Why does Hashmap internally use a LinkedList instead of an Arraylist when two objects are placed in the same bucket in the hash table?
Why does HashMap internally use s LinkedList instead of an Arraylist, when two objects are placed into the same bucket in the hash table?
Actually, it doesn't use either (!).
It actually uses a singly linked list implemented by chaining the hash table entries. (By contrast, a LinkedList is doubly linked, and it requires a separate Node object for each element in the list.)
So why am I nitpicking here? Because it is actually important ... because it means that the normal trade-off between LinkedList and ArrayList does not apply.
The normal trade-off is:
ArrayList uses less space, but insertion and removal of a selected element is O(N) in the worst case.
LinkedList uses more space, but insertion and removal of a selected element1 is O(1).
However, in the case of the private singly linked list formed by chaining together HashMap entry nodes, the space overhead is one reference (same as ArrayList), the cost of inserting a node is O(1) (same as LinkedList), and the cost of removing a selected node is also O(1) (same as LinkedList).
Relying solely on "big O" for this analysis is dubious, but when you look at the actual code, it is clear that what HashMap does beat ArrayList on performance for deletion and insertion, and is comparable for lookup. (This ignores memory locality effects.) And it also uses less memory for the chaining than either ArrayList or LinkedList was used ... considering that there are already internal entry objects to hold the key / value pairs.
But it gets even more complicated. In Java 8, they overhauled the HashMap internal data structures. In the current implementation, once a hash chain exceeds a certain length threshold, the implementation switches to using a binary tree representation if the key type implements Comparable.
1 - That is the insertion / deletion is O(1) if you have found the insertion / removal point. For example, if you are using the insert and remove methods on a LinkedList object's ListIterator.
This basically boils down to complexities of ArrayList and LinkedList.
Insertion in LinkedList (when order is not important) is O(1), just append to start.
Insertion in ArrayList (when order is not important) is O(N) ,traverse to end and there is also resizing overhead.
Removal is O(n) in LinkedList, traverse and adjust pointers.
Removal in arraylist could be O(n^2) , traverse to element and shift elements or resize the Arraylist.
Contains will be O(n) in either cases.
When using a HashMap we will expect O(1) operations for add, remove and contains. Using ArrayList we will incur higher cost for the add, remove operations in buckets
Short Answer : Java uses either LinkedList or ArrayList (whichever it finds appropriate for the data).
Long Answer
Although sorted ArrayList looks like the obvious way to go, there are some practical benefits of using LinkedList instead.
We need to remember that LinkedList chain is used only when there is collision of keys.
But as a definition of Hash function : Collisions should be rare
In rare cases of collisions we have to choose between Sorted ArrayList or LinkedList.
If we compare sorted ArrayList and LinkedList there are some clear trade-offs
Insertion and Deletion : Sorted ArrayList takes O(n), but LinkedList takes constant O(1)
Retrieval : Sorted ArrayList takes O(logn) and LinkedList takes 0(n).
Now, its clear that LinkedList are better than sorted ArrayList during insertion and deletion, but they are bad while retrieval.
In there are fewer collisions, sorted ArrayList brings less value (but more over head).
But when the collisions are more frequent and the collided elements list become large(over certain threshold) Java changes the collision data structure from LinkedList to ArrayList.
Related
As HashMap uses LinkedList when two different keys produces a same hashCode.But I was wondering what makes LinkedList a better candidate here over other implementation of List.Why not ArrayList because ArrayList uses Array internally and arrays have a faster iteration compared to a LinkedList.
Collisions in hash maps are an exception, rather than a rule. When your hash function is reasonably good, as it should be, there should be very few collisions.
If we used ArrayList for the buckets, with most lists being empty or having exactly one element, this would be a rather big waste of resources. With array lists allocating multiple members upfront, you would end up paying forward for multiple collisions that you may not have in the future.
Moreover, removing from array lists is cheap only when the last element gets deleted. When the first one gets deleted, you end up paying for the move of all elements.
Linked lists are free from these problems. Insertion is O(1), deletion is O(1), and they use exactly as many nodes as you insert. The memory overhead of the next/prior links is not too big a price to pay for this convenience.
The problem with an arrayList is that you can't fast remove an element: you have to move all the elements after the one you remove.
With a linkedList, removing an element is merely changing a reference from one node to the new next one, skipping the removed one.
The difference is huge. When you want to have a list and be able to fast remove elements, don't use an arraylist, the usual choice is the linked list.
Why not ArrayList because ArrayList uses Array internally and arrays have a faster iteration compared to a LinkedList.
