interview-index.md

Interview Question Index

Every Selenium, TestNG and framework question this lab answers with working code. For each one you get the short answer to give in an interview, the exact class and method that implements it, and the question number in interview-questions.pdf, which has the long explanation and an example.

Levels: 🟢 Beginner · 🟡 Intermediate · 🔴 Advanced · ⭐ Frequently asked

How to use it: find your question, say the short answer out loud, then open the implementation and read the comments. Every class starts with the interview question, the interview answer, the concept and the recommended approach; every line is commented. Trick questions are collected in Common interview traps; common mistakes next to the better code in Avoid vs prefer.

Where the code is:

Short name	Folder	What runs there
interview/<topic>/	src/test/java/com/automation/selenium/interview/	One example class per topic. Selenium topics run on the practice site the-internet.herokuapp.com; TestNG and framework topics need no site
tests/	src/test/java/com/automation/selenium/tests/	Application tests and scenarios on OrangeHRM, built with page objects
pages/	src/main/java/com/automation/selenium/pages/	Page objects for OrangeHRM

Run one topic with its group, e.g. mvn clean test -Dgroups=exceptions or -Dgroups=java.

Contents

1. WebDriver basics
2. Locators
3. Waits and synchronization
4. Selenium exceptions
5. Alerts, frames and windows
6. Drop-downs, checkboxes and web tables
7. Actions, JavaScript, screenshots, cookies and files
8. Page Object Model and framework design
9. TestNG core
10. TestNG data, parallel runs, listeners and retry
11. Reporting, build and run
12. Java coding
13. Not covered by code

---

1. WebDriver basics

#	Question	Level	Interview answer	Implementation	PDF
1	What is the difference between close() and quit()?	🟢 ⭐	close() closes the current window only; quit() closes every window and ends the driver session. Use quit() in teardown, or browsers and driver processes leak.	DriverManager.quit() · interview/windows/MultipleWindowTest (close one window)	Q6
2	What is the difference between findElement and findElements?	🟢 ⭐	findElement returns one element or throws NoSuchElementException; findElements returns a list that is empty when nothing matches. Use the list form to check that something is absent.	interview/exceptions/SeleniumExceptionsTest.noSuchElementException · AppPage.menuItems() · SeleniumScenariosTest.sideMenuListsTheCoreModules()	Q7, Q91
3	What is the difference between get() and navigate()?	🟢	get(url) loads a page and waits for its load event; navigate() also gives back(), forward() and refresh().	BasePage.openPath() (get) · LoginTest.backAfterLogoutStaysLoggedOut() (back + refresh)	Q8
4	How do you run tests headless, and what are browser options for?	🟢	Options configure the browser before it starts: headless mode, window size, flags. Headless needs no screen, so it suits CI; set the window size explicitly so the layout matches a normal run.	DriverManager.chromeOptions(), firefoxOptions(), edgeOptions()	Q9, Q10
5	What is Selenium Manager?	🟢	Since Selenium 4.6, Selenium Manager finds or downloads the right driver binary for the installed browser, so no chromedriver.exe or WebDriverManager is needed.	DriverManager.start(): just new ChromeDriver(options); there are no driver binaries in the repo	Q4

2. Locators

#	Question	Level	Interview answer	Implementation	PDF
6	Which locator strategies are there, and which do you prefer?	🟢 ⭐	id, name, className, tagName, linkText, partialLinkText, CSS and XPath. Prefer stable, meaningful attributes (id, name, data-*), then short CSS, and XPath when you need text or to move up the tree.	interview/locators/LocatorStrategiesTest.everyLocatorStrategyFindsAnElement · LoginPage @FindBy fields	Q12, Q13
7	CSS selector or XPath? What are XPath axes?	🟢 ⭐	CSS is shorter and usually faster but cannot match text or go to a parent; XPath can (normalize-space(), ancestor::, following-sibling::).	LocatorStrategiesTest.xpathAxesNavigateFromALabel · BasePage.inField()	Q14, Q16
8	How do you locate dynamic elements (values known only at run time)?	🟡 ⭐	Anchor on something stable (visible text, a label, a stable class) and build the locator from the value with String.format, instead of hard-coding generated ids.	LocatorStrategiesTest.dynamicLocatorIsBuiltFromAValue · BasePage.button(text), input(label)	Q17
9	What are relative locators (Selenium 4)?	🟡	RelativeLocator.with(By.tagName("input")).below(element): find an element by its position (above, below, toLeftOf, toRightOf, near). Layout-dependent, so a real attribute is still better when there is one.	LocatorStrategiesTest.relativeLocatorsFindFieldsByPosition	Q20
10	How do you find an input whose label is not linked to it?	🟡	Find the label by its text, go up to the container that holds both, then down to the input: //label[.='X']/ancestor::div[…]//input.	BasePage.inField(), input()	Q18

