Java Programming Cheatsheet: Core Java, OOP, Collections & Concurrency Guide

Section 1: Java Architecture (JDK vs JRE vs JVM)

• JDK (Java Development Kit): The full developer environment containing the Java Compiler (javac), debugger, and documentation tools.
• JRE (Java Runtime Environment): The runtime libraries required to execute compiled Java programs.
• JVM (Java Virtual Machine): The virtual processor that executes compiled platform-independent .class bytecode using JIT (Just-In-Time) compilation and automated Garbage Collection.

Section 2: The 4 Pillars of OOP in Java

// 1. Encapsulation & Abstraction
public abstract class Account {
    private String accountNumber;  // Encapsulated data
    protected double balance;

    public Account(String accNo, double initialBalance) {
        this.accountNumber = accNo;
        this.balance = initialBalance;
    }

    public abstract void calculateInterest(); // Abstract method

    public double getBalance() { return balance; }
}

// 2. Inheritance & Polymorphism
public class SavingsAccount extends Account {
    private double interestRate;

    public SavingsAccount(String accNo, double balance, double rate) {
        super(accNo, balance);
        this.interestRate = rate;
    }

    @Override
    public void calculateInterest() {  // Method Overriding (Runtime Polymorphism)
        double interest = balance * (interestRate / 100);
        balance += interest;
        System.out.println("Interest added: " + interest + " | New Balance: " + balance);
    }
}

Section 3: Java Collections Framework (JCF)

import java.util.*;

// 1. List: Ordered, allows duplicates (ArrayList vs LinkedList)
List courses = new ArrayList<>();
courses.add("DCA");
courses.add("Tally Prime");
courses.add("Python");

// 2. Set: Unique items only, no duplicates (HashSet vs TreeSet)
Set rollNumbers = new HashSet<>(Arrays.asList(101, 102, 101, 105)); // 101 only once

// 3. Map: Key-Value pairs (HashMap O(1) lookup)
Map feeMap = new HashMap<>();
feeMap.put("DCA", 4999.00);
feeMap.put("PGDCA", 9999.00);

for (Map.Entry entry : feeMap.entrySet()) {
    System.out.println(entry.getKey() + " -> ₹" + entry.getValue());
}

Section 4: Modern Java Streams API (Java 8+)

import java.util.stream.Collectors;

List filteredCourses = courses.stream()
    .filter(c -> c.startsWith("P"))
    .map(String::toUpperCase)
    .sorted()
    .collect(Collectors.toList());

// Java 17+ Modern Record:
public record StudentRecord(String name, String rollNo, int grade) {}