Java Fundamentals

Recap: What We've Learned

In the previous lectures, we covered:

• Lecture 1: OOP concepts, classes, objects, and UML class diagrams
• Lecture 2: Behavioral diagrams (Use Case, Activity, Sequence)

Quick Review: Class Relationships

classDiagram
    direction TB
    A -- B : Association
    C o-- D : Aggregation
    E *-- F : Composition
    G <|-- H : Inheritance
            

Our Development Tool: IntelliJ IDEA

IntelliJ IDEA is one of the most popular Java IDEs (Integrated Development Environment):

• Intelligent code completion - suggests methods and properties as you type
• Real-time error detection - catches mistakes before you run
• Powerful refactoring - safely rename and restructure code
• Built-in debugger - step through code line by line
• Version control - integrated Git support

IntelliJ IDEA Interface

• Project View (left): Shows your project structure and files
• Editor (center): Where you write code with intelligent completion
• Tool Windows (bottom/right): Terminal, Run, Debug, Version Control

Key shortcuts:

Creating Your First Project

1. Open IntelliJ IDEA
2. Click New Project
3. Select Java (left panel)
4. Choose your JDK (Java 17+ recommended)
   • If no JDK listed, click "Download JDK" and select a version
5. Name your project: JavaExercises
6. Click Create

Creating a Java Class

1. Right-click on the src folder
2. Select New → Java Class
3. Enter: fr.wayup.exercises.HelloWorld
   • This creates a package fr.wayup.exercises and class HelloWorld
4. Press Enter

package fr.wayup.exercises;

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

Click the green play button or press Shift+F10 to run!

Anatomy of a Java Class

package fr.wayup.exercises; ①

public class JavaSyntaxDemo { ②
    private String demoVersion = "1.0"; ③

    public JavaSyntaxDemo() { ④
        // Initialization code
    }

    public String getDemoVersion() { ⑤
        return this.demoVersion;
    }
}

Basic Code Elements

• Comments:

  // Single-line comment
  
  /* Multi-line comment
     spanning multiple lines */
  
  /** Javadoc comment - generates documentation */

• Statements: End with semicolon ;

  int x = 10;  // Statement ends with semicolon

• Blocks: Grouped with curly braces { }

  if (x > 5) {
      System.out.println("x is greater than 5");
  }

Packages and Imports

Packages organize your classes and prevent naming conflicts:

package fr.wayup.banking;  // This class belongs to this package

import java.util.ArrayList;  // Import a specific class
import java.time.*;          // Import all classes from a package

public class Account {
    private ArrayList transactions;
    private LocalDate openDate;
}

• Package names follow reverse domain convention: com.company.project
• Package structure matches folder structure on disk
• java.lang is imported automatically (String, System, etc.)

Two Kinds of Types in Java

int count = 10;              // Primitive
String message = "Hello";    // Reference (Object)

Primitive Types: Integers

Primitive Types: Floating Point

float price = 19.99f;      // Note the 'f' suffix
double pi = 3.14159265359; // Default for decimals
double scientific = 1.5e10; // Scientific notation: 1.5 × 10^10

Primitive Types: Boolean and Char

boolean isLoggedIn = false;
boolean hasPermission = true;

char firstLetter = 'A';       // Single quotes for char
char unicode = '\u0041';      // Unicode: also 'A'
char newline = '\n';          // Escape character

The String Type

String is a reference type (object), but it's used so often it has special syntax:

// String creation
String greeting = "Hello, World!";  // String literal
String name = new String("Alice");  // Using constructor (rarely needed)

// String operations
String fullName = "John" + " " + "Doe";  // Concatenation
int length = greeting.length();          // 13
String upper = greeting.toUpperCase();   // "HELLO, WORLD!"
boolean starts = greeting.startsWith("Hello"); // true
String sub = greeting.substring(0, 5);   // "Hello"

Local Variable Type Inference: var

Since Java 10, you can use var to let the compiler infer the type:

// Traditional declarations
String message = "Hello";
ArrayList<String> names = new ArrayList<String>();
Map<String, List<Integer>> data = new HashMap<String, List<Integer>>();

// With var - compiler infers the type
var message = "Hello";                    // Inferred as String
var names = new ArrayList<String>();      // Inferred as ArrayList<String>
var data = new HashMap<String, List<Integer>>();  // Much cleaner!

When to Use var

// Type is obvious from right side
var count = 0;
var name = "Alice";
var list = new ArrayList<String>();

// Long generic types
var map = new HashMap<String, List<Integer>>();

// With streams
var filtered = names.stream()
    .filter(n -> n.startsWith("A"))
    .toList();

// Type not clear
var result = getResult();  // What type?
var x = calculate();       // Unclear

// Numeric literals
var num = 3.14;  // double or float?
var id = 100;    // int or long?