3. Waits and synchronization

#	Question	Level	Interview answer	Implementation	PDF
11	Why avoid Thread.sleep()?	🟢 ⭐	It always waits the full time: too long when the app is fast, too short when it is slow. An explicit wait polls a condition and continues as soon as it is true.	Nowhere in the code: every action in BasePage waits explicitly · interview/synchronization/ExplicitWaitTest	Q21
12	What is an implicit wait? Can you mix it with explicit waits?	🟢 ⭐	implicitlyWait(t) makes every findElement retry for up to t; it is global and only waits for existence. Don't mix it with explicit waits: the timeouts add up. Use explicit waits only.	interview/synchronization/ImplicitWaitTest (without and with, reset afterwards)	Q22
13	Presence or visibility? Which ExpectedConditions do you use?	🟡 ⭐	Presence = in the DOM; visibility = in the DOM and displayed. A hidden element is present at once. Common conditions: visibilityOf…, elementToBeClickable, invisibilityOf…, textToBe, urlContains, alertIsPresent, frameToBeAvailableAndSwitchToIt.	ExplicitWaitTest.presenceIsNotVisibility · BasePage.click(), waitVisible()	Q23, Q24
14	How do you wait until a disabled element becomes usable?	🟢	elementToBeClickable = visible and enabled; wait for it before typing or clicking.	ExplicitWaitTest.waitUntilAnInputIsEnabled	Q23
15	How do you wait for a loading spinner (AJAX)?	🟡 ⭐	Give the spinner a moment to appear, then wait until it is invisible. Waiting only for "invisible" can pass before loading has even started.	ExplicitWaitTest.waitUntilTheLoadingIndicatorDisappears · BasePage.waitForLoader()	Q26
16	What is a FluentWait? How is it different from WebDriverWait?	🟡 ⭐	FluentWait lets you set the timeout, polling interval, ignored exceptions and message. WebDriverWait is a FluentWait<WebDriver> with defaults (500 ms, ignores NotFoundException).	interview/synchronization/FluentWaitTest.fluentWaitPollsUntilTheElementExists	Q22, Q27
17	How do you write a custom wait condition?	🟡	wait.until(driver -> …): return null/false to keep waiting, any other value to stop and use it. Add withMessage(...): a lambda has no readable name in the timeout.	FluentWaitTest.customConditionReturnsTheValueItWaitedFor · EmployeeProfilePage.fullName() · BasePage.tableRows()	Q27

4. Selenium exceptions

All in interview/exceptions/SeleniumExceptionsTest: each test reproduces the exception on purpose, then shows the fix; the Javadoc lists cause, fix and the workaround to avoid.

#	Question	Level	Interview answer	Implementation	PDF
18	NoSuchElementException: causes and handling?	🟢 ⭐	Wrong locator, element not there yet, or in another frame/window. Fix the locator, wait, or switch frame; check absence with findElements(...).isEmpty().	noSuchElementException	Q7, Q85
19	What is StaleElementReferenceException, and how do you handle it?	🟡 ⭐	The element you hold was removed from the DOM (reload, re-render, AJAX). Find it again after the change; wait with stalenessOf(old); keep locators, not elements.	staleElementReferenceException · BasePage (wait.ignoring(Stale…)) · SeleniumScenariosTest.findByFieldSurvivesAReloadButCacheLookupGoesStale	Q25
20	ElementNotInteractableException?	🟡	The element exists but is hidden or has no size. Wait for visibility; point the locator at the visible element. Not a JavaScript click: a user can't click a hidden element either.	elementNotInteractableException	Q24
21	ElementClickInterceptedException?	🟡 ⭐	Another element (overlay, spinner, banner) is on top. Wait for it to disappear; don't switch to a JavaScript click.	elementClickInterceptedException · AddEmployeePage.waitUntilLoaded()	Q86
22	What does TimeoutException tell you?	🟡	The condition was not met in time; the message names the condition and the time tried. Read it before raising timeouts.	timeoutException · BasePage.isVisible()	Q28, Q85
23	NoSuchWindowException / NoSuchFrameException?	🟢	Window: you used the driver after closing its window; switch to an open one. Frame: wrong name, or a nested frame; switch level by level.	noSuchWindowException, noSuchFrameException	Q32, Q33
24	InvalidSelectorException?	🟢	The CSS or XPath itself is malformed (e.g. an unclosed bracket). Try the expression in DevTools first ($x("…"), $$("…")).	invalidSelectorException	Q14
25	NoAlertPresentException / UnhandledAlertException?	🟡	No alert to switch to (wait with alertIsPresent), or the page was used while an alert was open (answer every alert you open).	alertExceptions	Q31

