Summary - Generic collections

Combine List, Map and Set to write a word frequency counter.

Summary - Generic collections

This group covered four core collection interfaces: List ordered, Map key-value, Set unique, Queue/Deque FIFO and double-ended. Each has concrete implementations: ArrayList, HashMap, HashSet, ArrayDeque.

The Collections class provides helpers like sort, reverse, max and min. Type parameters (<E>, <K,V>) give compile-time type safety; adding the wrong type is a compile error.

Exercise Build a score board

Combine three collections. Declare List<Integer> scores = new ArrayList<>();, add 70, 85, 70, 90 (70 is duplicated). Build Set<Integer> unique = new HashSet<>(scores); to drop duplicates. Print the list size with System.out.println(scores.size()); and the unique count with System.out.println(unique.size());.

Show example solution

import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        List<Integer> scores = new ArrayList<>();
        scores.add(70);
        scores.add(85);
        scores.add(70);
        scores.add(90);
        Set<Integer> unique = new HashSet<>(scores);
        System.out.println(scores.size());
        System.out.println(unique.size());
    }
}

Test yourself

Which collection rejects duplicate elements?
- List
- ArrayList
- Set
- Queue
Show answer

Set — Set holds unique elements; a duplicate add returns false.
Match interface to implementation

List

ArrayList
Map

HashMap
Set

HashSet
Queue

ArrayDeque
Match collection traits

List

Ordered, allows duplicates
Map

Key-value pairs
Set

Unique elements
Queue

FIFO order

Next lesson: Iterator

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies