9 Essential Selenium Interview Questions*

What functions/methods can you use to wait for a page to load, or some element on the page to show up?

To wait for a page to load, explicit wait can be used. Explicit waits stall until some specific condition is met. Although Thread.Sleep() is a form of explicit wait where the thread stalls for a specific duration of time, it may not be the most reliable way to wait for a page to load.

For example, rather than using something like:

Thread.Sleep(30000);

… where the thread sleeps for 30 seconds (30,000 milliseconds), the following can be used:

WebDriverWait wait= new WebDriverWait(webDriver,30);
wait.until(ExpectedConditions.visibilityOf(h1));

… where Selenium waits until the given element is visible on the page, or throws TimeoutException after waiting for 30 seconds.

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What is Page Object Model and Page Factory in Selenium?

Page Object Model in Selenium is a design pattern where web pages are represented using classes. Variables in the class can then be used to hold references to elements on the web page, and methods can be used to implement behaviors. This allows an elegant way of implementing test routines that are both readable and easier to maintain and extend in the future.

clickLoginButton();
setCredentials(user_name, user_password);
submitLoginForm();

Page Factory is used to initialize every WebElement variable with a reference to a corresponding element on the actual web page using configured “locators”. Annotations, such as @FindBy, can be used to define strategies for looking up elements, along with the necessary information for identifying them:

@FindBy(how=How.NAME, using="username")
private WebElement user_name;

More about Page Object Model and Page Factory can be found here.

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How do you deal with frame elements in Selenium on a page?

In order to manipulate frame and its content you must switch to it first. This is similar to how you have to switch to a different page before you can interact with it:

driver.switchTo().frame(index);

… where index is the zero-based index of the frame. Switching the frame directs all further interactions through the driver towards the selected frame. The frame method also works with name, element ID and reference to already located elements.

To switch back to the default frame, the defaultContent method can be used:

driver.switchTo().defaultContent();
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What are some differences between CSS selector and XPath based locators?

  • CSS selectors are often easier to read than XPath. Most front-end developers are more likely to be familiar with CSS selectors already.

  • Support for CSS selectors is quite consistent across various modern web browsers, which is not the case with their XPath engines.

  • CSS selectors work faster than XPath.

For various reasons, such as the ones outlined above, Selenium “best practices” advise the use of CSS selectors over XPath.

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How do you write a locator to identify paragraph elements that are the immediate child of a div element, or the descendent of a div element?

Immediate child:

An immediate child in XPath is defined indicated using “/”, while on CSS, it is indicated using “>”. For example, with XPath:

//div/p

… and with CSS:

div > p

Descendent:

To find paragraph elements that are descendent to any div element (i.e. the paragraph element appears in the subtree rooted at the div element), we can use “//” in XPath, and just a whitespace in CSS:

//div//p
div p
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Briefly explain what the following snippet of Java code does:

WebElement sample = driver.findElement(By.xpath("//*[contains(text(), 'data')]"));

It defines a variable sample of type WebElement, and uses an XPath search to initialize it with a reference to an element that contains the text value “data”.

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How can you send text input to a focused element?

This can be done by simulating key presses on the focused element. One way is to perform “actions” on the web driver object:

new Actions(webDriver).sendKeys(“some text”).perform();

An alternative way is to switch to the active element first, and send keys to it directly:

webDriver.switchTo().activeElement().sendKeys(“some text”);
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Why do we use headless drivers? How can you visually investigate test failure when using headless drivers?

Headless drivers are typically used in continuous integration (CI) setups. Headless drivers, such as PhantomJS, provide all standard web browser functionalities, but run in the command-line. These drivers are based on command-line tools and don’t produce screen output, making them ideal for completely automated setups.

To be able to visually investigate test failures, the developer needs to implement mechanisms to capture screenshots, otherwise rely on command line output.

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How are absolute XPaths different from relative XPaths? Why are relative XPaths typically preferred over absolute XPaths in automated tests?

Absolute XPaths, in terms of web pages, start with the root element:

html/head/body/table/tbody/tr/th

Relative XPaths, on the other hand, usually start with “//”:

//table/tbody/tr/th 

Even though both these XPaths probably refer to the same element on a certain web page, the former one is more likely to break with any change made to the page. For example, moving the table to inside of a div element will stop the absolute XPath from being able to locate the th element. On the other hand, the relative XPath will still continue to work.

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* There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work.
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Stratos Nikolaidis
Malta
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