5. Alerts, frames and windows

#	Question	Level	Interview answer	Implementation	PDF
26	How do you handle JavaScript alerts, confirms and prompts?	🟢 ⭐	They are browser dialogs, not page elements: wait.until(alertIsPresent()), then getText(), accept(), dismiss(), or sendKeys() for a prompt.	interview/alerts/AlertHandlingTest	Q31
27	How do you work with frames and iframes?	🟢 ⭐	Switch in by name/id, index or WebElement (frameToBeAvailableAndSwitchToIt waits too); parentFrame() goes one level up, defaultContent() back to the page.	interview/frames/IframeHandlingTest	Q32
28	How do you handle a link that opens a new window?	🟢 ⭐	Selenium doesn't follow it: remember the original handle, wait for the window count to grow, take the new handle by set difference (or by title), switch, work, close(), switch back.	interview/windows/MultipleWindowTest	Q33
29	How do you open a new tab yourself?	🟢	Selenium 4: driver.switchTo().newWindow(WindowType.TAB) opens it and switches to it; tabs share the session cookie.	SeleniumScenariosTest.dashboardOpensInASecondTab()	Q33, Q95

6. Drop-downs, checkboxes and web tables

#	Question	Level	Interview answer	Implementation	PDF
30	How do you handle a <select> drop-down?	🟢 ⭐	new Select(element): selectByVisibleText, selectByValue, selectByIndex, getOptions, getFirstSelectedOption, isMultiple. A disabled option is refused.	interview/dropdowns/SelectDropdownTest	Q29
31	How do you handle a drop-down that is not a <select>?	🟡 ⭐	Select only works on <select>. Click the box to open the list, then click the option by its text.	BasePage.choose(), options() · SeleniumScenariosTest.userRoleDropdownOffersAdminAndEss()	Q30, Q93
32	How do you handle checkboxes and radio buttons?	🟢	Read isSelected() first and click only if the state must change; a click toggles a checkbox. A selected radio button is never unselected by clicking it.	interview/elements/CheckboxTest	Q39
33	How do you read a web table?	🟡 ⭐	Read the headers, then each row's cells, and map them by position into column → value; then any question is plain Java. Prefer stable class names over positions when the cells have them.	interview/elements/WebTableTest · BasePage.tableRows()	Q19
34	How do you find a table row by its content and click something in it?	🟡 ⭐	Find the row by a cell's text, then search inside the row: row.findElement(By.xpath(".//a")). Without the dot, // searches the whole page.	WebTableTest.xpathInsideARowMustStartWithADot · EmployeeListPage.delete()	Q19