And ArrayList is much slower to modify. So they made a judgement call and went with LinkedList.
I'm thinking about filling a collection with a large amount of unique objects.
How is the cost of an insert in a Set (say HashSet) compared to an List (say ArrayList)?
My feeling is that duplicate elimination in sets might cause a slight overhead.
There is no "duplicate elimination" such as comparing to all existing elements. If you insert into hash set, it's really a dictionary of items by hash code. There's no duplicate checking unless there already are items with the same hash code. Given a reasonable (well-distributed) hash function, it's not that bad.
As Will has noted, because of the dictionary structure HashSet is probably a bit slower than an ArrayList (unless you want to insert "between" existing elements). It also is a bit larger. I'm not sure that's a significant difference though.
You're right: set structures are inherently more complex in order to recognize and eliminate duplicates. Whether this overhead is significant for your case should be tested with a benchmark.
Another factor is memory usage. If your objects are very small, the memory overhead introduced by the set structure can be significant. In the most extreme case (TreeSet<Integer> vs. ArrayList<Integer>) the set structure can require more than 10 times as much memory.
If you're certain your data will be unique, use a List. You can use a Set to enforce this rule.
Sets are faster than Lists if you have a large data set, while the inverse is true for smaller data sets. I haven't personally tested this claim.
Which type of List?
Also, consider which List to use. LinkedLists are faster at adding, removing elements.
ArrayLists are faster at random access (for loops, etc), but this can be worked around using the Iterator of a LinkedList. ArrayLists are are much faster at: list.toArray().
You have to compare concrete implementations (for example HashSet with ArrayList), because the abstract interfaces Set/List don't really tell you anything about performance.
Inserting into a HashSet is a pretty cheap operation, as long as the hashCode() of the object to be inserted is sane. It will still be slightly slower than ArrayList, because it's insertion is a simple insertion into an array (assuming you insert in the end and there's still free space; I don't factor in resizing the internal array, because the same cost applies to HashSet as well).
If the goal is the uniqueness of the elements, you should use an implementation of the java.util.Set interface. The class java.util.HashSet and java.util.LinkedHashSet have O(alpha) (close to O(1) in the best case) complexity for insert, delete and contains check.
ArrayList have O(n) for object (not index) contains check (you have to scroll through the whole list) and insertion (if the insertion is not in tail of the list, you have to shift the whole underline array).
You can use LinkedHashSet that preserve the order of insertion and have the same potentiality of HashSet (takes up only a bit more of memory).
I don't think you can make this judgement simply on the cost of building the collection. Other things that you need to take into account are:
Is the input dataset ordered? Is there a requirement that the output data structure preserves insertion order?
Is there a requirement that the output data structure is ordered (or reordered) based on element values?
Will the output data structure be subsequently modified? How?
Is there a requirement that the output data structure is duplicate free if other elements are added subsequently?
Do you know how many elements are likely to be in the input dataset?
Can you measure the size of the input dataset? (Or is it provided via an iterator?)
Does space utilization matter?
These can all effect your choice of data structure.
Java List:
If you don't have such requirement that you have to keep duplicate or not. Then you can use List instead of Set.
List is an interface in Collection framework. Which extends Collection interface. and ArrayList, LinkedList is the implementation of List interface.
When to use ArrayList or LinkedList
ArrayList: If you have such requirement that in your application mostly work is accessing the data. Then you should go for ArrayList. because ArrayList implements RtandomAccess interface which is Marker Interface. because of Marker interface ArrayList have capability to access the data in O(1) time. and you can use ArrayList over LinkedList where you want to get data according to insertion order.
LinkedList: If you have such requirement that your mostly work is insertion or deletion. Then you should use LinkedList over the ArrayList. because in LinkedList insertion and deletion happen in O(1) time whereas in ArrayList it's O(n) time.
Java Set:
If you have requirement in your application that you don't want any duplicates. Then you should go for Set instead of List. Because Set doesn't store any duplicates. Because Set works on the principle of Hashing. If we add object in Set then first it checks object's hashCode in the bucket if it's find any hashCode present in it's bucked then it'll not add that object.
i thought linkedlists were supposed to be faster than an arraylist when adding elements? i just did a test of how long it takes to add, sort, and search for elements (arraylist vs linkedlist vs hashset). i was just using the java.util classes for arraylist and linkedlist...using both of the add(object) methods available to each class.
arraylist out performed linkedlist in filling the list...and in a linear search of the list.
is this right? did i do something wrong in the implementation maybe?