// Diamond with var (fails)
var list = new ArrayList<>();  // Error!

var Best Practices

Arithmetic Operators

int a = 10, b = 3;
System.out.println(a / b);    // 3 (integer division!)
System.out.println(a % b);    // 1 (remainder)
System.out.println(10.0 / 3); // 3.333... (double division)

Increment and Compound Operators

int i = 10;

// Increment / Decrement
i++;      // i is now 11 (post-increment)
++i;      // i is now 12 (pre-increment)
i--;      // i is now 11

// Compound assignment operators
i += 5;   // Same as: i = i + 5  → i is 16
i -= 3;   // Same as: i = i - 3  → i is 13
i *= 2;   // Same as: i = i * 2  → i is 26
i /= 2;   // Same as: i = i / 2  → i is 13

// Pre vs Post increment
int a = 5;
int b = a++;  // b = 5, a = 6 (use then increment)
int c = ++a;  // c = 7, a = 7 (increment then use)

Comparison Operators

String a = "hello";
a.equals("hello")  // true - correct way
a == "hello"       // may be false - compares references!

Logical Operators

int age = 25;
boolean hasLicense = true;

// Combining conditions
boolean canDrive = age >= 18 && hasLicense;  // true
boolean isMinorOrNoLicense = age < 18 || !hasLicense; // false

// Short-circuit evaluation
if (name != null && name.length() > 0) {
    // Safe: second part only evaluated if first is true
}

Access Modifiers

Access modifiers control who can see and use your classes, fields, and methods:

Access Modifiers: Example

public class BankAccount {
    public String accountNumber;    // Accessible from anywhere
    protected double balance;       // Package + subclasses
    String bankName;                // Default: package only
    private String pin;             // Only within this class

    public void deposit(double amount) {  // Public method
        if (amount > 0) {
            this.balance += amount;
            logTransaction("Deposit: " + amount);
        }
    }

    private void logTransaction(String msg) {  // Private helper
        System.out.println(msg);
    }
}

Encapsulation: Protecting Your Data

Encapsulation = hiding internal details and providing controlled access.

classDiagram
    class BankAccount {
        -double balance
        -String pin
        +getBalance() double
        +deposit(amount) void
        +withdraw(amount) boolean
    }
            

Getters and Setters

public class BankAccount {
    private double balance;  // Private field

    // Getter - read access
    public double getBalance() {
        return this.balance;
    }

    // Setter - controlled write access
    public void setBalance(double balance) {
        if (balance >= 0) {  // Validation!
            this.balance = balance;
        } else {
            throw new IllegalArgumentException("Balance cannot be negative");
        }
    }
}

• Getter: getFieldName() - returns the value
• Setter: setFieldName(value) - sets with validation

Complete Encapsulation Example

public class Customer {
    private String name;
    private String email;

    public Customer(String name, String email) {
        setName(name);   // Use setters for validation
        setEmail(email);
    }

    public String getName() { return name; }
    public void setName(String name) {
        if (name != null && !name.isEmpty()) {
            this.name = name;
        }
    }

    public String getEmail() { return email; }
    public void setEmail(String email) {
        if (email != null && email.contains("@")) {
            this.email = email;
        }
    }
}

Type Casting

Converting values from one type to another:

• Widening (automatic): smaller → larger type, no data loss

  int myInt = 100;
  double myDouble = myInt;  // Automatic: 100.0

• Narrowing (manual): larger → smaller type, possible data loss

  double myDouble = 9.78;
  int myInt = (int) myDouble;  // Cast required: 9

Type Casting Examples

// Widening - automatic, safe
byte b = 10;
int i = b;       // OK
long l = i;      // OK
double d = l;    // OK

// Narrowing - requires cast, may lose data
double price = 19.99;
int roundedPrice = (int) price;  // 19 (decimal lost)

long bigNumber = 10_000_000_000L;
int smaller = (int) bigNumber;   // OVERFLOW! Wrong value

// String conversions
int age = Integer.parseInt("25");
double salary = Double.parseDouble("50000.50");
String ageStr = String.valueOf(age);  // "25"
String concat = "" + age;             // "25" (quick way)

Exercise 1: Create the Bank Classes

Create Java classes based on this class diagram:

classDiagram
    direction LR
    class Customer {
        -String name
        -String address
    }
    class Account {
        -String accountNumber
        -double balance
    }
    class SavingsAccount {
        -double interestRate
    }
    class InvestmentAccount

    Customer "1" -- "0..*" Account
    Account <|-- SavingsAccount
    Account <|-- InvestmentAccount
        

Exercise 1: Tasks

For each class in the diagram, implement:

1. Private fields as shown in the diagram
2. Constructor to initialize all fields
3. Getters and setters for each field with validation

Exercise 2: Add Behavior

Add the following methods to your SavingsAccount class:

1. deposit(double amount)
   • Add amount to balance (must be positive)
2. withdraw(double amount)
   • Subtract amount from balance (cannot go below 0)
3. computeInterest()
   • Return: balance × interestRate
4. applyInterest()
   • Add computed interest to balance

Key Takeaways

• IntelliJ IDEA is our development environment
  • Projects contain packages, packages contain classes
• Two type kinds: primitives (int, double) and objects (String)
• Operators: arithmetic, comparison, logical, assignment
• Access modifiers: private → default → protected → public
• Encapsulation: private fields + public getters/setters
• Type casting: widening (auto) vs narrowing (manual)