7. Actions, JavaScript, screenshots, cookies and files

#	Question	Level	Interview answer	Implementation	PDF
35	When do you use the Actions class?	🟡 ⭐	For low-level input: hover (moveToElement), right-click (contextClick), key combinations (keyDown/keyUp), drag and drop. Nothing happens until perform().	interview/actions/ActionsClassTest · LoginPage.loginWithKeyboard()	Q34, Q94
36	When do you use JavascriptExecutor? What does it return?	🟡 ⭐	For what WebDriver can't do: page state, scrolling. arguments[0…] go in; return values come back as Long, Double, String, Boolean, WebElement, List or null.	interview/javascript/JavaScriptExecutorTest · BasePage.runScript()	Q35, Q96
37	What is the risk of a JavaScript click?	🟡 ⭐	It skips what a user goes through (visibility, overlays), so a test can pass on a page a user cannot use.	JavaScriptExecutorTest.javascriptClickHidesAnOverlayProblem	Q35
38	clear() does not empty a field. Why, and what do you do?	🟡	Frameworks such as Vue or React keep their own model and may not see clear(). Send real keys instead: Ctrl+A, then Delete.	BasePage.type()	Q36
39	How do you take a screenshot of the page and of one element?	🟢 ⭐	Page: cast the driver to TakesScreenshot. Element (Selenium 4): element.getScreenshotAs(…). On failure, take it before cleanup.	BaseTest.attachScreenshot() · LoginPage.formScreenshot() · SeleniumScenariosTest.loginFormScreenshotIsAPng()	Q37, Q97
40	What is the difference between text, attribute and property?	🟡	getText() = visible text; getDomAttribute() = as written in the HTML; getDomProperty() = the live value (typed text, absolute href).	BasePage.valueOf() · LoginPage.placeholder() · AppPage.menuLinks()	Q39
41	How do you find broken links?	🟡 ⭐	Collect every href with Selenium, then request each URL with an HTTP client; Selenium cannot read status codes. 400 or more is broken.	Links · SeleniumScenariosTest.sideMenuHasNoBrokenLinks()	Q92
42	How do you work with cookies? Can you skip the login?	🟡	driver.manage(): getCookies, getCookieNamed, addCookie, deleteCookieNamed, deleteAllCookies (open a page of the site first). Saving the session cookie and adding it to a fresh session skips the login form.	interview/cookies/CookieTest	—
43	How do you upload a file?	🟢 ⭐	sendKeys(absolutePath) on the <input type="file">; never click it (OS dialog). On Grid add a LocalFileDetector. No AutoIt or Robot.	interview/upload/FileUploadTest	Q38

8. Page Object Model and framework design

#	Question	Level	Interview answer	Implementation	PDF
44	What is the Page Object Model, and why use it?	🟢 ⭐	One class per page holds its locators and actions; tests only call methods and assert. When the UI changes, you fix one page class instead of many tests.	pages/: the tests in tests/ contain no locators	Q40
45	Should page objects contain assertions?	🟡	Generally no: pages return values and states, tests decide what is correct. Pages may fail fast when they cannot do their job (e.g. login rejected).	LoginPage.isDisplayed() returns a boolean · LoginPage.loginAs() throws on a rejected login	Q41
46	What is PageFactory? Is it the same as POM?	🟡 ⭐	POM is a design pattern; PageFactory is one way to create the elements (@FindBy + initElements). Each field is a lazy proxy that finds the element on every use. @CacheLookup keeps the first element and goes stale after a re-render.	BasePage constructor · LoginPage.cachedUsernameField · SeleniumScenariosTest.findByFieldSurvivesAReloadButCacheLookupGoesStale()	Q42
47	What is method chaining (a fluent page API)?	🟢	Methods return a page object (this or the next page), so steps read like a sentence: openAdmin().filterByRole("Admin").search().	SystemUsersPage · SystemUsersTest.filterByRoleAndStatus()	Q43
48	What goes into a BasePage?	🟡	What every page needs: the driver, the wait, PageFactory set-up, safe actions (click, type, read) and the app's shared widgets (spinner, toast, tables).	BasePage, AppPage	Q44
49	How do you run Selenium tests in parallel safely? Is WebDriver "thread-safe"?	🔴 ⭐	Give every thread its own browser: keep the driver in a ThreadLocal<WebDriver>, never in a static field. WebDriver itself is still not thread-safe; ThreadLocal makes sure no two threads share one.	DriverManager · interview/framework/ThreadLocalDriverTest (two threads, two sessions)	Q45, Q87
50	How do you manage the driver's life cycle?	🔴	Start a fresh browser before each test, quit it after each test (in finally), remove() the ThreadLocal slot, and quit any leftover before starting a new one.	BaseTest.startBrowser() / stopBrowser() · DriverManager.start(), quit() · ThreadLocalDriverTest.quitEmptiesTheThreadsSlot	Q45
51	How do you manage configuration (URL, browser, timeouts)?	🟡	One properties file on the classpath, overridable by -Dkey=value, read through one class; a missing key fails at once with its name.	Config · interview/framework/ConfigOverrideTest	Q46, Q77
52	Where do you keep test data?	🟡	Out of the test logic: data providers for tables of cases, generators for unique data, config for credentials.	TestData (invalidCredentials, newEmployee())	Q47
53	How do you keep tests independent on a shared environment?	🔴 ⭐	Each test creates the data it needs with unique values, never relies on existing records or counts, and deletes what it made, even when it fails.	EmployeeTest.employeeCanBeAddedFoundAndDeleted() + cleanUp() · BaseTest.runCleanUp()	Q48, Q88
54	How do you log test steps?	🟡	Page methods log each user action; the steps are attached to the running test and shown in the report. Mask secrets.	Steps.log() · LoginPage.mask()	Q50

