Java Practice Lab

Collections · 312 words · 2 minute read

Choosing Java Collections: List, Set, Map, Queue, and Deque

By · Published 2026-09-04 · Updated 2026-09-04

The best Java collection is chosen by the questions your program needs to answer. Need ordered values with duplicates? Start with a List. Need membership or uniqueness? Use a Set. Need to associate a key with a value? Use a Map. Queue and Deque describe access order rather than a particular implementation, so code against those interfaces when possible.

Match the operation to the structure

ArrayList is an excellent default for indexed reads and append-heavy lists. HashSet gives expected constant-time membership but no meaningful iteration order. LinkedHashSet adds insertion order. TreeMap and TreeSet keep keys sorted at logarithmic cost. A HashMap is ideal for frequency counts and complement lookup.

import java.util.LinkedHashSet;
import java.util.Set;
public class Main {
    public static void main(String[] args) {
        Set<Integer> unique = new LinkedHashSet<>();
        unique.add(3); unique.add(1); unique.add(3); unique.add(2);
        System.out.println(unique);
    }
}

The output is [3, 1, 2]: the duplicate is removed and the first-occurrence order remains. Replacing this with HashSet would keep uniqueness but not promise that display order.

Complexity is a guide, not a substitute for contracts

Hash-based expected O(1) operations rely on correct equals and hashCode for custom keys. Trees require comparable keys or a Comparator. An ArrayList removal from the middle shifts later elements, even though its indexed read is fast. State whether output order, duplicate multiplicity, and null values matter before committing to a type.

Common mistakes

  • Depending on HashMap iteration order for user-visible output.
  • Calling Arrays.asList and expecting a growable list.
  • Changing a collection during for-each iteration instead of using an iterator.
  • Using a Set when the number of duplicate occurrences matters.

Practice next

Use remove duplicates from a list to compare HashSet and LinkedHashSet. Then solve word frequency counter, list intersection, and sliding window maximum. The collections compiler page lets you swap implementations and observe which guarantees change.

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