Difference between HashMap, LinkedHashMap and TreeMap in Java

if you are looking to store key value pairs in Java program,  you have wide range of choices available depending upon your requirement. Main difference between LinkedHashMap, TreeMap and HashMap comes in there internal implementation and specific features, which makes them useful in certain scenarios. For example, HashMap is a general purpose Map (hash table data structure), which should be used whenever you need a hashing based data structure for storing your mappings (key value pairs). TreeMap provides you sorting, on top of hashing offered by Map interface, which means you can not only retrieve elements in constant time i.e. O(1) time, but also iterate through those mapping in a predefined sorted order, but you need to pay heavy price to keep mappings in sorted order. On the other hand, LinkedHashMap is a compromise between these two, it doesn't provide sorting but unlike HashMap, it provides ordering e.g. maintaining mappings in a order they are inserted into Map, known as insertion order or order on which they are accessed, called access order. Apart from these three popular Map implementation, you also have some special purpose Map implementations e.g. EnumMap for storing mapping with enum constants as keys,  it is highly optimized for enum constants. You also have a special map called WeakHashMap for creating a Garbage Collector friendly Cache, where values become eligible for garbage collection as soon as there is no other reference to them apart from keys in WeakHashMap.Then there is IdentityHashMap for creating a Map which uses identity instead of equality for comparing keys, since identity equality is rare, you get less number of collision on this Map and finally JDK 5 introduced ConcurrentHashMap for better scalability in multi-threaded environment, wher When to use LinkedHashMap, TreeMap and HashMap in Java

You can use a LinkedHashMap, when you need to keep your mappings in either insertion order or access-order. LinkedHashMap by default keeps elements in the order, on which they are inserted, and this order is reflected when you traverse over LinkedHashMap, but it also provides a constructor, which allows you to keep entries in access-order, i.e. order in which they are accessed. One of the clever use of Java LinkedHashMap is to use it as Least Recently Use or LRU Cache.

TreeMap is your go to map implementation if you want to keep keys  in a sorted order, either in there natural order defined by Comparable interface or a custom order imposed by Comparator interface, though it's worth remembering that your compareTo() or compare() method must be consistent with equals() method, because Map interface is defined in terms of equals and TreeMap uses compareTo for comparing keys. So if keys compare() or compareTo() implementation is not consistent, then it will fail to obey Map's general contract.

HashMap is your general purpose hashing based collection, whenever you need to use a hash table data structure in Java to store key value pairs, first choice goes to HashMap in single threaded environment. If you happened to use a Map in a multi-threaded environment consider using Hashtable, synchronized HashMap or ConcurrentHashMap from Java Collection Framework.

Since LinkedHashMap solved problem of chaotic ordering provided by Hashtable and HashMap, without incurring high cost associated with TreeMap, you can also used LinkedHashMap to create a copy of a Map in Javae number of reader threads clearly out numbers number of writer threads.