Java Collections Framework: List, Set, Map

The Java Collections Framework (JCF) provides ready-made data structures and utility classes to store, manipulate, and process groups of objects efficiently.

The three most commonly used collection types are:

List → Ordered collection (duplicates allowed)

Set → Unordered collection (duplicates NOT allowed)

Map → Key–value pairs (keys unique)

1. List Interface

A List is an ordered collection where elements are stored by index.

Duplicates are allowed.

Common Implementations

ArrayList (most used)

LinkedList

Vector (legacy)

ArrayList Example

List<String> names = new ArrayList<>();

names.add("John");

names.add("Alice");

names.add("John");  // duplicates allowed

System.out.println(names); // [John, Alice, John]

Accessing elements

String first = names.get(0); // John

Looping through a list

for (String name : names) {

    System.out.println(name);

}

When to Use List

You need ordered data

You want to access items by index

You want to store duplicates

Useful for Selenium findElements() results

Example in Selenium:

List<WebElement> links = driver.findElements(By.tagName("a"));

2. Set Interface

A Set is a collection where duplicates are NOT allowed.

Order is not guaranteed (HashSet) or sorted (TreeSet).

Common Implementations

HashSet → no order

LinkedHashSet → insertion order

TreeSet → sorted order

HashSet Example

Set<String> cities = new HashSet<>();

cities.add("Delhi");

cities.add("Mumbai");

cities.add("Delhi");  // duplicate ignored

System.out.println(cities); // [Delhi, Mumbai] (order may vary)

Checking size

System.out.println(cities.size());

When to Use Set

You need unique values only

Order does NOT matter (or only sorted order needed)

Good for removing duplicates from a list

Example in Selenium:

Set<String> uniqueTexts = new HashSet<>(textsFromWebElements);

3. Map Interface

A Map stores data as key–value pairs.

Keys are unique, values can be duplicates.

Common Implementations

HashMap → fastest, no order

LinkedHashMap → insertion order

TreeMap → sorted by key

HashMap Example

Map<String, Integer> scores = new HashMap<>();

scores.put("John", 90);

scores.put("Alice", 95);

scores.put("John", 85); // Key 'John' overwritten

System.out.println(scores);  // {John=85, Alice=95}

Accessing values

int johnScore = scores.get("John");

Looping through a Map

for (Map.Entry<String, Integer> entry : scores.entrySet()) {

    System.out.println(entry.getKey() + " : " + entry.getValue());

}

When to Use Map

You need to store paired values (key → value)

You need fast lookups by key

Useful for:

storing configuration

mapping table headers to row values in Selenium

API testing key-value results

Example in Selenium:

Map<String, String> userData = new HashMap<>();

userData.put("Username", "Admin");

userData.put("Role", "Manager");

Comparison Table

Feature List Set Map

Stores Elements Unique elements Key–value pairs

Order Yes No (unless LinkedHashSet/TreeSet) Depends on implementation

Allows duplicates Yes No Keys: No, Values: Yes

Access by index Yes No Access by key

When to Use What?

Use List when:

✔ You need ordered data

✔ Duplicates are allowed

✔ Access by index is needed

Use Set when:

✔ You need unique values

✔ No duplicates allowed

Use Map when:

✔ You want to store data as key → value

✔ Need fast access by key

Real Selenium Example Using All Three

List<WebElement> elements = driver.findElements(By.tagName("a"));

List<String> linkTexts = new ArrayList<>();

for (WebElement e : elements) {

    linkTexts.add(e.getText());

}

Set<String> uniqueLinks = new HashSet<>(linkTexts);

Map<String, String> linkMap = new HashMap<>();

for (WebElement e : elements) {

    linkMap.put(e.getText(), e.getAttribute("href"));

}

Summary

List → ordered, duplicates allowed

Set → unique elements only

Map → key–value pairs

These structures make your code more efficient and readable

Especially useful for Selenium: storing elements, filtering, mapping, comparing values

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November 26, 2025

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