Java Basics Cheat Sheet

1. Introduction to Java

• Java is a high-level, object-oriented programming language.
• Known for Write Once, Run Anywhere (WORA) – Java code can run on any platform with a Java Virtual Machine (JVM).
• Used for web development, mobile apps (Android), desktop applications, and enterprise systems.

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2. Setup and Running Java

1. Install Java Development Kit (JDK):
   • Download from Oracle.
2. Verify Installation:

   java -version
   javac -version
   

3. Run a Java Program:

   javac HelloWorld.java  # Compile
   java HelloWorld        # Run
   

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3. Java Program Structure

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

• public class HelloWorld – Defines a class.
• public static void main(String[] args) – Main method, program entry point.
• System.out.println – Prints to the console.

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4. Variables and Data Types

Primitive Data Types

Non-Primitive Types

• String: String name = “Java”;
• Array: int[] arr = {1, 2, 3};
• Object: MyClass obj = new MyClass();

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5. Control Structures

1. Conditional Statements

int x = 10;
if (x > 0) {
    System.out.println("Positive");
} else if (x == 0) {
    System.out.println("Zero");
} else {
    System.out.println("Negative");
}

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2. Switch Statement

int day = 3;
switch (day) {
    case 1 -> System.out.println("Monday");
    case 2 -> System.out.println("Tuesday");
    default -> System.out.println("Other Day");
}

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3. Loops

For Loop:

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

While Loop:

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

Do-While Loop:

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

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6. Arrays

int[] numbers = {10, 20, 30, 40};
System.out.println(numbers[0]);  // Access element
numbers[1] = 25;                 // Modify element

• Array Length: numbers.length
• Iterate Array:

for (int num : numbers) {
    System.out.println(num);
}

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7. Methods (Functions)

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

    public static void main(String[] args) {
        int result = add(5, 3);
        System.out.println(result);
    }
}

• add – Custom method.
• Return Type: int (returns integer).
• void – No return value.

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8. Object-Oriented Programming (OOP)

1. Class and Object

public class Car {
    String brand;
    int speed;

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

    public void drive() {
        System.out.println(brand + " is driving at " + speed + " km/h");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 120);
        myCar.drive();
    }
}

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2. Inheritance

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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3. Polymorphism

Animal myDog = new Dog();
myDog.sound();  // Dog barks

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4. Encapsulation

public class Person {
    private String name;

    public String getName() {
        return name;
    }

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

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9. Exception Handling

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("End of try-catch");
}

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10. File I/O

import java.io.*;

public class FileExample {
    public static void main(String[] args) {
        try {
            FileWriter writer = new FileWriter("example.txt");
            writer.write("Hello Java");
            writer.close();
        } catch (IOException e) {
            System.out.println("Error occurred");
        }
    }
}

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Useful Commands and Shortcuts

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Tips for Java Development

• Practice OOP Concepts – Master classes, inheritance, and polymorphism.
• Work with APIs – Java has vast libraries like Spring and Hibernate.
• Understand JVM – Knowing how the JVM works will improve debugging and performance tuning.