One of the key attributes of an iterator is that
The iterator pattern allow us to:
access contents of a collection without exposing its internal structure.
What exactly is meant by that, what is the internal structure?
Every data structure is implemented differently. Some structures might use a linked design, some might be backed by a single array, or there could be some mix of the two. Imagine if every time you wanted to use a different List implementation, you needed to learn about how it works just to use it.
The Iterator interface (along with other Interfaces) provide(s) a consistent set of methods that allow you to use an iterable, even if you have no idea how it's implemented "under the hood".
The internal structure is the private member[s] that hold the contents of the collection.
For example, in ArrayList, the internal structure is a backing array holding the elements of the List:
transient Object[] elementData;
The Iterator returned by ArrayList's iterator() gives you access to the elements of the ArrayList without giving you access to the backing array, which means you cannot mutate the backing array directly.
For HashSet, the internal structure is a HashMap that holds the elements of the Set:
private transient HashMap<E,Object> map;
Again, the iterator gives you access to the elements of the Set without giving you access to that HashMap.
In this context internal structure means the list of member functions of the collection class or interface. But thinking further, as the member function list roughly makes up the class / interface, internal structure means the class / interface of the collection. And this statement is wide and can mean the sort of collection like array, bag, queue...
An iterator is an adapter often with a different interface than the collections it operates on. So using an iterator not only enables us to hide the collection's class declaration (which defines the collection's internal structure) but also enables us to hide the collection's public interface (which might be separate and smaller than the whole collection's class declaration). And this public interface is not internal structure. So your original statement without exposing its internal structure is ok but not completely comprehensive.
An iterator saves us not only from exposing internal structure but also from exposing the public interface of the collection. And this gives more value to the concept of iterator in context of code separation.
As a simple example, a TreeMap can be iterated to learn all members of the map. When using an iterator, I don't have to care whether the tree underlying the map is a binary tree, a trie, a b-tree, a red-black tree, etc. As a user I don't want to have to know how it maintains its internal links to read its data. I don't want to have to know anything about its implementation. Software that seeks maintainability should reduce the amount a user needs to know about its implementation.
"Structure" in this sense is used in the same manner as in "data structure"...a way of organizing data in memory to permit efficient lookup, insertion, deletion, or other operations. An iterator hides the details of the organization scheme used to make the operations efficient, by providing a simple way to get the data, item by item, just by calling next().
Suppose there is a string treeset (ts)of elemnent 1,2,3,4,5,6,7,8,9,10.
Is there is any in built method in treeset so that i can access an element.
For eg accessing 3 can i do ts.[2]and accessing 8 ts.[7].(something like that).
i used this method:
Iterator<String> it = ts.iterator();
int i=0;
while(it.hasNext()) {
String ele=it.next();
if(i==2){
System.out.println(ele+"");
}
i++;
}
though when i ran it didn't showed any o/p but if i did i=0 then it showed all the o/p i.e 1,2,3,4,5,6,7,8,9,10.
Secondly can anyone tell me that when it is best to use hashset,treeset and linkedhashset
If you wanna access elements in your collection like ts[2], then you should better convert your collection into array using collection inbuilt method.
Otherwise, using iterator is the standard and efficient way to access elements in collection.
For second question, Hashset is used as hash table ; LinkedHashSet is used as hash table with elements stored in same way as inserted; TreeSet is used for collection using navigations.
For complete knowledge you must check Oracle documentation.
TreeSet is a NavigableSetwhich means you have an order of items (natural ordering as default, but you can define your own ordering relationship by using Comparator or Comparable interface) and you can navigate through items by this order. However there is no index mechanism. Basically a TreeSet is based on a TreeMap which is a red-black tree. In such a data structure indexes (element indexes, not indexes in the sense of efficient access) are not much meaningfull.
HashSet on the other hand is based on a HashMap which is a classical hash table implementation. In this data structure there is no order defined. You can look up each item at O(1) time though due to hash function used.
LinkedHashSet is a subclass of HashSet. Other then HashSet methods no new method is defined, so LinkedHashSet does not allow any more capability like natural order or indexes. However it has an auxilary linked list that keeps track of the order in which elements are inserted. In this way when you iterate over a LinkedHashSet by .iterator() method or a for loop you get elements in the order you inserted.
