List vs Set vs Map in Java (2026): Differences

List vs Set vs Map in Java

Quick Answer

• List — an ordered collection that allows duplicates, accessed by index. "Many values."
• Set — a collection of unique values; duplicates are ignored. "Unique values."
• Map — key–value pairs with unique keys, accessed by key. "Look it up by name."

List and Set extend the Collection interface; Map is a separate interface — it isn't a Collection, because it stores pairs rather than single elements.

Comparison at a Glance

List — Ordered, Duplicates Allowed

List<String> browsers = new ArrayList<>(
        List.of("chrome", "firefox", "chrome")
);

System.out.println(
        browsers +
        " size=" + browsers.size() +
        " first=" + browsers.get(0)
);

// [chrome, firefox, chrome] size=3 first=chrome

• ArrayList — fast access by index; the default choice.
• LinkedList — cheaper insertions and removals at the ends; slower random access.

In Selenium: driver.findElements() returns a List<WebElement> because a page can have several matching elements, in document order:

List<WebElement> rows =
        driver.findElements(
                By.cssSelector("table#orders tbody tr")
        );

Assert.assertEquals(rows.size(), 5);

Set — Unique Values Only

Set<String> unique = new LinkedHashSet<>(browsers);

System.out.println(
        unique + " add chrome again? " + unique.add("chrome")
);

// [chrome, firefox] add chrome again? false

Set<String> sorted =
        new TreeSet<>(List.of("qa", "dev", "prod"));

System.out.println(sorted);

// [dev, prod, qa]

In Selenium: driver.getWindowHandles() returns a Set<String> because every window handle is unique:

String parent = driver.getWindowHandle();

for (String handle : driver.getWindowHandles()) {
    if (!handle.equals(parent)) {
        driver.switchTo().window(handle);
    }
}

add() returning false for a duplicate is also handy for checks like "are all product names on this page unique?"

Map — Key–Value Pairs

Map<String, String> config = new LinkedHashMap<>();

config.put("browser", "chrome");
config.put("env", "qa");

String old = config.put("browser", "firefox");
// Same key → value replaced

System.out.println(
        config +
        " replaced=" + old +
        " missing=" + config.getOrDefault("timeout", "10")
);

// {browser=firefox, env=qa} replaced=chrome missing=10

for (Map.Entry<String, String> e : config.entrySet()) {
    System.out.println(e.getKey() + "=" + e.getValue());
}

A classic interview program — character frequency — is a Map job:

Map<Character, Integer> freq = new TreeMap<>();

for (char c : "banana".toCharArray()) {
    freq.merge(c, 1, Integer::sum);
}

System.out.println(freq);

// {a=3, b=1, n=2}

The Data-Driven Pattern: List<Map<String, String>>

Each Map is one test record (field → value) and the List holds all the records — a structure you'll see in almost every data-driven framework, whether the data comes from Excel, JSON or a database:

List<Map<String, String>> testData = List.of(
        Map.of(
                "username", "asha",
                "password", "Secret@1"
        ),
        Map.of(
                "username", "ravi",
                "password", "Secret@2"
        )
);

for (Map<String, String> row : testData) {
    loginPage.login(
            row.get("username"),
            row.get("password")
    );
}

More: Data-Driven Testing.

Nulls and Performance

• Nulls: HashSet allows one null and HashMap allows one null key; TreeSet and TreeMap throw NullPointerException for null, because they must compare elements to sort them (verified in the demo).
• Performance: HashSet and HashMap give roughly constant-time add, remove and lookup; the Tree versions are logarithmic (they keep things sorted); ArrayList.get(index) is constant-time, but ArrayList.contains() scans the whole list.
• Rule of thumb: if you often ask "is this value in the collection?", use a Set, not a List.

Step through these examples line by line in the Java Code Visualizer to see how each collection changes.

Which Collection Should You Use?

📚 Official documentation: Oracle Java Tutorials: Collections · dev.java: Learn Java (Oracle)

Frequently Asked Questions

What is the difference between List, Set and Map?

A List keeps order and allows duplicates; a Set stores only unique values; a Map stores key–value pairs with unique keys.

Is Map part of the Collection interface?

No. List and Set extend Collection; Map is a separate interface in the Collections Framework because it stores pairs.

Why does Selenium return a Set for window handles?

Window handles are always unique, so a Set is the natural type — getWindowHandles() returns Set<String>.

Why does findElements() return a List?

A page can contain several matching elements, in a meaningful order, so a List is used — and it's empty (not an exception) when nothing matches.

What is the difference between HashSet and HashMap?

A HashSet stores unique values; a HashMap stores key–value pairs with unique keys. Internally, HashSet is backed by a HashMap.

HashMap vs LinkedHashMap vs TreeMap?

HashMap has no guaranteed order; LinkedHashMap keeps insertion order; TreeMap keeps keys sorted.

What is List<Map<String, String>> used for?

Data-driven testing: each Map is one test record and the List holds all the records.

Related

• Java Collections: List, Set & HashMap
• Java Collections in Your Automation Framework
• The Complete Java for Testers Guide
• Java Interview Questions for Testers