, known equally insertion order or gild on which they are accessed, called access order.
Apart from these iii pop Map implementation, you lot also convey roughly particular purpose Map implementations e.g. EnumMap for storing mapping amongst enum constants equally keys, it is highly optimized for enum constants. You also convey a particular map called WeakHashMap for creating a Garbage Collector friendly Cache, where values give-up the ghost eligible for garbage collection equally shortly equally at that topographic point is no other reference to them apart from keys inwards WeakHashMap.
Then at that topographic point is IdentityHashMap for creating a Map which uses identity instead of equality for comparison keys since identity equality is rare, you lot instruct less number of collisions on this Map together with finally, JDK v introduced ConcurrentHashMap for ameliorate scalability inwards a multi-threaded environment, where the number of reader threads clearly outnumbers a number of author threads.
LinkedHashMap tin move used to maintain insertion order, on which keys are inserted into Map or it tin also move used to maintain an access order, on which keys are accessed. This provides LinkedHashMap an border over HashMap without compromising likewise much performance.
TreeMap provides you lot consummate command over sorting elements past times passing custom Comparator of your choice, but amongst the expense of roughly performance. Since entries are stored inwards a tree-based information structure, it provides lower performance than HashMap together with LinkedHashMap.
On the other hand, TreeMap, which sorts elements inwards natural gild doesn't allow nada keys because compareTo() method throws NullPointerException if compared amongst null. If you lot are using TreeMap amongst user defined Comparator than it depends upon the implementation of compare() method.
By the way, it's worth remembering that apart from adding or removing to a greater extent than mappings, it tin also move whatever functioning which affects iteration gild of LinkedHashMap. In access-ordered LinkedHashMap, fifty-fifty querying the Map amongst get() method is a structural modification, because it changes the iteration order, on the other manus updating the value inwards an insertion-ordered linked hash map is non a structural modification.
Finally, the fail-fast take is non guaranteed, together with they throw ConcurrentModificationException on the best-effort basis, which agency practice non write code, which depends upon this behavior. It should solely move used to let out programming bugs.
BTW, constant fourth dimension performance is solely provided if mappings are distributed uniformly across bucket location. In the existent world, you lot ever convey collision together with HashMap handles collision past times using a linked listing to shop collided elements. This tin cut down worst instance performance of HashMap upwardly to O(n).
To mitigate the to a higher house performance issue, JDK 8 has introduced balanced tree instead of linked listing inwards instance of frequent collision inwards HashMap. It internally switches to balanced tree from linked listing if at that topographic point are to a greater extent than than 8 entries inwards ane bucket. See how does HashMap handles collisions inwards Java for to a greater extent than details.
Worth noting is that this take is solely applicable to HashMap, LinkedHashMap, together with ConcurrentHashMap, Hashtable is left behind to save its legacy iteration gild equally many legacy Java application relies on that together with this changes that order. This is also a skillful instance of why you lot should non rely on undocumented features of JDK e.g. iteration gild of HashMap because they tin modify inwards future.
but HashMap is for certain faster than Hashtable because it's non synchronized. Iteration over Map is straight proportional to the "capacity" + "size" of HashMap, that's why it's of import to ready the initial capacity high plenty if iteration performance is important. You tin farther job initial capacity together with load factor to fine melody your HashMap performance, to avoid rehashing of HashMap.
TreeMap is so it's costlier than HashMap if the order is non concerned. Since TreeMap is based on tree information construction (based upon Red-Black tree), it provides the log(n) fourth dimension for the get(), put(), containsKey() together with remove() operation, Algorithms are based upon those given inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms.
LinkedHashMap is a trade-off betwixt two, similar HashMap it also provides constant fourth dimension performance for add, contains together with remove, though it's slightly slower than HashMap, to maintain linked list. By the way, looping over Map inwards the instance of LinkedHashMap is slightly faster than HashMap because the fourth dimension required is proportional to size only. So if you lot necessitate insertion gild or access order, consider using LinkedHashMap over TreeMap inwards Java.
