Java cheatsheet

A one-page reference for Java. For JVM internals and interview Q&A, see the complete guide.

📖 Full guide: Java →

OOP essentials

public class Animal {
    protected String name;
    public Animal(String name) { this.name = name; }
    public String speak() { return name + " makes a sound"; }
}

public class Dog extends Animal implements Comparable<Dog> {
    public Dog(String name) { super(name); }
    @Override public String speak() { return name + " barks"; }
    public int compareTo(Dog o) { return name.compareTo(o.name); }
}

Generics

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

List<? extends Number> nums;   // producer, read-only
List<? super Integer> sink;    // consumer, write-only

Collections framework

List<String> list = new ArrayList<>();
Set<String> set = new HashSet<>();
Map<String, Integer> map = new HashMap<>();
Deque<String> stack = new ArrayDeque<>();  // push/pop

ArrayList — fast random access. LinkedList — fast insert/remove. HashMap — O(1) avg lookup, no order.

Streams & lambdas

List<String> names = users.stream()
    .filter(u -> u.getAge() > 18)
    .map(User::getName)
    .sorted()
    .collect(Collectors.toList());

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

Exception handling

try {
    risky();
} catch (IOException | SQLException e) {
    log.error("failed", e);
} finally {
    cleanup();
}

Checked exceptions must be declared/caught; unchecked (RuntimeException) don't.

Concurrency basics

ExecutorService pool = Executors.newFixedThreadPool(4);
Future<Integer> f = pool.submit(() -> compute());
f.get();   // blocks for result

synchronized (lock) { counter++; }   // mutual exclusion

Build tools

<!-- Maven: pom.xml -->
<dependency>
  <groupId>org.testng</groupId>
  <artifactId>testng</artifactId>
  <version>7.10.2</version>
</dependency>

mvn clean test
gradle test

Why Java for SDET

• Mature ecosystem: JUnit/TestNG, Selenium/Appium, Rest Assured all Java-first.
• Strong typing catches framework bugs at compile time, not runtime.
• Enterprise QA orgs standardize on it for shared tooling with backend teams.

See: Why Java Underpins the Mainstream SDET Stack