1Z0-811 Java Basics and Syntax Practice Question
You are developing a Java application for a library management system. The system must track the number of books in each genre. You need to store genre names (String) and their counts (int). The data will be accessed frequently and modified rarely. Which Java data structure should you use to store this mapping efficiently, while ensuring that genre names are unique?
⚠ Common exam trap
Oracle often tests the distinction between collections that store single elements (like HashSet) versus those that store key-value pairs (like HashMap), leading candidates to choose HashSet when a mapping is required.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
HashMap<String, Integer>
HashMap<String, Integer> is the correct choice because it stores key-value pairs, where each genre name (String) is a unique key mapped to its count (Integer). This provides O(1) average-time complexity for lookups and updates, which is ideal for frequent access and rare modifications. The HashMap ensures key uniqueness via its hash-based implementation, directly meeting the requirement of unique genre names.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ArrayList<Book>
Why it's wrong here
Not a mapping structure; duplicates allowed.
- ✗
LinkedList<String>
Why it's wrong here
No mapping, inefficient search.
- ✗
HashSet<String>
Why it's wrong here
Only keys, no count.
- ✓
HashMap<String, Integer>
Why this is correct
Maps unique genre to count.
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