cheat sheet java

 Here’s a complete Java cheat sheet for quick interview preparation. It’s organized by topic and designed for fast revision — ideal before interviews.

🧠 Java Interview Cheat Sheet

✅ Core Java Basics

Data Types

• Primitive Types: byte, short, int, long, float, double, char, boolean
• Wrapper Classes: Byte, Short, Integer, Long, Float, Double, Character, Boolean

Keywords

• static, final, abstract, synchronized, volatile, transient, default, this, super, instanceof, enum

Access Modifiers

Modifier	Same Class	Same Package	Subclass	Everywhere
private	✔️	❌	❌	❌
default	✔️	✔️	❌	❌
protected	✔️	✔️	✔️	❌
public	✔️	✔️	✔️	✔️

📦 OOP Concepts

• Encapsulation: Data hiding via private fields + public getters/setters.
• Abstraction: Using interfaces or abstract classes.
• Inheritance: extends keyword; single inheritance only.
• Polymorphism:
  
• Compile-time (method overloading)
• Runtime (method overriding)
• 
  

🧱 Class Structure

public class MyClass {

    private int value;

    

    public MyClass(int val) {

        this.value = val;

    }

    public void print() {

        System.out.println(value);

    }

}

🔁 Control Statements

Loops

for (int i = 0; i < 10; i++) {}

while (condition) {}

do {} while (condition);

Conditions

if (x > 5) {}

else if (x == 5) {}

else {}

switch(day) {

  case "MONDAY": break;

  default: break;

}

📚 Collections Framework

List

• ArrayList, LinkedList

Set

• HashSet, LinkedHashSet, TreeSet

Map

• HashMap, LinkedHashMap, TreeMap, Hashtable

Queue

• PriorityQueue, ArrayDeque

Key Methods

list.add("x"); list.get(0); list.contains("x");

map.put("key", "val"); map.get("key");

🎯 Java 8 Features

Lambda Expressions

List<String> list = Arrays.asList("a", "b");

list.forEach(x -> System.out.println(x));

Streams API

list.stream().filter(x -> x.startsWith("a")).collect(Collectors.toList());

Functional Interfaces

• Runnable, Callable, Predicate<T>, Function<T,R>, Consumer<T>

Default & Static Methods in Interfaces

interface A {

    default void print() {}

    static void show() {}

}

Optional

Optional<String> opt = Optional.of("Hello");

opt.ifPresent(System.out::println);

🧵 Multithreading

Creating Thread

Thread t = new Thread(() -> System.out.println("Running"));

t.start();

Synchronization

synchronized (this) {

    // thread-safe block

}

Executors

ExecutorService ex = Executors.newFixedThreadPool(2);

ex.submit(() -> System.out.println("task"));

ex.shutdown();

🗃️ Exception Handling

try {

    // code

} catch (Exception e) {

    e.printStackTrace();

} finally {

    // always runs

}

Types:

• Checked: IOException, SQLException
• Unchecked: NullPointerException, ArrayIndexOutOfBounds

🧩 Interfaces & Abstract Classes

Interface

interface Vehicle {

    void start();

}

Abstract Class

abstract class Animal {

    abstract void sound();

}

🌐 String Handling

Common Methods

str.length(); str.charAt(0); str.substring(1);

str.contains("abc"); str.equals("abc"); str.trim();

StringBuilder

StringBuilder sb = new StringBuilder("Hello");

sb.append(" World");

💾 File Handling (Java NIO)

Path path = Paths.get("file.txt");

Files.write(path, "data".getBytes());

List<String> lines = Files.readAllLines(path);

📄 Java Memory Areas

• Heap: Objects
• Stack: Method calls, local variables
• Method Area: Class-level metadata
• GC: Automatically collects unreachable objects

🚦 Keywords Recap

Keyword	Use
final	Constant, prevents override
static	Shared among all instances
this	Refers to current object
super	Refers to parent class
transient	Skip during serialization
volatile	Guarantees visibility between threads
synchronized	Lock mechanism for thread safety

🧪 Miscellaneous

• equals() vs ==: .equals() checks content; == checks reference.
• hashCode(): Used in hashing (HashMap, HashSet).
• Comparable vs Comparator: Natural vs custom sorting.
• Serialization: implements Serializable, use ObjectOutputStream.