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Technology

Core Java

Difficulty

Intermediate

Experience

1-2 Years

Type

Technical

AutomateQA Team

QA Automation Engineers

June 20, 2026

Interview Question

What is the difference between HashMap, LinkedHashMap, and TreeMap in Java?

Q: What is the difference between HashMap, LinkedHashMap, and TreeMap in Java?

HashMap vs LinkedHashMap vs TreeMap All implement the Map interface but differ in ordering and performance. HashMap — No Order Guaranteed java Map<String, Integer hashMap = new HashMap<(); hashMap.put("Banana", 2); hashMap.put("Apple", 1); hashMap.put("Cherry", 3); hashMap.put("Date", 4); // Iteration order is NOT predictable hashMap.forEach((k, v) - System.out.println(k + "=" + v)); // Could print in any order: Cherry=3, Date=4, Apple=1, Banana=2 Performance: O(1) average for get/put. LinkedHashMap — Insertion Order java Map<String, Integer linkedMap = new LinkedHashMap<(); linkedMap.put("Ban

Answer

HashMap vs LinkedHashMap vs TreeMap

All implement the Map interface but differ in ordering and performance.

HashMap — No Order Guaranteed

Java

Map<String, Integer> hashMap = new HashMap<>();
hashMap.put("Banana", 2);
hashMap.put("Apple", 1);
hashMap.put("Cherry", 3);
hashMap.put("Date", 4);

// Iteration order is NOT predictable
hashMap.forEach((k, v) -> System.out.println(k + "=" + v));
// Could print in any order: Cherry=3, Date=4, Apple=1, Banana=2

Performance: O(1) average for get/put.

LinkedHashMap — Insertion Order

Java

Map<String, Integer> linkedMap = new LinkedHashMap<>();
linkedMap.put("Banana", 2);
linkedMap.put("Apple", 1);
linkedMap.put("Cherry", 3);
linkedMap.put("Date", 4);

// Always prints in insertion order
linkedMap.forEach((k, v) -> System.out.println(k + "=" + v));
// Banana=2, Apple=1, Cherry=3, Date=4

Also supports access-order (LRU cache):

Java

// true = access-order (most-recently-accessed last)
Map<String, Integer> lruCache = new LinkedHashMap<>(16, 0.75f, true) {
    @Override
    protected boolean removeEldestEntry(Map.Entry<String, Integer> eldest) {
        return size() > 3;  // max 3 entries — evict oldest
    }
};

TreeMap — Sorted by Key

Java

Map<String, Integer> treeMap = new TreeMap<>();
treeMap.put("Banana", 2);
treeMap.put("Apple", 1);
treeMap.put("Cherry", 3);
treeMap.put("Date", 4);

// Always prints in natural (alphabetical) key order
treeMap.forEach((k, v) -> System.out.println(k + "=" + v));
// Apple=1, Banana=2, Cherry=3, Date=4

// Extra methods
System.out.println(treeMap.firstKey()); // "Apple"
System.out.println(treeMap.lastKey());  // "Date"
System.out.println(treeMap.headMap("Cherry")); // {Apple=1, Banana=2}
System.out.println(treeMap.tailMap("Cherry")); // {Cherry=3, Date=4}

Custom key order:

Java

Map<String, Integer> reverseMap = new TreeMap<>(Comparator.reverseOrder());
reverseMap.put("Banana", 2);
reverseMap.put("Apple", 1);
// Date=4, Cherry=3, Banana=2, Apple=1

Comparison Table

Feature	HashMap	LinkedHashMap	TreeMap
Order	None	Insertion order	Sorted (key)
Null key	✅ 1 null	✅ 1 null	❌ No null key
Performance	O(1)	O(1)	O(log n)
Thread-safe	❌	❌	❌
Implements	Map	Map	NavigableMap
Use when	Just key-value	Need insertion order	Need sorted keys

In Automation Testing

Java

// HashMap — store test data (order doesn't matter)
Map<String, String> testData = new HashMap<>();
testData.put("username", "alice");
testData.put("password", "pass123");

// LinkedHashMap — maintain form field order for ordered input
Map<String, String> formData = new LinkedHashMap<>();
formData.put("firstName", "John");
formData.put("lastName", "Doe");
formData.put("email", "john@example.com");
// Fill form fields in order
formData.forEach((field, value) ->
    driver.findElement(By.name(field)).sendKeys(value));

// TreeMap — sorted test parameter names for reports
Map<String, String> sortedParams = new TreeMap<>(testParameters);
sortedParams.forEach((k, v) -> report.addParameter(k, v));