When using java.util.Map.of(), it does not preserve ordering. Is there a way to have immutable Map which does preserve ordering?
I believe what you are looking for is LinkedHashMap.
Linked Hashmap is a like a regular HashMap that preserves insertion order.
tl;dr
Make a LinkedHashMap, wrap with a call to Collections.umodifiableMap, as discussed in Unmodifiable View Collections.
Map.of unmodifiable static factory maps
The Map implementation used by Map.of methods is clearly documented in the Javadoc. One of the bullet points makes clear that the keys are not sorted:
Unmodifiable Maps
…
The iteration order of mappings is unspecified and is subject to change.
…
So you must use another implementation of Map if need sorting.
SortedMap
By "preserve order" did you mean sorted order?
Java includes the SortedMap interface to define behavior of Map implementations that maintain a total ordering across all its keys.
There are two such implementations bundled with Java: TreeMap and ConcurrentSkipListMap.
Original insertion order preserved
Your Question is not clear, but I would guess that what you meant by "preserve ordering" is that you want the key maintained in the order in which they where added to the map. LinkedHashMap does just that, as mentioned in the correct Answer by randypaq13.
To quote the Javadoc:
… predictable iteration order …
… maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map.
Unmodifiable maps
The LinkedHashMap meets your need for original-insertion-order, but not your need for being unmodifiable. For that, use the Collections class with its utility method unmodifiableMap.
Map< String , String > map = new LinkedHashMap<>() ;
map.put( "this" , "that" ) ;
map.put( "other" , "thing" ) ;
Map< String , String > unmodMap = Collections.unmodifiableMap( map ) ;
Be sure to study the documentation on Unmodifiable View Collections to understand their behavior. Click to zoom.
Here is a table giving an overview of all the Map implementations bundled with Java 11.
The following does NOT preserve ordering:
Map.of("a", 1, "b", 2);
If you want to have an immutable ordered Map, then you must go this roundabout way:
var map = new LinkedHashMap<String, String>();
map.put("a", 1);
map.put("b", 2);
map = Collections.unmodifiableMap(map);
Not very optimal, because you have to create two maps, one for the ordering and the other for being immutable..
If Google Guava already exists then
ImmutableMap will please your needs.
Immutability (not an unmodifiable view) and
Iteration order = creation order
More about ImmutableCollection types.
I am inserting four values with different keys in a HashMap.
Code Snippet :
HashMap<Integer, String> choice = new HashMap<Integer, String>();
choice.put(1, "1917");
choice.put(2, "1791");
choice.put(3, "1902");
choice.put(4, "1997");
But when I am printing that map values,it returns a result something like :
{4=1997, 1=1917, 2=1791, 3=1902}
How can I get the map values in a sequential order the way I have put/inserted?
You can use a LinkedHashMap instead, which will keep the insertion order:
This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).
You can modify your code like this:
Map<Integer, String> choice = new LinkedHashMap<Integer, String>();
//rest of the code is the same
LinkedHashMap, which is a Hash table and linked list implementation of the Map interface can be used.
It maintains the same order in which the values are inserted using doubly linked list.
Reference : http://docs.oracle.com/javase/1.4.2/docs/api/java/util/LinkedHashMap.html
[Edit]What the OP asks is not entirely clear to me. The OP could be asking, "How do I retrieve items from a map in insertion order?" If that is what the OP meant, then the following information is not a solution.
If the OP is asking, "How do I retrieve items from a Hash sorted by the key?" then the following is on-point. Getting the sorted keys of a hash/associative array is a common operation.
The answer can be found in how to sort Map values by key in Java:
List sortedKeys=new ArrayList(yourMap.keySet());
Collections.sort(sortedKeys);
If you care about the order in which the keys are stored and retrieved, consider using a TreeMap instead. Unless you have a very large number of elements (millions+), the performance difference is not likely to be noticeable.
I was wondering if the Collection view of the values contained in a HashMap is kept ordered when the HashMap changes.
For example if I have a HashMap whose values() method returns L={a, b, c}
What happened to L if I add a new element "d" to the map?
Is it added at the end, i.e. if I iterate through the elements, it's the order kept?
In particular, if the addition of the new element "d" causes a rehash, will the order be kept in L?
Many thanks!
I was wondering if the Collection view of the values contained in a HashMap is kept ordered when the HashMap changes.
No, there is no such guarantee.
If this was the case, then the following program would output and ordered sequence from 1-100
HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
for (int i = 0; i < 100; i++)
map.put(i, i);
System.out.println(map.values());
(and it doesn't).