When using synchronized Map e.g. synchronized LinkedHashMap or SortedMap, you lot must practice at the fourth dimension or creating the map to forestall accidental non-synchronized access. You tin job the next idiom to create Synchronized Map inwards Java:
Synchronized LinkedHashMap
Synchronized TreeMap
Remember to job Collections.synchronizedMap() for synchronizing HashMap, LinkedHashMap together with Collections.synchronizedSortedMap() method for synchronizing TreeMap. If you lot are non comfortable thence come across this guide on how to synchronize HashMap inwards Java.
On the other hand, LinkedHashMap extends HashMap together with uses linked listing to render insertion gild guarantee. It uses doubly-linked listing running through all of its entries, which tin also move used to maintain access-order. Remember, insertion gild is non affected if a substitution is re-inserted into LinkedHashMap, but access gild is affected if LinkedHashMap is created to maintain access-order.
TreeMap is internally based upon Red-Black Tree together with NavigableMap, introduced inwards JDK 6. The Red-Black tree is used to maintain the sorting gild imposed past times Comparable or Comparator, provided at the fourth dimension of creation. TreeMap provides guaranteed log(n) fourth dimension cost for the get, put, containsKey together with take operations. Algorithms are adaptations of those inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms.
TreeMap is your give-up the ghost to map implementation if you lot desire to give-up the ghost along keys in a sorted order, either inwards their natural gild defined past times Comparable interface or a custom gild imposed past times Comparator interface, though it's worth remembering that your compareTo() or compare() method must move consistent amongst equals() method, because Map interface is defined inwards price of equals together with TreeMap uses compareTo for comparison keys. So if keys compare() or compareTo() implementation is non consistent, thence it volition neglect to obey Map's full general contract.
HashMap is your full general purpose hashing based collection, whenever you lot necessitate to job a hash tabular array information construction inwards Java to shop key-value pairs, the start selection goes to HashMap inwards a unmarried threaded environment. If you lot happened to job a Map inwards a multi-threaded surroundings consider using Hashtable, synchronized HashMap or ConcurrentHashMap from Java Collection Framework.
Since LinkedHashMap solved the job of chaotic ordering provided past times Hashtable together with HashMap, without incurring the high cost associated amongst TreeMap, you lot tin also job LinkedHashMap to create a re-create of a Map inwards Java, equally shown inwards below example.
You tin come across that TreeMap has sorted mappings inwards contrary order, because of contrary Comparator provided to it. Also, LinkedHashMap has created a re-create of TreeMap together with gild of entries are retained.
That's all on the difference betwixt LinkedHashMap, TreeMap, together with HashMap inwards Java. Though all iii are Map implementation, they convey a different purpose together with used accordingly. Use LinkedHashMap, if you lot necessitate to maintain insertion or access gild of mappings e.g. inwards LRU Cache. Use TreeMap, if you lot necessitate to maintain mappings inwards a sorted order, either inwards their natural gild or a custom gild defined past times Comparator together with job HashMap for all your full general purpose hashing based collection requirement. HashMap allows you lot to retrieve an object inwards O(1) fourth dimension if you lot know the key.
Further Learning
Java In-Depth: Become a Complete Java Engineer
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TreeMap is a Red-Black tree based NavigableMap implementation provides you lot sorting, on top of hashing offered past times Map interface. This agency you lot tin non solely retrieve elements inwards guaranteed log(n) fourth dimension (Algorithms are adaptations of those inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms), but also iterate through those mapping inwards a predefined sorted order, but you lot necessitate to pay a heavy toll to give-up the ghost along mappings inwards sorted order.
On the other hand, LinkedHashMap is a compromise betwixt these two, it doesn't render sorting but dissimilar HashMap, it provides ordering e.g. maintaining mappings inwards an gild they are inserted into Map, known equally insertion order or gild on which they are accessed, called access order.
Apart from these iii pop Map implementation, you lot also convey roughly particular purpose Map implementations e.g. EnumMap for storing mapping amongst enum constants equally keys, it is highly optimized for enum constants. You also convey a particular map called WeakHashMap for creating a Garbage Collector friendly Cache, where values give-up the ghost eligible for garbage collection equally shortly equally at that topographic point is no other reference to them apart from keys inwards WeakHashMap.
