Difference Betwixt Hashmap, Linkedhashmap Together With Treemap Inwards Java
Sunday, August 5, 2018
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The java.util.Map is 1 of the almost of import interfaces from Java Collection Framework. It provides hash tabular array information construction functionality past times its implementations similar HashMap, Hashtable, LinkedHashMap as well as a niggling fleck of sorting amongst the TreeMap. So if you lot are looking to shop key-value pairs inwards Java program, you bring a broad make of choices available depending upon your requirement. The principal deviation betwixt LinkedHashMap, TreeMap as well as HashMap comes inwards their internal implementation as well as 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 live on used whenever you lot need 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 way you lot tin non solely retrieve elements inwards guaranteed log(n) fourth dimension (Algorithms are adaptations of those inwards Cormen, Leiserson, as well as Rivest's Introduction to Algorithms), but also iterate through those mapping inwards a predefined sorted order, but you lot need to pay a heavy cost to give-up the ghost along mappings inwards sorted order.
On the other hand, LinkedHashMap is a compromise betwixt these two, it doesn't furnish sorting but different HashMap, it provides ordering e.g. maintaining mappings inwards an social club they are inserted into Map, known equally insertion order or social club on which they are accessed, called access order.
Apart from these 3 pop Map implementation, you lot also bring about especial purpose Map implementations e.g. EnumMap for storing mapping amongst enum constants equally keys, it is highly optimized for enum constants. You also bring a especial 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 teach less number of collisions on this Map as well as finally, JDK v introduced ConcurrentHashMap for improve scalability inwards a multi-threaded environment, where the number of reader threads clearly outnumbers a number of author threads.
LinkedHashMap tin live on used to maintain insertion order, on which keys are inserted into Map or it tin also live on used to maintain an access order, on which keys are accessed. This provides LinkedHashMap an border over HashMap without compromising equally good much performance.
TreeMap provides you lot consummate command over sorting elements past times passing custom Comparator of your choice, but amongst the expense of about performance. Since entries are stored inwards a tree-based information structure, it provides lower performance than HashMap as well as LinkedHashMap.
On the other hand, TreeMap, which sorts elements inwards natural social club doesn't allow cipher 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 live on whatever functioning which affects iteration social club 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 demeanour is non guaranteed, as well as they throw ConcurrentModificationException on the best-effort basis, which way do non write code, which depends upon this behavior. It should solely live on used to abide by programming bugs.
BTW, constant fourth dimension performance is solely provided if mappings are distributed uniformly across bucket location. In the existent world, you lot e'er bring collision as well as HashMap handles collision past times using a linked listing to shop collided elements. This tin trim back worst instance performance of HashMap upward to O(n).
To mitigate the higher upward 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 1 bucket. See how does HashMap handles collisions inwards Java for to a greater extent than details.
Worth noting is that this demeanour is solely applicable to HashMap, LinkedHashMap, as well as ConcurrentHashMap, Hashtable is left behind to save its legacy iteration social club equally many legacy Java application relies on that as well as this changes that order. This is also a practiced instance of why you lot should non rely on undocumented features of JDK e.g. iteration social club of HashMap because they tin modify inwards future.
but HashMap is sure enough 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 utilisation initial capacity as well as 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() as well as remove() operation, Algorithms are based upon those given inwards Cormen, Leiserson, as well as Rivest's Introduction to Algorithms.
LinkedHashMap is a trade-off betwixt two, similar HashMap it also provides constant fourth dimension performance for add, contains as well as 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 need insertion social club or access order, consider using LinkedHashMap over TreeMap inwards Java.
When using synchronized Map e.g. synchronized LinkedHashMap or SortedMap, you lot must do at the fourth dimension or creating the map to forbid accidental non-synchronized access. You tin utilisation the next idiom to create Synchronized Map inwards Java:
Synchronized LinkedHashMap
Synchronized TreeMap
Remember to utilisation Collections.synchronizedMap() for synchronizing HashMap, LinkedHashMap as well as Collections.synchronizedSortedMap() method for synchronizing TreeMap. If you lot are non comfortable as well as then encounter this lead on how to synchronize HashMap inwards Java.
On the other hand, LinkedHashMap extends HashMap as well as uses linked listing to furnish insertion social club guarantee. It uses doubly-linked listing running through all of its entries, which tin also live on used to maintain access-order. Remember, insertion social club is non affected if a telephone substitution is re-inserted into LinkedHashMap, but access social club is affected if LinkedHashMap is created to maintain access-order.
TreeMap is internally based upon Red-Black Tree as well as NavigableMap, introduced inwards JDK 6. The Red-Black tree is used to maintain the sorting social club 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 as well as take operations. Algorithms are adaptations of those inwards Cormen, Leiserson, as well as 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 social club defined past times Comparable interface or a custom social club imposed past times Comparator interface, though it's worth remembering that your compareTo() or compare() method must live on consistent amongst equals() method, because Map interface is defined inwards price of equals as well as TreeMap uses compareTo for comparison keys. So if keys compare() or compareTo() implementation is non consistent, as well as then it volition neglect to obey Map's full general contract.