So basically a HashSet is more appropriate if you will access elements individually. Or being the simplest Set implementation you can use HashSet in generic cases. If you need to keep the order of insertion you need to use LinkedHashSet and if you have to enforce any custom ordering or natural ordering of items you should use TreeSet.
I need a data structure to do a get / find in Log N time and iterate starting from the object that was returned by the get operation. The iterator should iterate in the same order in which elements are inserted into the data structure.
Can I achieve this using TreeSet ? Or any other data structure?
Thanks!
This answer assumes that you want to get / find items by value, as opposed to access by insertion sequence number. I assume that this value is completely unrelated to the order in which items are inserted.
The closest you can get with standard Java foundation classes is a LinkedHashSet. This allows fast searching and iteration in insertion order. But it does not give you an iterator starting at a given position, so you'll have to implement that yourself. Either based on the LinkedHashSet, or using your own set implementation. I guess the easiest way would be using a HashSet and implementing the linking yourself. That way, you could use the set methods to look up the starting element, and use that to construct an iterator following the links. You could hide the links inside a wrapper object, so you won't have to expose them in your API.
If you start with a SortedMap<Integer, Object> and use keys for keeping track of the insertion order, you'll be able to use the fast tailMap operation for your needs.
If you need to find the position of an object by a key (or maybe by the object itself), then introduce another WeakHashMap<Object, Integer> that will map from your key to the position of the object. You'll then use the retrieved sequence number as the key into the former map.
Provided you are using Java 6, you can have a look at ConcurrentSkipListSet and ConcurrentSkipListMap.
i use a hashmap to store some data, but i need to keep it in ascending order whenever new data saved to the hashmap or old data move out of the hashmap. but hashmap itself doesn't suppport order, what data structure i can use to support order? Thanks
TreeMap would be the canonical sorted map implementation. Note that this is sorted on the keys, which I presume is what you're after, but if not it won't be suitable.
Since Java 6 also comes with a SortedMap interface, you can look at the list of classes which implement it (on the linked Javadoc page), and choose between those. Implementing this method only guarantees that they have some sort of defined iteration order, you'd have to read the descriptions of each class to see if it's what you like.
TreeMap isn't a hashmap, in that it isn't backed by a hashtable to provide amortised O(1) inserts. However, it's not possible to maintain a sorted map with O(1) inserts anyway (since you have to inspect at least some of the existing elements to work out where the new element should go), and hence the O(lg n) performance of TreeMap is as good as you'll get in this case.
LinkedHashMap may be what you're looking for.
http://download.oracle.com/javase/1.4.2/docs/api/java/util/LinkedHashMap.html
I've been using HashMaps since I started programming again in Java without really understanding these Collections thing.
Honestly I am not really sure if using HashMaps all the way would be best for me or for production code. Up until now it didn't matter to me as long as I was able to get the data I need the way I called them in PHP (yes, I admit whatever negative thing you are thinking right now) where $this_is_array['this_is_a_string_index'] provides so much convenience to recall an array of variables.
So now, I have been working with java for more than 3 months and came across the Interfaces I specified above and wondered, why are there so many of these things (not to mention, vectors, abstractList {oh well the list goes on...})?
I mean how are they different from each other?
And more importantly, what is the best Interface to use in my case?
The API is pretty clear about the differences and/or relations between them:
Collection
The root interface in the collection hierarchy. A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not. Some are ordered and others unordered.
http://download.oracle.com/javase/6/docs/api/java/util/Collection.html
List
An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.
http://download.oracle.com/javase/6/docs/api/java/util/List.html
Set
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.
http://download.oracle.com/javase/6/docs/api/java/util/Set.html
Map
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
http://download.oracle.com/javase/6/docs/api/java/util/Map.html
Is there anything in particular you find confusing about the above? If so, please edit your original question. Thanks.
A short summary of common java collections:
'Map': A 'Map' is a container that allows to store key=>value pair. This enables fast searches using the key to get to its associated value. There are two implementations of this in the java.util package, 'HashMap' and 'TreeMap'. The former is implemented as a hastable, while the latter is implemented as a balanced binary search tree (thus also having the property of having the keys sorted).
'Set': A 'Set' is a container that holds only unique elements. Inserting the same value multiple times will still result in the 'Set' only holding one instance of it. It also provides fast operations to search, remove, add, merge and compute the intersection of two sets. Like 'Map' it has two implementations, 'HashSet' and 'TreeSet'.