9. TestNG core

#	Question	Level	Interview answer	Implementation	PDF
55	In what order do TestNG annotations run? @BeforeTest or @BeforeMethod?	🟢 ⭐	Suite → Test → Class → Method → @Test → and back out. @BeforeTest runs once per <test> tag in testng.xml; @BeforeMethod before every test method.	interview/testng/AnnotationOrderTest (records and checks the order) · BaseTest	Q52, Q53
56	What does alwaysRun = true do?	🟡 ⭐	Configuration methods don't inherit class groups; under -Dgroups=… a method without alwaysRun is left out (no browser!). alwaysRun also runs teardown after failures.	BaseTest.startBrowser(), stopBrowser() · AnnotationOrderTest	Q54
57	What are groups, and how do you run them?	🟢 ⭐	Labels on tests (groups = "smoke"), selected with -Dgroups=… or <include>/<exclude> in testng.xml. A class-level group applies to every method.	Class-level groups on every test class (e.g. {"interview", "exceptions"}) · excludedGroups in pom.xml	Q55, Q75
58	What do priority and dependsOnMethods do? What if the first test fails?	🟢 ⭐	priority sorts methods before they are scheduled (lower first); it is not a dependency, and with parallel classes there is no order between classes. dependsOnMethods runs a test only if the other passed; otherwise it is skipped, not failed.	TestNgFeaturesTest.loginPageIsShown() → adminLogsInAfterThePageCheck() · ReportDemoTest.skippedBecauseDependencyFailed()	Q56–Q58, Q100
59	What are dependsOnGroups and enabled = false?	🟡	dependsOnGroups waits for a whole group to pass; alwaysRun = true makes a dependency "soft". enabled = false switches a test off: not run, not reported.	interview/testng/DependencyTest	Q58
60	What does invocationCount do?	🟢	Runs the same test N times and reports each run, e.g. to catch a check that only sometimes passes.	TestNgFeaturesTest.wrongPasswordIsRejectedEveryTime() · ParallelExecutionTest	Q56, Q101
61	What does timeOut do?	🟡	Fails the test if it runs longer than the limit: a safety net against hangs, not a replacement for waits.	TestNgFeaturesTest.loginFinishesWithinOneMinute()	Q56, Q104
62	Hard vs soft assertions?	🟢 ⭐	A hard assert stops at the first failure; SoftAssert collects every failure and reports them together at assertAll(). Forget assertAll() and the failures are lost.	TestNgFeaturesTest.softAssertChecksTheLoginPage()	Q59, Q98
63	How do you skip a test at run time?	🟢	Throw SkipException with the reason; the test is reported as skipped, with that message.	ReportDemoTest.skippedWhenPreconditionIsMissing()	Q60, Q73
64	How do you test that an exception is thrown?	🟡	@Test(expectedExceptions = X.class, expectedExceptionsMessageRegExp = "…"), or inside a test assertThrows / expectThrows (which returns the exception to inspect).	TestNgFeaturesTest.wrongPasswordThrowsTheLoginError() · SeleniumExceptionsTest (assertThrows, expectThrows)	Q61, Q99