HashMap is your full general purpose hashing based collection, whenever you lot need to utilisation a hash tabular array information construction inwards Java to shop key-value pairs, the kickoff pick goes to HashMap inwards a unmarried threaded environment. If you lot happened to utilisation a Map inwards a multi-threaded surroundings consider using Hashtable, synchronized HashMap or ConcurrentHashMap from Java Collection Framework.
Since LinkedHashMap solved the occupation of chaotic ordering provided past times Hashtable as well as HashMap, without incurring the high cost associated amongst TreeMap, you lot tin also utilisation LinkedHashMap to create a re-create of a Map inwards Java, equally shown inwards below example.
You tin encounter that TreeMap has sorted mappings inwards contrary order, because of contrary Comparator provided to it. Also, LinkedHashMap has created a re-create of TreeMap as well as social club of entries are retained.
That's all on the difference betwixt LinkedHashMap, TreeMap, as well as HashMap inwards Java. Though all 3 are Map implementation, they bring a different purpose as well as used accordingly. Use LinkedHashMap, if you lot need to maintain insertion or access social club of mappings e.g. inwards LRU Cache. Use TreeMap, if you lot need to maintain mappings inwards a sorted order, either inwards their natural social club or a custom social club defined past times Comparator as well as utilisation 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 way you lot tin non solely retrieve elements inwards guaranteed log(n) fourth dimension (Algorithms are adaptations of those inwards Cormen, Leiserson, as well as Rivest's Introduction to Algorithms), but also iterate through those mapping inwards a predefined sorted order, but you lot need to pay a heavy cost to give-up the ghost along mappings inwards sorted order.
On the other hand, LinkedHashMap is a compromise betwixt these two, it doesn't furnish sorting but different HashMap, it provides ordering e.g. maintaining mappings inwards an social club they are inserted into Map, known equally insertion order or social club on which they are accessed, called access order.
Apart from these 3 pop Map implementation, you lot also bring about especial purpose Map implementations e.g. EnumMap for storing mapping amongst enum constants equally keys, it is highly optimized for enum constants. You also bring a especial 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 teach less number of collisions on this Map as well as finally, JDK v introduced ConcurrentHashMap for improve 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 3 classes implement java.util.Map interface as well as follows full general contract of a Map interface, defined inwards price of equals() as well as hashCode() method, they also bring several differences inwards price of Ordering, Sorting, permitting cipher elements, Iteration, Performance, Speed as well as internal implementation. Let's bring a quick expect on each of these properties.Ordering as well as Sorting
HashMap doesn't furnish whatever ordering guarantee for entries, which means, you lot tin non assume whatever social club piece iterating over keys as well as values of HashMap. This demeanour of HashMap is similar to Hashtable piece other 2 Map implementation provides ordering guarantee.LinkedHashMap tin live on used to maintain insertion order, on which keys are inserted into Map or it tin also live on used to maintain an access order, on which keys are accessed. This provides LinkedHashMap an border over HashMap without compromising equally good much performance.
TreeMap provides you lot consummate command over sorting elements past times passing custom Comparator of your choice, but amongst the expense of about performance. Since entries are stored inwards a tree-based information structure, it provides lower performance than HashMap as well as LinkedHashMap.
Null keys as well as Values
HashMap allows one null telephone substitution as well as multiple null values. It keeps cipher telephone 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 cipher keys. Since LinkedHashMap is a subclass of HashMap, it also allows null keys as well as values.On the other hand, TreeMap, which sorts elements inwards natural social club doesn't allow cipher 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 stance methods e.g. values() or keySet() is fail-fast iterators, which way they volition throw ConcurrentModificatoinException if Collection is modified structurally 1 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 live on whatever functioning which affects iteration social club 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 demeanour is non guaranteed, as well as they throw ConcurrentModificationException on the best-effort basis, which way do non write code, which depends upon this behavior. It should solely live on used to abide by programming bugs.
Performance as well as Speed
Since HashMap is a barebone implementation of java.util.Map interface, it provides constant fourth dimension performance for the get() as well as put() operation, where put() method is used to shop entries (key-value pairs) as well as 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 e'er bring collision as well as HashMap handles collision past times using a linked listing to shop collided elements. This tin trim back worst instance performance of HashMap upward to O(n).
To mitigate the higher upward 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 1 bucket. See how does HashMap handles collisions inwards Java for to a greater extent than details.