Then at that topographic point is IdentityHashMap for creating a Map which uses identity instead of equality for comparison keys since identity equality is rare, you lot instruct less number of collisions on this Map together with finally, JDK v introduced ConcurrentHashMap for ameliorate scalability inwards a multi-threaded environment, where the number of reader threads clearly outnumbers a number of author threads.
LinkedHashMap vs TreeMap vs HashMap
Though all iii classes implement java.util.Map interface together with follows full general contract of a Map interface, defined inwards price of equals() together with hashCode() method, they also convey several differences inwards price of Ordering, Sorting, permitting nada elements, Iteration, Performance, Speed together with internal implementation. Let's convey a quick expect on each of these properties.Ordering together with Sorting
HashMap doesn't render whatever ordering guarantee for entries, which means, you lot tin non assume whatever gild spell iterating over keys together with values of HashMap. This take of HashMap is similar to Hashtable spell other ii Map implementation provides ordering guarantee.LinkedHashMap tin move used to maintain insertion order, on which keys are inserted into Map or it tin also move used to maintain an access order, on which keys are accessed. This provides LinkedHashMap an border over HashMap without compromising likewise much performance.
TreeMap provides you lot consummate command over sorting elements past times passing custom Comparator of your choice, but amongst the expense of roughly performance. Since entries are stored inwards a tree-based information structure, it provides lower performance than HashMap together with LinkedHashMap.
Null keys together with Values
HashMap allows one null substitution together with multiple null values. It keeps nada substitution based entries on index[0] on an internal bucket. If you lot expect at the put() method of HashMap, you lot tin see, it doesn't throw NullPointerException for nada keys. Since LinkedHashMap is a subclass of HashMap, it also allows null keys together with values.On the other hand, TreeMap, which sorts elements inwards natural gild doesn't allow nada keys because compareTo() method throws NullPointerException if compared amongst null. If you lot are using TreeMap amongst user defined Comparator than it depends upon the implementation of compare() method.
Iterators
Iterators returned past times all these Map's collection sentiment methods e.g. values() or keySet() is fail-fast iterators, which agency they volition throw ConcurrentModificatoinException if Collection is modified structurally ane time Iteration begins, except past times using remove() method of Iterator.By the way, it's worth remembering that apart from adding or removing to a greater extent than mappings, it tin also move whatever functioning which affects iteration gild of LinkedHashMap. In access-ordered LinkedHashMap, fifty-fifty querying the Map amongst get() method is a structural modification, because it changes the iteration order, on the other manus updating the value inwards an insertion-ordered linked hash map is non a structural modification.
Finally, the fail-fast take is non guaranteed, together with they throw ConcurrentModificationException on the best-effort basis, which agency practice non write code, which depends upon this behavior. It should solely move used to let out programming bugs.
Performance together with Speed
Since HashMap is a barebone implementation of java.util.Map interface, it provides constant fourth dimension performance for the get() together with put() operation, where put() method is used to shop entries (key-value pairs) together with get() is used to retrieve a value based on a key.BTW, constant fourth dimension performance is solely provided if mappings are distributed uniformly across bucket location. In the existent world, you lot ever convey collision together with HashMap handles collision past times using a linked listing to shop collided elements. This tin cut down worst instance performance of HashMap upwardly to O(n).
To mitigate the to a higher house performance issue, JDK 8 has introduced balanced tree instead of linked listing inwards instance of frequent collision inwards HashMap. It internally switches to balanced tree from linked listing if at that topographic point are to a greater extent than than 8 entries inwards ane bucket. See how does HashMap handles collisions inwards Java for to a greater extent than details.
Worth noting is that this take is solely applicable to HashMap, LinkedHashMap, together with ConcurrentHashMap, Hashtable is left behind to save its legacy iteration gild equally many legacy Java application relies on that together with this changes that order. This is also a skillful instance of why you lot should non rely on undocumented features of JDK e.g. iteration gild of HashMap because they tin modify inwards future.
but HashMap is for certain faster than Hashtable because it's non synchronized. Iteration over Map is straight proportional to the "capacity" + "size" of HashMap, that's why it's of import to ready the initial capacity high plenty if iteration performance is important. You tin farther job initial capacity together with load factor to fine melody your HashMap performance, to avoid rehashing of HashMap.
