Cheat sheet

Selenium WebDriver

Waits, locators, stale elements, and framework answers interviewers keep asking.

Waits

Implicit wait: A global polling time for finding elements. Avoid mixing it with explicit waits; the combined timeout becomes hard to predict.; driver.manage().timeouts().implicitlyWait(Duration.ofSeconds(5));
Explicit wait: Waits for one condition on one element. The default choice.; new WebDriverWait(driver, Duration.ofSeconds(10)).until(ExpectedConditions.elementToBeClickable(by));
Fluent wait: Explicit wait with a custom poll interval and ignored exceptions.
Thread.sleep: A fixed delay. Slows every run and still flakes. Say you replace it with a condition.

id / test id: Stable and fast when it is unique. Ask developers for data-testid.
CSS selector: Fast and readable. Prefer over XPath unless you need text or parent traversal.; button[data-testid='submit']
XPath: Powerful for text and axes; brittle when it uses absolute paths or indexes.; //tr[td[text()='Alice']]//button
Link text: Fine for simple links; breaks on copy changes or translation.

StaleElementReferenceException: The element was re-rendered. Re-find it after the action instead of caching it.
ElementClickInterceptedException: Something covers the element (overlay, sticky header). Wait for it to go or scroll into view.
NoSuchElementException: Wrong locator, element not loaded yet, or it is inside an iframe or shadow DOM.

Iframe: Switch in, act, switch back.; driver.switchTo().frame("id"); driver.switchTo().defaultContent();
Window or tab: Store the original handle, loop handles to find the new one.
Alert: switchTo().alert() then accept(), dismiss(), or getText().

Page Object Model: One class per page or component holding locators and actions. Tests read as business steps; a UI change touches one file.
Parallel runs: One driver per thread (ThreadLocal), no shared mutable data, independent tests.
Selenium Grid: Runs tests on many browsers and machines in parallel through a hub and nodes.