'List': The 'List' interface is implemented by the 'Vector', 'ArrayList' and 'LinkedList' classes. A 'List' is basically a collection of elements that preserve their relative order. You can add/remove elements to it and access individual elements at any given position. Unlike a 'Map', 'List' items are indexed by an int that is their position is the 'List' (the first element being at position 0 and the last at 'List.size()'-1). 'Vector' and 'ArrayList' are implemented using an array while 'LinkedList', as the name implies, uses a linked list. One thing to note is, unlike php's associative arrays (which are more like a Map), an array in Java and many other languages actually represents a contiguous block of memory. The elements in an array are basically laid out side by side on adjacent "slots" so to speak. This gives very fast lookup and write times, much faster than associative arrays which are implemented using more complex data structures. But they can't be indexed by anything other than the numeric positions within the array, unlike associative arrays.
To get a really good idea of what each collection is good for and their performance characteristics I would recommend getting a good idea about data structures like arrays, linked lists, binary search trees, hashtables, as well as stacks and queues. There is really no substitute to learning this if you want to be an effective programmer in any language.
You can also read the Java Collections trail to get you started.
In Brief (and only looking at interfaces):
List - a list of values, something like a "resizable array"
Set - a container that does not allow duplicates
Map - a collection of key/value pairs
A Map vs a List.
In a Map, you have key/value pairs. To access a value you need to know the key. There is a relationship that exists between the key and the value that persists and is not arbitrary. They are related somehow. Example: A persons DNA is unique (the key) and a persons name (the value) or a persons SSN (the key) and a persons name (the value) there is a strong relationship.
In a List, all you have are values (a persons name), and to access it you need to know its position in the list (index) to access it. But there is no permanent relationship between the position of the value in the list and its index, it is arbitrary.
■ List — An ordered collection of elements that allows duplicate entries
Concrete Classes:
ArrayList — Standard resizable list.
LinkedList — Can easily add/remove from beginning or end.
Vector — Older thread-safe version of ArrayList.
Stack — Older last-in, first-out class.
■ Set — Does not allow duplicates
Concrete Classes:
HashSet—Uses hashcode() to find unordered elements.
TreeSet—Sorted and navigable. Does not allow null values.
■ Queue — Orders elements for processing
Concrete Classes:
LinkedList — Can easily add/remove from beginning or end.
ArrayDeque—First-in, first-out or last-in, first-out. Does not allow null values.
■ Map — Maps unique keys to values
Concrete Classes:
HashMap — Uses hashcode() to find keys.
TreeMap — Sorted map. Does not allow null keys.
Hashtable — Older version of hashmap. Does not allow null keys or values.
That is a question that ultimately has a very complex answer--there are entire college classes dedicated to data structures. The short answer is that they all have trade-offs in memory usage and the speed of various operations.
What would be really healthy is some time with a nice book on data structures--I can almost guarantee that your code will improve significantly if you get a nice understanding of data structures.
That said, I can give you some quick, temporary advice from my experience with Java. For most simple internal things, ArrayList is generally preferred. For passing collections of data about, simple arrays are generally used. HashMap is only really used for cases when there is some logical reason to have special keys corresponding to values--I haven't seen anyone use them as a general data structure for everything. Other structures are more complicated and tend to be used in special cases.
As you already know, they are containers for objects. Reading their respective APIs will help you understand their differences.
Since others have described what are their differences about their usage, I will point you to this link which describes complexity of various data structures.
This list is programming language agnostic, and, as always, real world implementations will vary.
It is useful to understand complexity of various operations for each of these structures, since in the real world, it will matter if you're constantly searching for an object in your 1,000,000 element linked list that's not sorted. Performance will not be optimal.
List Vs Set Vs Map
1) Duplicity: List allows duplicate elements. Any number of duplicate elements can be inserted into the list without affecting the same existing values and their indexes.
Set doesn’t allow duplicates. Set and all of the classes which implements Set interface should have unique elements.
Map stored the elements as key & value pair. Map doesn’t allow duplicate keys while it allows duplicate values.
2) Null values: List allows any number of null values.
Set allows single null value at most.
Map can have single null key at most and any number of null values.
3) Order: List and all of its implementation classes maintains the insertion order.
Set doesn’t maintain any order; still few of its classes sort the elements in an order such as LinkedHashSet maintains the elements in insertion order.
