Collections in Java

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Collections in Java

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обновлено: 30 September 2026

Collections are lists, queues, dictionaries, and other classes for adding, removing, and sorting items.

All collections are defined in the java.util.* package

When creating a collection, the <E> type is used, which means that all elements are of the same type.
Since the collection uses the <E> type when it is created, so the collection is called generic (or in other words, typed).

Java Collections

Java Collections based on interface List<E> (This is an array, i.e. add and remove elements, get an element by index)

Java

class ArrayList<E>

class ArrayList<E> is an array (in other words, a collection of elements).
Any element can be gotten by integer index.
You can add and delete elements.
Is not synchronized (not thread-safe).
Read more class ArrayList<E> ...

synchronized for threads
❌

Java

class Vector<E>

class Vector<E> is an array (in other words, a collection of elements).
Any element can be gotten by integer index.
You can add and delete elements.
Vector is fully synchronized (thread-safe).
Note! Vector is the same as ArrayList, and the difference is that Vector is fully synchronized (thread-safe). Vector is slower than ArrayList.
Read more class Vector<E> ...

synchronized for threads
✅

Java

class Stack<E>

class Stack<E> extends class Vector<E> with the "last in - first out" algorithm:
Each new element is placed at the very top.
The first element is the one at the very top.
Stack is fully synchronized (thread-safe).
Read more class Stack<E> ...

synchronized for threads
✅

Java

class CopyOnWriteArrayList<E>

CopyOnWriteArrayList it is the same as ArrayList but safe in threads.
In CopyOnWriteArrayList all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.
Read more class CopyOnWriteArrayList<E> ...

synchronized for threads
✅

Java Collections based on interface Queue<E> (queue, i.e. adding, deleting elements according to the algorithm: first in - first out)

Java
class PriorityQueue<E>

PriorityQueue oracle.com ...

Java Collections based on interface Deque<E> (two-way queue i.e. adding, removing elements from the beginning and end)

Java

class LinkedList<E>

class LinkedList is a set of Nodes. Each Node has 2 links: a link to the next Node and a link to the previous Node.
In LinkedList can add and remove elements.
LinkedList is not synchronized.
Read more class LinkedList<E> ...

synchronized for threads
❌

Java
class ArrayDeque<E>

ArrayDeque oracle.com ...

Java
class LinkedBlockingDeque<E>

LinkedBlockingDeque oracle.com ...

Java
class ConcurrentLinkedDeque<E>

ConcurrentLinkedDeque oracle.com ...

Java Collections based on interface Set<E> (collection of unique elements)

Java

class HashSet<E>

HashSet contains a non-repeating collection of items.
HashSet contains unsorted items.
HashSet allows you to quickly determine whether such an element exists or not (fast because the hash code of the element is calculated).
Read more class HashSet<E> ...

synchronized for threads
❌

Java
class LinkedHashSet<E>

LinkedHashSet implementation the same as HashSet and additionally maintains a doubly-linked list running through all of its elements. This linked list makes guarantees order of items in the set.
class LinkedHashSet<E> is not synchronized.

Java Collections based on interface SortedSet<E> (Collection of unique elements. Items are stored in a sorted view)

Java
class TreeSet<E>

TreeSet contains a non-repeating collection of items.
TreeSet Items are stored in a sorted form.[/rus]
TreeSet oracle.com ...

Java
class ConcurrentSkipListSet<E>

ConcurrentSkipListSet oracle.com ...

Java Collections based on interface Map<K, V> (contains keys and values. Each key is associated with only one value, i.e. from the key we get the value )

Java
class HashMap<K,V>

HashMap oracle.com ...

Java
class Hashtable<K,V>

Hashtable oracle.com ...

There are several differences between Hashtable and HashMap in Java:

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.

One of HashMap subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Java
class LinkedHashMap<K,V>

LinkedHashMap oracle.com ...

Java
class WeakHashMap<K,V>

WeakHashMap oracle.com ...

Java
class Properties<K,V>

Properties oracle.com ...

Java
class Provider<K,V>

Provider oracle.com ...

Java
class EnumMap<K,V>

EnumMap oracle.com ...

Java
class IdentityHashMap<K,V>

IdentityHashMap oracle.com ...

Java Collections based on interface SortedMap<E> (contains keys and values. Each key is associated with only one value, i.e. we get the value from the key. Keys are stored in a sorted form.)

Java
class TreeMap<K,V>

TreeMap oracle.com ...

Java
class ConcurrentSkipListMap<K,V>

ConcurrentSkipListMap oracle.com ...

Java Collections based on interface ConcurrentMap<K, V> (contains keys and values. Each key is associated with only one value, i.e. we get the value from the key. Functions in ConcurrentMap are thread-safe and support fast multi-thread reads and writes.)

Java
class ConcurrentHashMap<K,V>

It is used when several threads can read and update data at the same time, while the card remains in the correct state. This is especially useful for storing intermediate or computed values that are frequently accessed by multiple threads.
ConcurrentHashMap oracle.com ...

Java
class ConcurrentSkipListMap<K,V>

ConcurrentSkipListMap oracle.com ...

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