47Java for SDET

47.1 Why Java for SDET

Java is the lingua franca of enterprise test automation. Selenium, REST Assured, TestNG/JUnit, Cucumber-JVM, Allure — all assume a JVM. Understanding the language well separates someone who copies WebDriver snippets from someone who designs maintainable frameworks.

47.2 The four OOP pillars

Encapsulation

public class LoginPage {
  private final WebDriver driver;
  private final By userField = By.id("user");
  public LoginPage(WebDriver driver) { this.driver = driver; }
  public DashboardPage login(String u, String p) {
    driver.findElement(userField).sendKeys(u);
    /* ... */
    return new DashboardPage(driver);
  }
}

Inheritance

public abstract class BasePage {
  protected final WebDriver driver;
  protected BasePage(WebDriver driver) { this.driver = driver; }
  protected void waitVisible(By l) { /* shared */ }
}
public class LoginPage extends BasePage {
  public LoginPage(WebDriver d) { super(d); }
}

Polymorphism — overloading vs overriding

void click(By l) { ... }
void click(WebElement el) { ... }
void click(By l, int t) { ... }

class BaseTest { void setup() {} }
class ApiTest extends BaseTest {
  @Override void setup() { /* replaces */ }
}

Abstraction — abstract class vs interface

47.3 Access modifiers

47.4 Collections

HashMap internals (must know)

• Array of buckets; key's hashCode() indexes into the array
• Collisions chain as linked list; Java 8+ converts to tree when bucket length > 8 (TREEIFY_THRESHOLD)
• Load factor 0.75 — when size exceeds capacity×0.75, capacity doubles and rehash happens
• equals() + hashCode() contract: equal objects MUST have equal hash codes

47.5 Generics

public class Stack<T> {
  private final List<T> data = new ArrayList<>();
  public void push(T x) { data.add(x); }
  public T pop() { return data.remove(data.size() - 1); }
}

// Bounded
public <T extends Comparable<T>> T max(List<T> xs) {
  return xs.stream().max(Comparable::compareTo).orElseThrow();
}

// Wildcards
List<? extends Number> numbers;        // read-only (covariant)
List<? super Integer> integers;        // write-only (contravariant)

47.6 Exceptions

// try-with-resources — auto-closes
try (Connection c = DriverManager.getConnection(url);
     PreparedStatement ps = c.prepareStatement(sql)) {
  /* use */
} catch (SQLException e) {
  throw new RuntimeException("DB failed", e);
}

47.7 Streams + lambdas

List<User> admins = users.stream()
  .filter(u -> u.role().equals("admin"))
  .sorted(Comparator.comparing(User::createdAt))
  .collect(Collectors.toList());

// Group by
Map<String, List<User>> byRole = users.stream()
  .collect(Collectors.groupingBy(User::role));

// Reduce
int total = orders.stream().mapToInt(Order::total).sum();

// Parallel — be careful
long count = bigList.parallelStream().filter(predicate).count();

47.8 String immutability + equals/hashCode

String a = "hello";
String b = "hello";
a == b;          // true — string pool
a.equals(b);     // true — value comparison
new String("hello") == "hello";   // false — different object

// Always use .equals() for content comparison
"alice".equals(user.name());

// equals/hashCode contract for custom classes
@Override public boolean equals(Object o) {
  if (this == o) return true;
  if (!(o instanceof User u)) return false;
  return Objects.equals(email, u.email);
}
@Override public int hashCode() { return Objects.hash(email); }

47.9 Threading + ThreadLocal for WebDriver

// Each thread gets its own WebDriver instance — required for parallel TestNG
public class DriverFactory {
  private static final ThreadLocal<WebDriver> driver = new ThreadLocal<>();
  public static void set(WebDriver d) { driver.set(d); }
  public static WebDriver get() { return driver.get(); }
  public static void remove() {
    WebDriver d = driver.get();
    if (d != null) d.quit();
    driver.remove();
  }
}