Java Basics – Part 1

1 Structure and Hello World

Every Java program needs at least one class and one main() method.
Code blocks use curly braces { } instead of indentation.

public class HelloWorld {
    // Entry point of every Java program
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Key points

• File name = class name (HelloWorld.java)
• Each statement ends with ;
• System.out.println() prints text followed by a newline.
  Use System.out.print() if you don’t want a newline.

2 Strong Typing and Variables

Java is strongly typed:
you must declare each variable’s data type once and cannot later change it.

int number = 7;
double pi = 3.14;
boolean isJavaFun = true;
char letter = 'A';
String text = "Hello Java!";

Extra notes:

• var (since Java 10) can infer the type: var city = "Berlin"; // automatically a String
• Use final for constants (cannot be reassigned): final double TAX_RATE = 0.19;

3 Basic Operators

Arithmetic:

System.out.println(21 / 3);     // 7
System.out.println(23 / 3);     // 7 (integer division)
System.out.println(23.0 / 3);   // 7.666...

Increment / Decrement:

int x = 7;
x++;        // post-increment
++x;        // pre-increment
x--;        // decrement

Logical operators:

System.out.println(true && false);  // false
System.out.println(true || false);  // true
System.out.println(!true);          // false

Comparison operators: ==, !=, <, >, <=, >=.

4 Random Numbers

Two common ways:

// 0.0 ≤ value < 1.0
double r1 = Math.random();

// 1–6 (dice)
int dice = (int)(Math.random() * 6) + 1;

Or with the Random class:

import java.util.Random;

Random rnd = new Random();
int dice2 = rnd.nextInt(1, 7);   // from 1 (inclusive) to 7 (exclusive)

5 Type Casting

Convert between numeric types manually:

int a = 5;
double b = a;          // implicit widening
int c = (int)3.9;      // explicit cast → 3

6 Comments

// single-line comment
/* multi-line
   comment */
/** Javadoc comment for API documentation */

Summary

• Java uses braces for code blocks.
• Data types: int, double, boolean, char, String.
• System.out.println() for output.
• Operators: arithmetic, logical, increment/decrement.
• Random numbers via Math.random() or Random.
• Strong typing + explicit casting.