Java.md

Java

Data Types

int	        Whole numbers (32-bit)	            int x = 10;
double     	Decimal numbers (64-bit)	          double y = 3.14;
char	      Single character (16-bit Unicode)   char grade = 'A';
boolean	    True or False                	      boolean isJavaFun = true;
String	    Sequence of characters (Object)	    String name = "Utkarsh";

//Type Casting
double myDouble = 9.78;
int myInt = (int) myDouble;     // Manual casting: 9

String s = "100";
int i = Integer.parseInt(s);     // String to int

Arithmetic Operators:
+ , - , * , / , % , a++ , ++a , a-- , --a. for power Math.pow(a, b) return double datatye

Logical & Comparison Operators:
Logical: && (AND), || (OR), ! (NOT). Comparison: ==, !=, >, <, >=, <=.

Basic code for "Hello, World!"

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

String Methods Strings in Java are Immutable Objects.

String s = " Java Programming ";
s.length();          // 17
s.charAt(0);         // ' '
s.toUpperCase();     // " JAVA PROGRAMMING "
s.trim();            // "Java Programming" (removes leading/trailing spaces)
s.substring(1, 5);   // "Java" (start index, end index)
s.contains("Java");  // true
s.replace('a', 'o'); // " Jovo Progomming "

Arrays & Lists (Collections)

1. Array(Fixed Size)

int[] nums = {1, 2, 3};
System.out.println(nums.length);             // 3

2. ArrayList (Dynamic Size - similar to Python List)

import java.util.ArrayList;
ArrayList<String> list = new ArrayList<>();
list.add("Apple");            // append
list.add(0, "Mango");         // insert at index
list.remove("Apple");         // remove by value
list.get(0);                  // access element (list[0])
list.size();                  // length

Dictionaries (HashMap) In Java, "Dictionaries" are implemented using the HashMap class.

import java.util.HashMap;
HashMap<String, Integer> map = new HashMap<>();
map.put("Alice", 25);
map.get("Alice");           // 25
map.containsKey("Bob");     // false
map.remove("Alice");
map.keySet();               // Get all keys
map.values();               // Get all values

Sets Unique elements, unordered.

import java.util.HashSet;
HashSet<Integer> set = new HashSet<>();
set.add(1);
set.add(1); // Won't be added (duplicate)
set.contains(1); // true

Control Flow Conditional:

if (x > 0) {
    System.out.println("Positive");
} else if (x < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

// Ternary
String result = (10 > 5) ? "Greater" : "Smaller";

Loops:

// For loop
for (int i = 0; i < 5; i++) { ... }

// Enhanced For loop (For-each)
for (String s : list) { ... }

// While loop
while (condition) { ... }

Functions (Methods) Methods must be inside a class.

public class Main {
    // static means we can call it without creating an object
    public static int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        System.out.println(add(5, 10));
    }
}

Object-Oriented Programming (OOP)

1. Class & Object:

class Student {
    String name;
    // Constructor
    Student(String name) {
        this.name = name;
    }
}
Student s1 = new Student("Utkarsh");

2. Encapsulation: Use private variables and public Getters/Setters.
3. Inheritance: Use extends keyword.

class Animal { void eat() { } }
class Dog extends Animal { void bark() { } }

4. Abstraction: Use abstract classes or interfaces.

interface Shape {
    double getArea(); // Method signature only
}