| Class | Data access particulars | Operations | Thread-safe version | Notes | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Individual | Key-value | Duplicate- element | Order iteration | Random access | |||||||||
| FIFO | Sorted | LIFO | By key | By value | By index | ||||||||
| HashMap | ✓ | ✓ | ConcurrentHashMap | ||||||||||
| LinkedHashMap | ✓ | ✓ | ✓ | Collections.synchronizedMap | Retains insertion order. | ||||||||
| TreeMap | ✓ | ✓ | ✓ | ConcurrentSkipListMap | Ordered via Comparable (or Comparator). | ||||||||
| HashSet | ✓ | ✓ | ConcurrentHashMap< Key, Key > | ||||||||||
| LinkedHashSet | ✓ | ✓ | ✓ | ConcurrentHashMap< Key, Key > | Retains insertion order. | ||||||||
| ArrayList | ✓ | ✓ | ✓ | ✓ | ✓ | CopyOnWriteArrayList | Retains insertion order. | ||||||
| PriorityQueue | ✓ | ✓ | ✓ | PriorityBlockingQueue | |||||||||
| LinkedList | ✓ | ✓ | ✓ | ✓ | ✓ | (not thread-safe) | |||||||
| ArrayDeque | ✓ | ✓ | ✓ | ✓ | ArrayBlockingQueue | ||||||||
Some notes on the table above:
Maps have key-value pairs whereas Lists and Queues have individual values. Some lists commonly contain multiple, identical elements (a list of a box of crayons might justifiably contain two blues).
Some sorted collections will behave as first-in, first-out, others last-in, last-out, and still others are just sorted.
Maps are typically accessed "by key" (why they can't contain duplicates), but LinkedList is accessed by index (position of the element in the list).
There are other, third-party maps and lists not illustrated here.