Java Collections Framework is a set of interfaces + implementations.

• Interfaces define behavior
• Classes define implementation
• Code to interfaces, not implementations

Iterable
  └── Collection
        ├── List
        ├── Set
        └── Queue
	    // Array is not collection (does not implement Collection)

Map (separate hierarchy)

// code:
List<String> list = new ArrayList<>();

Array vs Collection

Array

• Fixed size
• Fast access (index-based)
  • Cache friendly
• Memory efficient
  • Contiguous memory
• No rich API

  • No dynamic resizing

    No built-in algorithms (like add, …)

int[] arr = new int[5];

Collection

• Dynamic size
• Rich API
• Generics support