Worth noting is that this demeanour is solely applicable to HashMap, LinkedHashMap, as well as ConcurrentHashMap, Hashtable is left behind to save its legacy iteration social club equally many legacy Java application relies on that as well as this changes that order. This is also a practiced instance of why you lot should non rely on undocumented features of JDK e.g. iteration social club of HashMap because they tin modify inwards future.
but HashMap is sure enough 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 utilisation initial capacity as well as 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() as well as remove() operation, Algorithms are based upon those given inwards Cormen, Leiserson, as well as Rivest's Introduction to Algorithms.
LinkedHashMap is a trade-off betwixt two, similar HashMap it also provides constant fourth dimension performance for add, contains as well as 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 need insertion social club or access order, consider using LinkedHashMap over TreeMap inwards Java.
Thread-safety as well as Synchronization
All 3 Map implementation are not thread-safe, which way you lot tin non utilisation 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 utilisation their concurrent counterpart e.g. ConcurrentHashMap which is also a improve pick than HashMap inwards a concurrent Java application.When using synchronized Map e.g. synchronized LinkedHashMap or SortedMap, you lot must do at the fourth dimension or creating the map to forbid accidental non-synchronized access. You tin utilisation 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 utilisation Collections.synchronizedMap() for synchronizing HashMap, LinkedHashMap as well as Collections.synchronizedSortedMap() method for synchronizing TreeMap. If you lot are non comfortable as well as then encounter this lead on how to synchronize HashMap inwards Java.
Internal Implementation
TreeMap is Red-Black tree based NavigableMap implementation piece HashMap is internally backed past times an array. It uses index[0] to shop entries corresponding to cipher keys. In fact, questions related to the inner working of HashMap is rattling pop inwards Java, for example, How does get() method of HashMap industrial plant internally is 1 of the ofttimes used questions to Senior Java developers.On the other hand, LinkedHashMap extends HashMap as well as uses linked listing to furnish insertion social club guarantee. It uses doubly-linked listing running through all of its entries, which tin also live on used to maintain access-order. Remember, insertion social club is non affected if a telephone substitution is re-inserted into LinkedHashMap, but access social club is affected if LinkedHashMap is created to maintain access-order.
TreeMap is internally based upon Red-Black Tree as well as NavigableMap, introduced inwards JDK 6. The Red-Black tree is used to maintain the sorting social club 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 as well as take operations. Algorithms are adaptations of those inwards Cormen, Leiserson, as well as Rivest's Introduction to Algorithms.
When to utilisation LinkedHashMap, TreeMap, as well as HashMap
You tin utilisation a LinkedHashMap when you lot need 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, as well as this social club 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. social club inwards which they are accessed. One of the clever utilisation of Java LinkedHashMap is to utilisation 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 social club defined past times Comparable interface or a custom social club imposed past times Comparator interface, though it's worth remembering that your compareTo() or compare() method must live on consistent amongst equals() method, because Map interface is defined inwards price of equals as well as TreeMap uses compareTo for comparison keys. So if keys compare() or compareTo() implementation is non consistent, as well as then it volition neglect to obey Map's full general contract.
HashMap is your full general purpose hashing based collection, whenever you lot need to utilisation a hash tabular array information construction inwards Java to shop key-value pairs, the kickoff pick goes to HashMap inwards a unmarried threaded environment. If you lot happened to utilisation a Map inwards a multi-threaded surroundings consider using Hashtable, synchronized HashMap or ConcurrentHashMap from Java Collection Framework.
Since LinkedHashMap solved the occupation of chaotic ordering provided past times Hashtable as well as HashMap, without incurring the high cost associated amongst TreeMap, you lot tin also utilisation LinkedHashMap to create a re-create of a Map inwards Java, equally shown inwards below example.
An instance of using LinkedHashMap, TreeMap as well as HashMap inwards Java
Let's encounter an instance of how to utilisation these Map implementations. In this example, nosotros volition utilisation HashMap to create a full general purpose Cache, TreeMap to create a sorted Cache as well as nosotros volition utilisation LinkedHashMap for copying a Map (cache) as well as 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 utilisation LinkedHashMap, TreeMap as well as HashMap. * It shows that HashMap doesn't guarantee whatever order, TreeMap keeps them in * sorted social club determined past times default using Comparable or explicit Comparator * provided past times client, as well as LinkedHashMap also give-up the ghost along mapping inwards social club 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 social club {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 encounter that TreeMap has sorted mappings inwards contrary order, because of contrary Comparator provided to it. Also, LinkedHashMap has created a re-create of TreeMap as well as social club of entries are retained.
Summary
Here is the summary of differences betwixt HashMap, LinkedHashMap, as well as TreeMap inwards Java:
That's all on the difference betwixt LinkedHashMap, TreeMap, as well as HashMap inwards Java. Though all 3 are Map implementation, they bring a different purpose as well as used accordingly. Use LinkedHashMap, if you lot need to maintain insertion or access social club of mappings e.g. inwards LRU Cache. Use TreeMap, if you lot need to maintain mappings inwards a sorted order, either inwards their natural social club or a custom social club defined past times Comparator as well as utilisation 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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