10. TestNG data, parallel runs, listeners and retry

#	Question	Level	Interview answer	Implementation	PDF
65	How does a @DataProvider work? How do you share one?	🟡 ⭐	It returns Object[][]; TestNG runs the test once per row with the row as parameters. Share it by putting it in another class and naming it with dataProviderClass.	TestData.invalidCredentials() · LoginTest.loginIsRejectedForInvalidCredentials()	Q62, Q63
66	What other forms can a data provider take?	🔴	A lazy Iterator<Object[]>; a provider with a Method parameter that serves several tests; parallel = true to run rows at the same time (the test must then be thread-safe).	interview/testng/DataProviderVariantsTest	Q62
67	@DataProvider or @Parameters?	🟡	@Parameters passes single values from testng.xml (environment-like settings); @DataProvider feeds many rows of test data from code. @Optional gives a default when the XML has no value.	TestNgFeaturesTest.titleMatchesTheSuiteParameter() · testng.xml	Q64, Q103
68	What goes into testng.xml?	🟢	Suites, tests, classes or packages, group include/exclude, parameters, listeners and parallel settings.	testng.xml	Q65
69	How do you run tests in parallel? Why parallel=classes?	🔴 ⭐	Set parallel (methods, classes, tests, instances) and thread-count; on one method, invocationCount + threadPoolSize. classes keeps each class's methods on one thread, so its fields and dependsOnMethods stay safe.	Surefire parallel/threadCount in pom.xml · interview/testng/ParallelExecutionTest (threads forced to overlap; ThreadLocal keeps them apart)	Q66, Q67
70	Which listeners do you know? How do you register one?	🔴	ITestListener, ISuiteListener, IReporter, IRetryAnalyzer, IAnnotationTransformer. Register with @Listeners (applies to the whole suite!), in testng.xml, or through ServiceLoader.	interview/testng/ListenerTest + TestEventsListener · HtmlReportListener via META-INF/services	Q68, Q69
71	How do you retry a failed test? Should you retry every failure?	🔴 ⭐	Implement IRetryAnalyzer and attach it with retryAnalyzer = …. Retry only known environment hiccups; retrying everything hides real defects.	RetryOnce · TestNgFeaturesTest.adminLoginIsRetriedOnceOnFailure()	Q71, Q102
72	Why does a test show as skipped?	🟡	A failed dependency, a SkipException, a failed @Before… configuration method, or a retried attempt (the earlier try is reported as skipped).	ReportDemoTest (both skip types)	Q73

11. Reporting, build and run

#	Question	Level	Interview answer	Implementation	PDF
73	How do you capture a screenshot on failure and show it in the report?	🟡 ⭐	In @AfterMethod, check that the result failed, take the screenshot, store it on the ITestResult as an attribute, and let the report read it from there.	BaseTest.stopBrowser() / attachScreenshot() · HtmlReportListener.renderTest()	Q37, Q82
74	How do you build a custom HTML report? Why would you?	🔴	Implement IReporter; after the run, TestNG hands you every result. Write one self-contained file with what your team needs: steps, errors, screenshots, filters.	HtmlReportListener	Q70, Q81
75	How does Maven run TestNG? How do you run one class, method or group?	🟢	Surefire finds and runs the tests: -Dtest=Class#method, -Dgroups=…. With suiteXmlFiles set, these switches are ignored, which is why this project runs without a suite file.	pom.xml (Surefire)	Q74–Q76
76	What is a Maven profile, and when do you use one?	🟢	A named set of settings switched on with -P<id>, e.g. a different group filter or environment.	demo profile in pom.xml: mvn clean test -Pdemo	Q78

12. Java coding

Common SDET coding questions on strings, arrays, collections and streams, in interview/java/. Each class has the solution, its edge cases and, where one exists, a check against the library way. No browser; the whole group runs in about 2 seconds (mvn clean test -Dgroups=java). Not in the PDF, which covers Selenium and TestNG.

