Java Programming Language - Beginner Cheat Sheet

(Basics → OOP → Advanced → Professional)

1. What Is Java?

Java is a high-level, object-oriented programming language mainly used for backend systems, APIs, enterprise apps, and Android.
It follows Write Once, Run Anywhere because Java runs on JVM.

2. Basic Program Structure

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

• class defines your program.

• main() is where execution starts.

3. Variables

Variables store data in memory.

int age = 25;
String name = "Sumit";
boolean active = true;

Each variable has a type + name + value.

4. Constants

Use final to prevent value changes.

final int MAX = 100;

Constants improve safety and readability.

5. Data Types

Primitive (store direct values)

int, double, char, boolean

Non-Primitive (store object references)

String, Array, Class

Primitive = faster.
Non-primitive = more powerful.

6. Operators

Used to perform calculations and comparisons.

Arithmetic

+ - * / %

Comparison

== != > <

Logical

&& || !

7. Conditional Statements

if / else

if(age > 18){}
else{}

Used to make decisions based on conditions.

switch

switch(day){
 case 1: break;
 default:
}

Used when checking many fixed values.

8. Loops

Loops repeat code automatically.

for

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

while

while(condition){}

do while

do{} while(condition);

9. Arrays

Arrays store multiple values of same type.

int[] nums = {1,2,3};

Access using index: nums[0].

10. Methods

Methods are reusable blocks of code.

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

They improve modularity and clarity.