TreeMap is so it's costlier than HashMap if the order is non concerned. Since TreeMap is based on tree information construction (based upon Red-Black tree), it provides the log(n) fourth dimension for the get(), put(), containsKey() together with remove() operation, Algorithms are based upon those given inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms.
LinkedHashMap is a trade-off betwixt two, similar HashMap it also provides constant fourth dimension performance for add, contains together with remove, though it's slightly slower than HashMap, to maintain linked list. By the way, looping over Map inwards the instance of LinkedHashMap is slightly faster than HashMap because the fourth dimension required is proportional to size only. So if you lot necessitate insertion gild or access order, consider using LinkedHashMap over TreeMap inwards Java.
Thread-safety together with Synchronization
All iii Map implementation are not thread-safe, which agency you lot tin non job them safely inwards a multi-threaded application. Though you lot tin synchronize them externally past times using Collections.synchronizedMap(Map map) method. Alternatively, you lot tin also job their concurrent counterpart e.g. ConcurrentHashMap which is also a ameliorate selection than HashMap inwards a concurrent Java application.When using synchronized Map e.g. synchronized LinkedHashMap or SortedMap, you lot must practice at the fourth dimension or creating the map to forestall accidental non-synchronized access. You tin job the next idiom to create Synchronized Map inwards Java:
Synchronized LinkedHashMap
Map<Integer, Integer> numbers = Collections.synchronizedMap(new LinkedHashMap<>());
Synchronized TreeMap
SortedMap<Integer, String> sorted = Collections.synchronizedSortedMap(new TreeMap<>());
Remember to job Collections.synchronizedMap() for synchronizing HashMap, LinkedHashMap together with Collections.synchronizedSortedMap() method for synchronizing TreeMap. If you lot are non comfortable thence come across this guide on how to synchronize HashMap inwards Java.
Internal Implementation
TreeMap is Red-Black tree based NavigableMap implementation spell HashMap is internally backed past times an array. It uses index[0] to shop entries corresponding to nada keys. In fact, questions related to the inner working of HashMap is really pop inwards Java, for example, How does get() method of HashMap plant internally is ane of the oft used questions to Senior Java developers.On the other hand, LinkedHashMap extends HashMap together with uses linked listing to render insertion gild guarantee. It uses doubly-linked listing running through all of its entries, which tin also move used to maintain access-order. Remember, insertion gild is non affected if a substitution is re-inserted into LinkedHashMap, but access gild is affected if LinkedHashMap is created to maintain access-order.
TreeMap is internally based upon Red-Black Tree together with NavigableMap, introduced inwards JDK 6. The Red-Black tree is used to maintain the sorting gild imposed past times Comparable or Comparator, provided at the fourth dimension of creation. TreeMap provides guaranteed log(n) fourth dimension cost for the get, put, containsKey together with take operations. Algorithms are adaptations of those inwards Cormen, Leiserson, together with Rivest's Introduction to Algorithms.
When to job LinkedHashMap, TreeMap, together with HashMap
You tin job a LinkedHashMap when you lot necessitate to give-up the ghost along your mappings inwards either insertion order or access order. LinkedHashMap past times default keeps elements inwards the order, on which they are inserted, together with this gild is reflected when you lot traverse over LinkedHashMap, but it also provides a constructor, which allows you lot to give-up the ghost along entries inwards access order, the. gild inwards which they are accessed. One of the clever job of Java LinkedHashMap is to job it equally Least Recently Use or LRU Cache.TreeMap is your give-up the ghost to map implementation if you lot desire to give-up the ghost along keys in a sorted order, either inwards their natural gild defined past times Comparable interface or a custom gild imposed past times Comparator interface, though it's worth remembering that your compareTo() or compare() method must move consistent amongst equals() method, because Map interface is defined inwards price of equals together with TreeMap uses compareTo for comparison keys. So if keys compare() or compareTo() implementation is non consistent, thence it volition neglect to obey Map's full general contract.