There is a class that does precisely what you're asking for, and that is LinkedHashMap:
Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).
If it doesn't say it in the JavaDoc then there are no guarantees about it. Different versions of Java could do different things. Don't depend on undocumented behaviour.
You might want to look at LinkedHashMap.
HashMap in Java aren't ordered, so I think it will be safe to say that values() won't return an ordered Collection.
LinkedHashMap is an ordered version of HashMap (insertion order), but I don't know it values() will return an ordered Collection. I think the best is to try.
Generally they is no guarantee of order when you are using HashMap. It might be in the order in which you add elements for a few elements but it would get reshuffled when there is a possibility of collision and it has to go with a collision resolution strategy.
What is the difference between HashMap, LinkedHashMap and TreeMap in Java?
I don't see any difference in the output as all the three has keySet and values. What are Hashtables?
Map m1 = new HashMap();
m1.put("map", "HashMap");
m1.put("schildt", "java2");
m1.put("mathew", "Hyden");
m1.put("schildt", "java2s");
print(m1.keySet());
print(m1.values());
SortedMap sm = new TreeMap();
sm.put("map", "TreeMap");
sm.put("schildt", "java2");
sm.put("mathew", "Hyden");
sm.put("schildt", "java2s");
print(sm.keySet());
print(sm.values());
LinkedHashMap lm = new LinkedHashMap();
lm.put("map", "LinkedHashMap");
lm.put("schildt", "java2");
lm.put("mathew", "Hyden");
lm.put("schildt", "java2s");
print(lm.keySet());
print(lm.values());
I prefer visual presentation:
Property
HashMap
TreeMap
LinkedHashMap
Iteration Order
no guaranteed order, will remain constant over time
sorted according to the natural ordering
insertion-order
Get / put / remove / containsKey
O(1)
O(log(n))
O(1)
Interfaces
Map
NavigableMap, Map, SortedMap
Map
Null values/keys
allowed
only values
allowed
Fail-fast behavior
Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification
Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification
Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification
Implementation
buckets
Red-Black Tree
double-linked buckets
Is synchronized
implementation is not synchronized
implementation is not synchronized
implementation is not synchronized
All three classes implement the Map interface and offer mostly the same functionality. The most important difference is the order in which iteration through the entries will happen:
HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added.
TreeMap will iterate according to the "natural ordering" of the keys according to their compareTo() method (or an externally supplied Comparator). Additionally, it implements the SortedMap interface, which contains methods that depend on this sort order.
LinkedHashMap will iterate in the order in which the entries were put into the map
"Hashtable" is the generic name for hash-based maps. In the context of the Java API,
Hashtable is an obsolete class from the days of Java 1.1 before the collections framework existed. It should not be used anymore, because its API is cluttered with obsolete methods that duplicate functionality, and its methods are synchronized (which can decrease performance and is generally useless). Use ConcurrentHashMap instead of Hashtable.
All three represent mapping from unique keys to values, and therefore implement the Map interface.
HashMap is a map based on hashing of the keys. It supports O(1) get/put operations. Keys must have consistent implementations of hashCode() and equals() for this to work.
LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added (or accessed), so the iteration order is the same as insertion order (or access order, depending on construction parameters).
TreeMap is a tree based mapping. Its put/get operations take O(log n) time. It requires items to have some comparison mechanism, either with Comparable or Comparator. The iteration order is determined by this mechanism.
See where each class is in the class hierarchy in the following diagram (bigger one). TreeMap implements SortedMap and NavigableMap while HashMap doesn't.
HashTable is obsolete and the corresponding ConcurrentHashMap class should be used.
HashMap
It has pair values(keys,values)
NO duplication key values
unordered unsorted
it allows one null key and more than one null values
HashTable
same as hash map
it does not allows null keys and null values
LinkedHashMap
It is ordered version of map implementation
Based on linked list and hashing data structures
TreeMap
Ordered and sortered version
based on hashing data structures
Just some more input from my own experience with maps, on when I would use each one:
HashMap - Most useful when looking for a best-performance (fast) implementation.
TreeMap (SortedMap interface) - Most useful when I'm concerned with being able to sort or iterate over the keys in a particular order that I define.
LinkedHashMap - Combines advantages of guaranteed ordering from TreeMap without the increased cost of maintaining the TreeMap. (It is almost as fast as the HashMap). In particular, the LinkedHashMap also provides a great starting point for creating a Cache object by overriding the removeEldestEntry() method. This lets you create a Cache object that can expire data using some criteria that you define.
