Java

Mac Java Installation Steps Download and Install Java Check installed JDKs /usr/libexec/java_home -V [for Mac] java -version [for Windows] Open .zshrc and set JAVA_HOME vi ~/.zshrc [using terminal] export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-17.jdk/Contents/Home [add this] esc → :wq [Save and exit] Save and reload shell config in terminal source ~/.zshrc Check Java HOME is setup properly from terminal echo $JAVA_HOME Windows Java Installation Steps Download and Install Java Check installed JDKs java -version [for Windows] Set JAVA_HOME Environment Variable Open System Properties → Advanced → Environment Variables Under System variables, click New and add: Variable name: JAVA_HOME Variable value: C:\Program Files\Java\jdk-17 [or your JDK path] Click OK to save Update PATH Variable In the same Environment Variables window, select Path → Edit → New, then add: %JAVA_HOME%\bin Click OK to save and close Verify Configuration in Command Prompt java -version echo %JAVA_HOME% Class-level/Instance variables/field + Local Variable 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 public class Car { String brand; // Class-level/Instance variables/field (each Car object has its own copy) int year; static int wheels = 4; //Static variable (shared by ALL Car objects) public Car(String brand, int year) { // Constructor (sets instance variables) this.brand = brand; this.year = year; } public void displayInfo() { // Method with a local variable String info = "Car Brand: " + brand + ", Year: " + year + ", Wheels: " + wheels; // Local variable (exists only while this method runs) System.out.println(info); } public static void main(String[] args) { Car myCar = new Car("Toyota", 2022); // Local variable in main() myCar.displayInfo(); // Calling instance method using the object System.out.println("All cars have " + Car.wheels + " wheels."); // Accessing static variable Car anotherCar = new Car("Honda", 2020); // Creating another car to show that static variable is shared anotherCar.displayInfo(); } } When to use static You’d only make it static if the method doesn’t depend on any specific object’s data. For example: public static void showWheelsInfo() { System.out.println(“All cars have " + wheels + " wheels.”); } This one is static because it only uses the wheels variable — which is also static — and doesn’t need to know about a specific Car object. Perfect! Here’s your fully formatted version with bold definitions and added emojis for each concept to make it visually pop: ...

Java Collections and Key Programming Comparisons This document provides a comprehensive comparison of Java collections, interfaces, classes, and key programming concepts. 1. HashMap vs. Hashtable Feature HashMap Hashtable Synchronization Non-synchronized; not thread-safe without external synchronization. Synchronized; thread-safe by default. Nulls Allows one null key and multiple null values. Does not allow null keys or values. Version Introduced in JDK 1.2 (new class). Legacy class. Performance Fast. Slower. Synchronization Option Can be made synchronized with Collections.synchronizedMap(hashMap). Internally synchronized; cannot be unsynchronized. Traversal Uses Iterator. Uses Enumeration and Iterator. Fail-Fast Behavior Iterator is fail-fast. Enumerator is not fail-fast. Inheritance Inherits AbstractMap. Inherits Dictionary. 2. ArrayList vs. LinkedList Both implement the List interface and maintain insertion order. Both are non-synchronized classes. ...

Complete Java Interview Prep Sheet (Final Version) 1. Core OOP Concepts 4 Fundamental Principles: Abstraction: Hide implementation details, show only essential features (abstract classes/interfaces). (what to implement). Encapsulation: Bundle data + methods, hide internals using private fields + public getters/setters (data hiding). Inheritance: Child class acquires parent class properties (is-a relationship, single inheritance only). Polymorphism: Same interface, multiple forms (compile-time: method overloading; runtime: method overriding). Key Points: Java not pure OOP (uses primitives, no multiple class inheritance). Object class is root of all classes: equals(), hashCode(), toString(), wait()/notify(). Multiple inheritance via interfaces only (no diamond problem). 2. Java Basics Class/Object/Method: ...