HashMap is your full general purpose hashing based collection, whenever you lot necessitate to job a hash tabular array information construction inwards Java to shop key-value pairs, the start selection goes to HashMap inwards a unmarried threaded environment. If you lot happened to job a Map inwards a multi-threaded surroundings consider using Hashtable, synchronized HashMap or ConcurrentHashMap from Java Collection Framework.
Since LinkedHashMap solved the job of chaotic ordering provided past times Hashtable together with HashMap, without incurring the high cost associated amongst TreeMap, you lot tin also job LinkedHashMap to create a re-create of a Map inwards Java, equally shown inwards below example.
An instance of using LinkedHashMap, TreeMap together with HashMap inwards Java
Let's come across an instance of how to job these Map implementations. In this example, nosotros volition job HashMap to create a full general purpose Cache, TreeMap to create a sorted Cache together with nosotros volition job LinkedHashMap for copying a Map (cache) together with maintaining orders inwards the master copy Map.import java.util.Collections; import java.util.HashMap; import java.util.LinkedHashMap; import java.util.Map; import java.util.SortedMap; import java.util.TreeMap; /** * Java Program to demonstrate How to job LinkedHashMap, TreeMap together with HashMap. * It shows that HashMap doesn't guarantee whatever order, TreeMap keeps them in * sorted gild determined past times default using Comparable or explicit Comparator * provided past times client, together with LinkedHashMap also give-up the ghost along mapping inwards gild they * are added or accessed.,
* * @author Javin Paul */ public class MapTest { public static void main(String args[]){ //Using HashMap equally full general purpose unmarried threaded cache Map<Integer, String> cache = new HashMap<>(); cache.put(1, "Stuart"); cache.put(2, "Steven"); cache.put(3, "James"); cache.put(4, "Ian"); System.out.printf("Name of Employee amongst id %d is %s %n", 1, cache.get(1)); System.out.println("Order of Entries inwards HashMap - Not guaranteed"); System.out.println(cache); //Using TreeMap to create a sorted cache, sorting keys on contrary order SortedMap<Integer, String> sortedCache = new TreeMap<>(Collections.reverseOrder()); sortedCache.putAll(cache); System.out.println("Order of Entries inwards TreeMap - Sorted inwards contrary order"); System.out.println(sortedCache); //Using LinkedHashMap to create re-create of a Map inwards Java Map<Integer, String> re-create = new LinkedHashMap<>(sortedCache); System.out.println("Order of Entries inwards a re-create Map created past times LinkedHashMap"); System.out.println(copy); } } Output: Name of Employee amongst id 1 is Stuart Order of Entries inwards HashMap - Not guaranteed {1=Stuart, 2=Steven, 3=James, 4=Ian} Order of Entries inwards TreeMap - Sorted inwards contrary gild {4=Ian, 3=James, 2=Steven, 1=Stuart} Order of Entries inwards a re-create Map created past times LinkedHashMap {4=Ian, 3=James, 2=Steven, 1=Stuart}
You tin come across that TreeMap has sorted mappings inwards contrary order, because of contrary Comparator provided to it. Also, LinkedHashMap has created a re-create of TreeMap together with gild of entries are retained.
Summary
Here is the summary of differences betwixt HashMap, LinkedHashMap, together with TreeMap inwards Java:
That's all on the difference betwixt LinkedHashMap, TreeMap, together with HashMap inwards Java. Though all iii are Map implementation, they convey a different purpose together with used accordingly. Use LinkedHashMap, if you lot necessitate to maintain insertion or access gild of mappings e.g. inwards LRU Cache. Use TreeMap, if you lot necessitate to maintain mappings inwards a sorted order, either inwards their natural gild or a custom gild defined past times Comparator together with job HashMap for all your full general purpose hashing based collection requirement. HashMap allows you lot to retrieve an object inwards O(1) fourth dimension if you lot know the key.
Further Learning
Java In-Depth: Become a Complete Java Engineer
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