All three classes HashMap, TreeMap and LinkedHashMap implements java.util.Map interface, and represents mapping from unique key to values.
HashMap
A HashMap contains values based on the key.
It contains only unique elements.
It may have one null key and multiple null values.
It maintains no order.
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
LinkedHashMap
A LinkedHashMap contains values based on the key.
It contains only unique elements.
It may have one null key and multiple null values.
It is same as HashMap instead maintains insertion order. //See class deceleration below
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>
TreeMap
A TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
It contains only unique elements.
It cannot have null key but can have multiple null values.
It is same as HashMap instead maintains ascending order(Sorted using the natural order of its key.).
public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, Serializable
Hashtable
A Hashtable is an array of list. Each list is known as a bucket. The position of bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key.
It contains only unique elements.
It may have not have any null key or value.
It is synchronized.
It is a legacy class.
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable
Ref: http://javarevisited.blogspot.in/2015/08/difference-between-HashMap-vs-TreeMap-vs-LinkedHashMap-Java.html
HashMap makes absolutely not guarantees about the iteration order. It
can (and will) even change completely when new elements are added.
TreeMap will iterate according to the "natural ordering" of the keys
according to their compareTo() method (or an externally supplied
Comparator). Additionally, it implements the SortedMap interface,
which contains methods that depend on this sort order. LinkedHashMap
will iterate in the order in which the entries were put into the map
Look at how performance varying..
Tree map which is an implementation of Sorted map. The complexity of the put, get and containsKey operation is O(log n) due to the Natural ordering
#Amit: SortedMap is an interface whereas TreeMap is a class which implements the SortedMap interface. That means if follows the protocol which SortedMap asks its implementers to do.
A tree unless implemented as search tree, can't give you ordered data because tree can be any kind of tree. So to make TreeMap work like Sorted order, it implements SortedMap ( e.g, Binary Search Tree - BST, balanced BST like AVL and R-B Tree , even Ternary Search Tree - mostly used for iterative searches in ordered way ).
public class TreeMap<K,V>
extends AbstractMap<K,V>
implements SortedMap<K,V>, Cloneable, Serializable
In NUT-SHELL
HashMap : gives data in O(1) , no ordering
TreeMap : gives data in O(log N), base 2. with ordered keys
LinkedHashMap : is Hash table with linked list (think of indexed-SkipList) capability to store data in the way it gets inserted in the tree. Best suited to implement LRU ( least recently used ).
Hash map doesn't preserves the insertion order.
Example. Hashmap
If you are inserting keys as
1 3
5 9
4 6
7 15
3 10
It can store it as
4 6
5 9
3 10
1 3
7 15
Linked Hashmap preserves the insertion order.
Example.
If you are inserting keys
1 3
5 9
4 6
7 15
3 10
It will store it as
1 3
5 9
4 6
7 15
3 10
same as we insert.
Tree map stores the vales in Increasing Order Of Keys.
Example.
If you are inserting keys
1 3
5 9
4 6
7 15
3 10
It will store it as
1 3
3 10
4 6
5 9
7 15
HashMap:
Order not maintains
Faster than LinkedHashMap
Used for store heap of objects
LinkedHashMap:
LinkedHashMap insertion order will be maintained
Slower than HashMap and faster than TreeMap
If you want to maintain an insertion order use this.
TreeMap:
TreeMap is a tree-based mapping
TreeMap will follow the natural ordering of key
Slower than HashMap and LinkedHashMap
Use TreeMap when you need to maintain natural(default) ordering
Following are major difference between HashMap and TreeMap
HashMap does not maintain any order. In other words , HashMap does not provide any guarantee that the element inserted first will be printed first, where as Just like TreeSet , TreeMap elements are also sorted according to the natural ordering of its elements
Internal HashMap implementation use Hashing and TreeMap internally uses Red-Black tree implementation.
HashMap can store one null key and many null values.TreeMap can not contain null keys but may contain many null values.
HashMap take constant time performance for the basic operations like get and put i.e O(1).According to Oracle docs , TreeMap provides guaranteed log(n) time cost for the get and put method.
HashMap is much faster than TreeMap, as performance time of HashMap is constant against the log time TreeMap for most operations.
HashMap uses equals() method in comparison while TreeMap uses compareTo() method for maintaining ordering.
HashMap implements Map interface while TreeMap implements NavigableMap interface.
These are different implementations of the same interface. Each implementation has some advantages and some disadvantages (fast insert, slow search) or vice versa.