#	Question	Level	Interview answer	Implementation
77	Reverse a string without StringBuilder.reverse().	🟢 ⭐	Copy to a char array and swap from both ends towards the middle, O(n); in real code new StringBuilder(s).reverse(). Avoid result = c + result in a loop (O(n²)).	ReverseStringTest
78	Check whether a string is a palindrome.	🟢 ⭐	Two indexes from both ends, stop at the first mismatch; for phrases skip non-letters and ignore case. Ask whether case and punctuation count.	PalindromeTest
79	Count how often each character occurs.	🟢 ⭐	One pass with map.merge(c, 1, Integer::sum) into a LinkedHashMap (keeps first-appearance order), O(n).	CharacterFrequencyTest
80	Find the duplicate characters in a string.	🟢 ⭐	A seen set: seen.add(c) returns false for a repeat; collect repeats in a LinkedHashSet so each appears once.	DuplicateCharactersTest
81	Find the first non-repeating character.	🟡 ⭐	Count first (LinkedHashMap), then return the first entry with count 1; Optional.empty() when there is none.	FirstNonRepeatingCharacterTest
82	Are two strings anagrams?	🟢 ⭐	Normalise (lower case, no spaces), sort both char arrays, compare; different lengths → no at once. Counting letters is the O(n) alternative.	AnagramTest
83	Reverse the words of a sentence.	🟢	trim().split("\\s+"), then join from the last word back; \\s+ avoids empty words from repeated spaces.	ReverseWordsTest
84	Count the words in a sentence.	🟢	Count where words start (non-space after space), or trim().split("\\s+").length with a blank-input check (it returns 1 for "").	CountWordsTest
85	Remove duplicate characters, keeping the first of each.	🟢	Append a character only when seen.add(c) is true; or text.chars().distinct().	RemoveDuplicateCharactersTest
86	Find the duplicate elements in an array.	🟢 ⭐	seen.add(n) false → duplicate, collected in a LinkedHashSet; O(n) time, O(n) space (sort and compare neighbours: less space, O(n log n)).	FindDuplicateElementsTest
87	Find the second-highest number without sorting.	🟡 ⭐	One pass with highest and second; skip values equal to the highest; empty result when there are fewer than two distinct values.	SecondHighestNumberTest
88	Find the missing number in 1..n.	🟡 ⭐	n(n+1)/2 minus the array's sum, added up in a long (an int overflows for large n); XOR is the overflow-free alternative.	FindMissingNumberTest
89	Find the largest and smallest value in one pass.	🟢	Start both from the first element (not 0: wrong for negatives), update in one loop; reject an empty array. Library: summaryStatistics().	FindMaxMinTest
90	Sort an array without Arrays.sort.	🟢 ⭐	Bubble sort with an early exit when a pass makes no swap, O(n²); say that real code uses Arrays.sort (O(n log n)).	SortArrayTest
91	Generate the Fibonacci series / the n-th number.	🟢 ⭐	A loop keeping the last two values, O(n); plain recursion recomputes values (O(2ⁿ)); long overflows after the 92nd number.	FibonacciTest
92	Check whether a number is prime.	🟢 ⭐	Below 2 no, 2 yes, other evens no; then odd divisors while i * i <= n (√n). Sieve of Eratosthenes for many numbers.	PrimeNumberTest
93	Swap two numbers without a third variable.	🟢	a = a + b; b = a - b; a = a - b; or XOR; works even on int overflow (wrap-around). A temporary variable is clearer in real code.	SwapNumbersTest
94	Find the common elements of two arrays.	🟢	Put one array in a HashSet, keep elements of the other that it contains, in a LinkedHashSet; O(n + m) instead of nested loops.	ArrayIntersectionTest
95	Remove duplicates from a list, keeping the order.	🟢 ⭐	new ArrayList<>(new LinkedHashSet<>(list)) or stream().distinct(); a plain HashSet loses the order.	RemoveDuplicatesFromListTest
96	Count word frequency with streams.	🟡 ⭐	groupingBy(Function.identity(), LinkedHashMap::new, counting()) after lower-casing and splitting on \\W+; counting() returns Long.	StreamFrequencyTest

13. Not covered by code

Topics this lab explains but does not run, and why:

Topic	Why not	Where it is explained
Radio buttons	The practice site has none; they behave like checkboxes except that clicking a selected one does not unselect it	CheckboxTest Javadoc
HTML5 drag and drop	Actions.dragAndDrop has long been unreliable with HTML5 (draggable) drag events in Chrome, so it does not make a dependable example; it works for mouse-based drag (sliders, jQuery UI)	PDF Q34
Table sorting by header click	The practice site's sorting plugin does not load in the browser	WebTableTest
Selenium Grid, Docker, CI	Out of scope for this interview lab; they belong in a production-style project	PDF Q79, Q80