Java Cheat Sheet - This is a concise Java cheat sheet based on core concepts and syntax. It is designed to help you quickly reference Java fundamentals.

1. Basic Syntax

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

• public class Main: Defines a class named Main.
• public static void main(String[] args): Entry point of the program.
• System.out.println(): Prints output to the console.

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2. Data Types

Data Type	Size	Description
byte	1 byte	Stores whole numbers (-128 to 127)
short	2 bytes	Stores whole numbers (-32,768 to 32,767)
int	4 bytes	Stores whole numbers (-2^31 to 2^31-1)
long	8 bytes	Stores whole numbers (-2^63 to 2^63-1)
float	4 bytes	Stores fractional numbers (6-7 decimal digits)
double	8 bytes	Stores fractional numbers (15 decimal digits)
boolean	1 bit	Stores true or false
char	2 bytes	Stores a single character/letter

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3. Variables

int num = 10;               // Integer
double pi = 3.14;           // Double
char letter = 'A';          // Character
boolean isJavaFun = true;   // Boolean
String name = "Java";       // String

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4. Operators

Type	Operators
Arithmetic	+, -, *, /, %, ++, --
Assignment	=, +=, -=, *=, /=
Comparison	==, !=, >, <, >=, <=
Logical	&&, `

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5. Control Flow

If-Else

if (condition) {
    // code
} else if (condition) {
    // code
} else {
    // code
}

Switch

switch (variable) {
    case value1:
        // code
        break;
    case value2:
        // code
        break;
    default:
        // code
}

Loops

for (int i = 0; i < 5; i++) {
    // code
}

while (condition) {
    // code
}

do {
    // code
} while (condition);

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6. Arrays

int[] numbers = {1, 2, 3, 4, 5};  // Array declaration
System.out.println(numbers[0]);   // Access first element
numbers[1] = 10;                  // Modify second element
int length = numbers.length;      // Get array length

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7. Methods

public static int add(int a, int b) {
    return a + b;
}

// Method call
int result = add(5, 10);

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8. Object-Oriented Programming (OOP)

Class and Object

class Car {
    String brand;  // Field

    // Constructor
    public Car(String brand) {
        this.brand = brand;
    }

    // Method
    public void drive() {
        System.out.println("Driving " + brand);
    }
}

// Create an object
Car myCar = new Car("Toyota");
myCar.drive();

Inheritance

class Vehicle {
    void run() {
        System.out.println("Vehicle is running");
    }
}

class Bike extends Vehicle {
    void run() {
        System.out.println("Bike is running");
    }
}

Encapsulation

class Person {
    private String name;  // Private field

    // Getter
    public String getName() {
        return name;
    }

    // Setter
    public void setName(String name) {
        this.name = name;
    }
}

Polymorphism

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

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9. Exception Handling

try {
    int[] arr = new int[5];
    System.out.println(arr[10]);  // Throws ArrayIndexOutOfBoundsException
} catch (Exception e) {
    System.out.println("An error occurred: " + e.getMessage());
} finally {
    System.out.println("This will always execute");
}

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10. File Handling

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        try {
            File file = new File("example.txt");
            FileWriter writer = new FileWriter(file);
            writer.write("Hello, Java!");
            writer.close();
        } catch (IOException e) {
            System.out.println("An error occurred: " + e.getMessage());
        }
    }
}

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11. Common String Methods

String str = "Hello, Java!";
int length = str.length();                // Length of string
String upper = str.toUpperCase();         // Convert to uppercase
String lower = str.toLowerCase();         // Convert to lowercase
boolean contains = str.contains("Java");  // Check if string contains substring
String substring = str.substring(0, 5);   // Extract substring

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12. Collections Framework

ArrayList

import java.util.ArrayList;

ArrayList<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
System.out.println(fruits.get(0));  // Access element
fruits.remove(1);                   // Remove element

HashMap

import java.util.HashMap;

HashMap<String, Integer> map = new HashMap<>();
map.put("Apple", 10);
map.put("Banana", 20);
System.out.println(map.get("Apple"));  // Access value
map.remove("Banana");                  // Remove key-value pair

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13. Lambda Expressions

interface MathOperation {
    int operate(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        MathOperation add = (a, b) -> a + b;
        System.out.println(add.operate(5, 10));  // Output: 15
    }
}

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14. Threading

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread thread = new MyThread();
        thread.start();  // Start the thread
    }
}