For details look at the javadoc of TreeMap, HashMap, LinkedHashMap.
While there are plenty of excellent Answers here, I'd like to present my own table describing the various Map implementations bundled with Java 11.
We can see these differences listed on the table graphic:
HashMap is the general-purpose Map commonly used when you have no special needs.
LinkedHashMap extends HashMap, adding this behavior: Maintains an order, the order in which the entries were originally added. Altering the value for key-value entry does not alter its place in the order.
TreeMap too maintains an order, but uses either (a) the “natural” order, meaning the value of the compareTo method on the key objects defined on the Comparable interface, or (b) invokes a Comparator implementation you provide.
TreeMap implements both the SortedMap interface, and its successor, the NavigableMap interface.
NULLs: TreeMap does not allow a NULL as the key, while HashMap & LinkedHashMap do.
All three allow NULL as the value.
HashTable is legacy, from Java 1. Supplanted by the ConcurrentHashMap class. Quoting the Javadoc: ConcurrentHashMap obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable.
The most important among all the three is how they save the order of the entries.
HashMap - Does not save the order of the entries.
eg.
public static void main(String[] args){
HashMap<String,Integer> hashMap = new HashMap<>();
hashMap.put("First",1);// First ---> 1 is put first in the map
hashMap.put("Second",2);//Second ---> 2 is put second in the map
hashMap.put("Third",3); // Third--->3 is put third in the map
for(Map.Entry<String,Integer> entry : hashMap.entrySet())
{
System.out.println(entry.getKey()+"--->"+entry.getValue());
}
}
LinkedHashMap : It save the order in which entries were made. eg:
public static void main(String[] args){
LinkedHashMap<String,Integer> linkedHashMap = new LinkedHashMap<>();
linkedHashMap.put("First",1);// First ---> 1 is put first in the map
linkedHashMap.put("Second",2);//Second ---> 2 is put second in the map
linkedHashMap.put("Third",3); // Third--->3 is put third in the map
for(Map.Entry<String,Integer> entry : linkedHashMap.entrySet())
{
System.out.println(entry.getKey()+"--->"+entry.getValue());
}
}
TreeMap : It saves the entries in ascending order of the keys. eg:
public static void main(String[] args) throws IOException {
TreeMap<String,Integer> treeMap = new TreeMap<>();
treeMap.put("A",1);// A---> 1 is put first in the map
treeMap.put("C",2);//C---> 2 is put second in the map
treeMap.put("B",3); //B--->3 is put third in the map
for(Map.Entry<String,Integer> entry : treeMap.entrySet())
{
System.out.println(entry.getKey()+"--->"+entry.getValue());
}
}
All offer a key->value map and a way to iterate through the keys. The most important distinction between
these classes are the time guarantees and the ordering of the keys.
HashMap offers 0(1) lookup and insertion. If you iterate through the keys, though, the ordering of the
keys is essentially arbitrary. It is implemented by an array of linked lists.
TreeMap offers O(log N) lookup and insertion. Keys are ordered, so if you need to iterate through
the keys in sorted order, you can. This means that keys must implement the Comparable interface.TreeMap is implemented by a Red-Black Tree.
LinkedHashMap offers 0(1) lookup and insertion. Keys are ordered by their insertion order. It is
implemented by doubly-linked buckets.
Imagine you passed an empty TreeMap, HashMap, and LinkedHashMap into the following function:
void insertAndPrint(AbstractMap<Integer, String> map) {
int[] array= {1, -1, 0};
for (int x : array) {
map.put(x, Integer.toString(x));
}
for (int k: map.keySet()) {
System.out.print(k + ", ");
}
}
The output for each will look like the results below.
For HashMap, the output was, in my own tests, { 0, 1, -1}, but it could be any ordering. There is no guarantee on the
ordering.
Treemap,the output was,{ -1, 0, 1}
LinkedList,the output was,{ 1, -1, 0}
HashMap
can contain one null key.
HashMap maintains no order.
TreeMap
TreeMap can not contain any null key.
TreeMap maintains ascending order.
LinkedHashMap
LinkedHashMap can be used to maintain insertion order, on which keys are inserted into Map or it can also be used to maintain an access order, on which keys are accessed.
Examples::
1) HashMap map = new HashMap();
map.put(null, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");`enter code here`
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}
2) TreeMap map = new TreeMap();
map.put(1, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}
3) LinkedHashMap map = new LinkedHashMap();
map.put(1, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}