What is the difference between Lists, ArrayLists, Maps, Hashmaps, Collections etc..?

Question

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

Answer 1

The API is pretty clear about the differences and/or relations between them:

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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

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Is there anything in particular you find confusing about the above? If so, please edit your original question. Thanks.

Answer 2

■ 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.

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■ Set — Does not allow duplicates

Concrete Classes:

HashSet—Uses hashcode() to find unordered elements.
TreeSet—Sorted and navigable. Does not allow null values.

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■ 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.

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■ 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.

Answer 3

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.

Answer 4

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.