RKTechGame | Java Common Collection Classes

ArrayList

ArrayList is a resizable array implementation of the List interface. It stores elements in index-based order and allows duplicates. It provides fast random access.

Key features:

Dynamic resizing as elements are added or removed.
Allows duplicate elements.
Provides constant-time positional access (get and set).
Slower insertions and removals in the middle due to array shifting.

Example:

ArrayList<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Cherry");
System.out.println(list.get(1)); // Output: Banana

LinkedList

LinkedList implements the List and Deque interfaces. It uses a doubly-linked list under the hood. It allows duplicate elements and insertion order is maintained.

Key features:

Faster insertions and removals compared to ArrayList, especially in the middle of the list.
Slower random access compared to ArrayList as it requires traversal.
Can be used as list, stack, or queue.

Example:

LinkedList<String> ll = new LinkedList<>();
ll.add("Dog");
ll.add("Cat");
ll.add("Bird");
System.out.println(ll.remove()); // Removes first element: Dog

HashSet

HashSet implements the Set interface backed by a hash table. It stores unique elements and does not maintain any order.

Key features:

No duplicate elements allowed.
Does not guarantee order of elements.
Provides constant-time performance for add, remove, contains operations.

Example:

HashSet<String> hs = new HashSet<>();
hs.add("Java");
hs.add("Python");
hs.add("Java"); // Duplicate ignored
System.out.println(hs); // Output: [Java, Python]

HashMap

HashMap implements the Map interface and stores key-value pairs. Keys are unique but values can be duplicated.

Key features:

Provides constant-time performance for basic operations (get and put).
Does not guarantee any order of keys or values.
Allows one null key and multiple null values.

Example:

HashMap<Integer, String> map = new HashMap<>();
map.put(1, "Red");
map.put(2, "Green");
map.put(3, "Blue");
System.out.println(map.get(2)); // Output: Green