***************EDIT*****************
i just want to make sure i'm using these things right. here's what i'm doing:
public class LinkedListTest {
private List<String> Names;
public LinkedListTest(){
Names = new LinkedList<String>();
}
Then I just using linkedlist methods ie "Names.add(strings)". And when I tested arraylists, it's nearly identical:
public class ArrayListTest {
private List<String> Names;
public ArrayListTest(){
Names = new ArrayList<String>();
}
Am I doing it right?
Yes, that's right. LinkedList will have to do a memory allocation on each insertion, while ArrayList is permitted to do fewer of them, giving it amortized O(1) insertion. Memory allocation looks cheap, but may be actually be very expensive.
The linear search time is likely slower in LinkedList due to locality of reference: the ArrayList elements are closer together, so there are fewer cache misses.
When you plan to insert only at the end of a List, ArrayList is the implementation of choice.
Remember that:
there's a difference in "raw" performance for a given number of elements, and in how different structures scale;
different structures perform differently at different operations, and that's essentially part of what you need to take into account in choosing which structure to use.
So, for example, a linked list has more to do in adding to the end, because it has an additional object to allocate and initialise per item added, but whatever that "intrinsic" cost per item, both structures will have O(1) performance for adding to the end of the list, i.e. have an effectively "constant" time per addition whatever the size of the list, but that constant will be different between ArrayList vs LinkedList and likely to be greater for the latter.
On the other hand, a linked list has constant time for adding to the beginning of the list, whereas in the case of an ArrayList, the elements must be "shuftied" along, an operation that takes some time proportional to the number of elements. But, for a given list size, say, 100 elements, it may still be quicker to "shufty" 100 elements than it is to allocate and initialise a single placeholder object of the linked list (but by the time you get to, say, a thousand or a million objects or whatever the threshold is, it won't be).
So in your testing, you probably want to consider both the "raw" time of the operations at a given size and how these operations scale as the list size grows.
Why did you think LinkedList would be faster? In the general case, an insert into an array list is simply a case of updating the pointer for a single array cell (with O(1) random access). The LinkedList insert is also random access, but must allocate an "cell" object to hold the entry, and update a pair of pointers, as well as ultimately setting the reference to the object being inserted.
Of course, periodically the ArrayList's backing array may need to be resized (which won't be the case if it was chosen with a large enough initial capacity), but since the array grows exponentially the amortized cost will be low, and is bounded by O(lg n) complexity.
Simply put - inserts into array lists are much simpler and therefore much faster overall.
Linked list may be slower than array list in these cases for a few reasons. If you are inserting into the end of the list, it is likely that the array list has this space already allocated. The underlying array is usually increased in large chunks, because this is a very time-consuming process. So, in most cases, to add an element in the back requires only sticking in a reference, whereas the linked list needs the creation of a node. Adding in the front and the middle should give different performance in for both types of list.
Linear traversal of the list will always be faster in an array based list because it must only traverse the array normally. This requires one dereferencing operation per cell. In the linked list, the nodes of the list must also be dereferenced, taking double the amount of time.
When adding an element to the back of a LinkedList (in Java LinkedList is actually a doubly linked list) it is an O(1) operation as is adding an element to the front of it. Adding an element on the ith position is roughly an O(i) operation.
So, if you were adding to the front of the list, a LinkedList would be significantly faster.
ArrayList is faster in accessing random index data, but slower when inserting elements in the middle of the list, because using linked list you just have to change reference values. But in an array list you have to copy all elements after the inserted index, one index behind.
EDIT: Is not there a linkedlist implementation which keeps the last element in mind? Doing it this way would speed up inserting at the end using linked list.
What are the advantages of each structure?
In my program I will be performing these steps and I was wondering which data structure above I should be using:
Taking in an unsorted array and
adding them to a sorted structure1.
Traversing through sorted data and removing the right one
Adding data (never removing) and returning that structure as an array
When do you know when to use a TreeSet or LinkedList? What are the advantages of each structure?
In general, you decide on a collection type based on the structural and performance properties that you need it to have. For instance, a TreeSet is a Set, and therefore does not allow duplicates and does not preserve insertion order of elements. By contrast a LinkedList is a List and therefore does allow duplicates and does preserve insertion order. On the performance side, TreeSet gives you O(logN) insertion and deletion, whereas LinkedList gives O(1) insertion at the beginning or end, and O(N) insertion at a selected position or deletion.
The details are all spelled out in the respective class and interface javadocs, but a useful summary may be found in the Java Collections Cheatsheet.
In practice though, the choice of collection type is intimately connected to algorithm design. The two need to be done in parallel. (It is no good deciding that your algorithm requires a collection with properties X, Y and Z, and then discovering that no such collection type exists.)