Similar to Set Map also doesn’t stores the elements in an order, however few of its classes does the same. For e.g. TreeMap sorts the map in the ascending order of keys and LinkedHashMap sorts the elements in the insertion order, the order in which the elements got added to the LinkedHashMap.enter code here
List Vs Set Vs Map
1) Duplicity: List allows duplicate elements. Any number of duplicate elements can be inserted into the list without affecting the same existing values and their indexes.
Set doesn’t allow duplicates. Set and all of the classes which implements Set interface should have unique elements.
Map stored the elements as key & value pair. Map doesn’t allow duplicate keys while it allows duplicate values.
2) Null values: List allows any number of null values.
Set allows single null value at most.
Map can have single null key at most and any number of null values.
3) Order: List and all of its implementation classes maintains the insertion order.
Set doesn’t maintain any order; still few of its classes sort the elements in an order such as LinkedHashSet maintains the elements in insertion order.
Similar to Set Map also doesn’t stores the elements in an order, however few of its classes does the same. For e.g. TreeMap sorts the map in the ascending order of keys and LinkedHashMap sorts the elements in the insertion order, the order in which the elements got added to the LinkedHashMap.
Difference between Set, List and Map in Java -
Set, List and Map are three important interface of Java collection framework and Difference between Set, List and Map in Java is one of the most frequently asked Java Collection interview question. Some time this question is asked as When to use List, Set and Map in Java. Clearly, interviewer is looking to know that whether you are familiar with fundamentals of Java collection framework or not. In order to decide when to use List, Set or Map , you need to know what are these interfaces and what functionality they provide. List in Java provides ordered and indexed collection which may contain duplicates. Set provides an un-ordered collection of unique objects, i.e. Set doesn't allow duplicates, while Map provides a data structure based on key value pair and hashing. All three List, Set and Map are interfaces in Java and there are many concrete implementation of them are available in Collection API. ArrayList and LinkedList are two most popular used List implementation while LinkedHashSet, TreeSet and HashSet are frequently used Set implementation. In this Java article we will see difference between Map, Set and List in Java and learn when to use List, Set or Map.
Set vs List vs Map in Java
As I said Set, List and Map are interfaces, which defines core contract e.g. a Set contract says that it can not contain duplicates. Based upon our knowledge of List, Set and Map let's compare them on different metrics.
Duplicate Objects
Main difference between List and Set interface in Java is that List allows duplicates while Set doesn't allow duplicates. All implementation of Set honor this contract. Map holds two object per Entry e.g. key and value and It may contain duplicate values but keys are always unique. See here for more difference between List and Set data structure in Java.
Order
Another key difference between List and Set is that List is an ordered collection, List's contract maintains insertion order or element. Set is an unordered collection, you get no guarantee on which order element will be stored. Though some of the Set implementation e.g. LinkedHashSet maintains order. Also SortedSet and SortedMap e.g. TreeSet and TreeMap maintains a sorting order, imposed by using Comparator or Comparable.
Null elements
List allows null elements and you can have many null objects in a List, because it also allowed duplicates. Set just allow one null element as there is no duplicate permitted while in Map you can have null values and at most one null key. worth noting is that Hashtable doesn't allow null key or values but HashMap allows null values and one null keys. This is also the main difference between these two popular implementation of Map interface, aka HashMap vs Hashtable.
Popular implementation
Most popular implementations of List interface in Java are ArrayList, LinkedList and Vector class. ArrayList is more general purpose and provides random access with index, while LinkedList is more suitable for frequently adding and removing elements from List. Vector is synchronized counterpart of ArrayList. On the other hand, most popular implementations of Set interface are HashSet, LinkedHashSet and TreeSet. First one is general purpose Set which is backed by HashMap , see how HashSet works internally in Java for more details. It also doesn't provide any ordering guarantee but LinkedHashSet does provides ordering along with uniqueness offered by Set interface. Third implementation TreeSet is also an implementation of SortedSet interface, hence it keep elements in a sorted order specified by compare() or compareTo() method. Now the last one, most popular implementation of Map interface are HashMap, LinkedHashMap, Hashtable and TreeMap. First one is the non synchronized general purpose Map implementation while Hashtable is its synchronized counterpart, both doesn' provide any ordering guarantee which comes from LinkedHashMap. Just like TreeSet, TreeMap is also a sorted data structure and keeps keys in sorted order.