Introduction

Ludovic Deneuville

Introduction to Java

  • 6h CM
  • 12h TP
  • ludovic.deneuville@ensai.fr
  • office 161
  • class based on those by Ikko Yamane, Benjamin Girault, Romaric Gaudel, Samuel Toubon and Olivier Levitt

Objectives

  • Learn the basic syntax of Java
  • Understand key OOP concepts
  • Learn how to use collections
  • Best practices

TP

  • 4 x 3h
  • Datalab SSPCloud
  • Follow instructions precisely
  • Don’t use GenAI

Evaluation

  • Graded TP
    • 2 students max per group
    • to be completed at home
  • Table-top exam
    • duration: 1h30
    • a handwritten A4 sheet allowed

Why Java

History / Version

  • Created in 1991
    • by James Gosling and Mike Sheridan (Sun Microsystems)
  • Java 8 (2014) : Lambda, Stream, Optional
  • Java 17 (2021)
  • Java 21 (2023)
  • Java 25 (sept. 2025)

https://www.java.com/

Portability

  • Compiled to bytecode
  • Runs on any machine with a JVM
  • “Write Once, Run Anywhere”

Robust / Secure

  • Automatic memory management
  • Strong Static Typing
  • Compile-Time Checks

Static Typing

Static Typing Dynamic Typing
Type checked at compile time Type checked at runtime
Variable type is declared Variable type inferred during execution
Errors detected earlier More flexible but errors appear later

Strong Typing

Strong Typing Weak Typing
Types are strictly enforced Implicit conversions are common
Few automatic casts Many implicit conversions
Safer and more predictable More flexible but error-prone

Compiler

  • Transform java code into bytecode
  • Detects syntax and type errors
  • Optimization
  • javac MyClass.java ➡️ create file MyClass.class

JVM

Definition

The Java Virtual Machine (JVM) is the runtime environment for Java programs. It acts as an interpreter between Java code and the operating system.

The JVM :

  • Executes bytecode
  • Interacts with OS
  • Manages Memory (Garbage Collection)
  • java MyClass ➡️ run main method

JVM, JDK, JRE

  • JRE: Java Runtime Environment
  • JDK: Java Development Kit
Component Purpose
JVM Runs Java bytecode
JRE Contains JVM + libraries to run Java apps
JDK Includes JRE + compiler + dev tools

https://openjdk.org/projects/jdk/

Java Vs Python

Java Python
Typing Strongly typed, static Strongly typed, dynamic
Compilation Compiled into bytecode Interpreted
Performance Very fast Slower
Syntax Verbose, requires {} Concise, uses indentation
Paradigm Object-oriented Multi-paradigm
Ecosystem Huge (Spring, Hibernate…) Large (Django, Flask, Pandas…)

High-level languages

  • Human-readable syntax
  • Hardware abstraction
  • Faster, easier development
  • Indirect performance (VM/runtime)

Low-level languages

  • Close to hardware
  • Fine control over memory/resources
  • Maximum performance
  • Higher complexity

High/Low-level

Level Priority Typical Uses
High Productivity Web, data, scripting
Mid Balance Enterprise, backend
Low Control & speed OS, embedded, real-time

Resources

Practice java language