In your use-case, it looks like TreeSet would be a better fit. There is no efficient way (i.e. better than O(N^2)) to sort a large LinkedList that doesn't involve turning it into some other data structure to do the sorting. There is no efficient way (i.e. better than O(N)) to insert an element into the correct position in a previously sorted LinkedList. The third part (copying to an array) works equally well with a LinkedList or TreeSet; it is an O(N) operation in both cases.
[I'm assuming that the collections are large enough that the big O complexity predicts the actual performance accurately ... ]
The genuine power and advantage of TreeSet lies in interface it realizes - NavigableSet
Why is it so powerfull and in which case?
Navigable Set interface add for example these 3 nice methods:
headSet(E toElement, boolean inclusive)
tailSet(E fromElement, boolean inclusive)
subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
These methods allow to organize effective search algorithm(very fast).
Example: we need to find all the names which start with Milla and end with Wladimir:
TreeSet<String> authors = new TreeSet<String>();
authors.add("Andreas Gryphius");
authors.add("Fjodor Michailowitsch Dostojewski");
authors.add("Alexander Puschkin");
authors.add("Ruslana Lyzhichko");
authors.add("Wladimir Klitschko");
authors.add("Andrij Schewtschenko");
authors.add("Wayne Gretzky");
authors.add("Johann Jakob Christoffel");
authors.add("Milla Jovovich");
authors.add("Taras Schewtschenko");
System.out.println(authors.subSet("Milla", "Wladimir"));
output:
[Milla Jovovich, Ruslana Lyzhichko, Taras Schewtschenko, Wayne Gretzky]
TreeSet doesn't go over all the elements, it finds first and last elemenets and returns a new Collection with all the elements in the range.
TreeSet:
TreeSet uses Red-Black tree underlying. So the set could be thought as a dynamic search tree. When you need a structure which is operated read/write frequently and also should keep order, the TreeSet is a good choice.
If you want to keep it sorted and it's append-mostly, TreeSet with a Comparator is your best bet. The JVM would have to traverse the LinkedList from the beginning to decide where to place an item. LinkedList = O(n) for any operations, TreeSet = O(log(n)) for basic stuff.
The most important point when choosing a data structure are its inherent limitations. For example if you use TreeSet to store objects and during run-time your algorithm changes attributes of these objects which affect equal comparisons while the object is an element of the set, get ready for some strange bugs.
The Java Doc for Set interface state that:
Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.
Interface Set Java Doc
Maybe the title is not appropriate but I couldn't think of any other at this moment. My question is what is the difference between LinkedList and ArrayList or HashMap and THashMap .
Is there a tree structure already for Java(ex:AVL,red-black) or balanced or not balanced(linked list). If this kind of question is not appropriate for SO please let me know I will delete it. thank you
ArrayList and LinkedList are implementations of the List abstraction. The first holds the elements of the list in an internal array which is automatically reallocated as necessary to make space for new elements. The second constructs a doubly linked list of holder cells, each of which refers to a list element. While the respective operations have identical semantics, they differ considerably in performance characteristics. For example:
The get(int) operation on an ArrayList takes constant time, but it takes time proportional to the length of the list for a LinkedList.
Removing an element via the Iterator.remove() takes constant time for a LinkedList, but it takes time proportional to the length of the list for an ArrayList.
The HashMap and THashMap are both implementations of the Map abstraction that are use hash tables. The difference is in the form of hash table data structure used in each case. The HashMap class uses closed addressing which means that each bucket in the table points to a separate linked list of elements. The THashMap class uses open addressing which means that elements that hash to the same bucket are stored in the table itself. The net result is that THashMap uses less memory and is faster than HashMap for most operations, but is much slower if you need the map's set of key/value pairs.
For more detail, read a good textbook on data structures. Failing that, look up the concepts in Wikipedia. Finally, take a look at the source code of the respective classes.
Read the API docs for the classes you have mentioned. The collections tutorial also explains the differences fairly well.
java.util.TreeMap is based on a red-black tree.
Regarding the lists:
Both comply with the List interface, but their implementation is different, and they differ in the efficiency of some of their operations.
ArrayList is a list stored internally as an array. It has the advantage of random access, but a single item addition is not guaranteed to run in constant time. Also, removal of items is inefficient.
A LinkedList is implemented as a doubly connected linked list. It does not support random access, but removing an item while iterating through it is efficient.
As I remember, both (LinkedList and ArrayList) are the lists. But they have defferent inner realization.