Java Basics

Ludovic Deneuville

First of all

Important

In Java, all executable code must be inside a class.

• Not anymore !

Hello World

HelloWord.java

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

• compile: javac HelloWord.java
  • create file HelloWord.class
• run: java HelloWord
• exit: CTRL + C

• you only write .java files
• main method

Syntax

• A Line of code ends with a semicolon ;
• Blocs are delimited with curly braces: { }
• Comments
  • // one line
  • /* multi-lines */

Naming Conventions

• Classes: PascalCase
• Variables, Methods: camelCase
• Constants: UPPERCASE_WITH_UNDERSCORES
• Packages: lowercase, dot-separated
  • package com.example.utils;

Variables and Data Types

int age = 25;
final double PI = 3.14;        // final : Immutable

boolean isJavaFun = true;

char grade = 'A';             // single quote
String name = "Léa";          // double quote

var i = 2F;                   // type inference -> float

• strong typing, immutable type
• primitive, object
• String
  • not a primitive data type
  • String is a Class
  • immutable: Any modification creates a new string in memory

Primitive type vs Object

	int	Integer
Nature	Primitive	Object (Wrapper)
Nullable	❌ No	✅ Yes
Performance	⭐ Very fast	Slightly slower
Memory usage	Low	Higher (object overhead)
Typical uses	Calculations, loops	Collections, APIs

• int stores the value directly
• Integer wraps an int inside an object
• Required in collections (e.g., ArrayList<Integer>)

int a = 5;
Integer b = 5;   // Autoboxing
int c = b;       // Unboxing

Boolean

boolean isJavaFun = true;
boolean hasBrother = false;

boolean isFast = 20 > 15;

isJavaFun && !false || true;

• lowercase booleans
• No need for == true

String

String s = "Java";
String fullName = "Alain" + " " + "Philippe";   // concat

s.length();
s.charAt(0);          // Get character at index 0
s.contains("va");
s.toLowerCase();      // also toUpperCase
s.endsWith("Java");   // also startsWith

s.equals("Java");     // Use equals to compare, not ==

String textBlock = """
        First line
        Second line
        """;

Others: replace(), split()

• String objects are immutable
  • ➡️ their content can never be modified after creation

When an operation appears to modify a string, Java actually creates a new object.

String s = "hello";

s.toUpperCase();      // dont modify s
s = s.toUpperCase();  // creates a new String and affect it to variable s

Format String

• Prettier than concatenating
• creates a formatted string using placeholders

String name = "Tiphaine";
int age = 5;
String s = String.format("My name is %s and i'm %d years old", name, age);

double price = 19.90;
String s = String.format("Price: %.2f", price);

Type casting

double num1 = 9.78;
int num2 = (int) num1;                   // Casting Narrowing

String str1 = "123";
int num1 = Integer.parseInt(str1);       // String to int

String str2 = String.valueOf(num1);      // int to String
String str3 = num1 + ""

toString() for Object types

Print

• System.out.print("Hello");
• System.out.println("Hello");
  • adds a new line
• System.out ➡️ Standard output stream

• \n: carriage return
• Keyboard shortcut

Structures

if-else

int number = 10;

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

⚠️ Don’t forget the brackets ()

Ternary Operator

• Shorter alternative to if-else

String status = (number > 0) ? "Positive" : "Negative";

Switch

String fruit = "lemon";

switch (fruit) {
    case "cherry", "strawberry" -> System.out.println("Red fruit");
    case "banana", "lemon" -> System.out.println("Yellow fruit");
    default -> System.out.println("Unknown fruit");
}

• switch can return a value

String color = switch (fruit) {
    case "cherry", "strawberry" -> "Red fruit";
    case "banana", "lemon" -> "Yellow fruit";
    default -> "Unknown fruit";
};

For

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration: " + i);
}

• initialization: i = 0
• condition: i < 5
• update: i++

• condition: checked before entering
• update: after each iteration

casimir.java

int i = 0;
for ( ; ; ) {
    if ( i < 5)
        break;
    System.out.println("Iteration: " + i);
    i++;
}

For each

• Iterating through arrays, lists

int[] numbers = {1, 2, 3, 4};

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

first and last time I’m going to talk about arrays

While

int i = 10;
while (i >= 0) {
    System.out.println(i);
    i--;
}

do-while to check after each iteration

do { ...
} while { ... }

break / continue

for (int i = 0; i < 10; i++) {
    if (i == 2)
        continue;
    else if (i == 4)
        break;
    System.out.println("Iteration: " + i);
}

Iteration: 0
Iteration: 1
Iteration: 3

break: exit a single loop

Function

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

• Access modifier: public
• Return type: int
• Method name: add
• Parameters: (int a, int b)

• reminder: we are still in a class
• Method name : camelCase

Void

• Indicate that a method does not return any value

public void printIsOdd(int n) {
    System.out.println(n % 2 != 0);
}

Access Modifiers

Methods and Attributes Accessiblility

Modifier	Same Class	Same Package	Subclasses	Everywhere
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
default (no modifier)	✅	✅	❌	❌
private	✅	❌	❌	❌

Various

int i = 0;

i++;                // increment by 1
i += 3;             // i = i + 3 

Math.sqrt(3);
Math.max(8, 5);
Math.pow(2, 10);   // 1024

• ++i
• += works with Strings
• `%: modulo