Class Structure

public class ClassName {
    // 1. Instance variables (fields) — the DATA
    private type variableName;
    
    // 2. Constructors — INITIALIZATION
    public ClassName(parameters) {
        // assign values to instance variables
    }
    
    // 3. Methods — BEHAVIOR
    public returnType methodName(parameters) {
        // code that uses instance variables
    }
}

public class BankAccount {
    // Instance variables — each BankAccount object has its own copy
    private String owner;
    private double balance;
    
    // Constructor — called when you create a new BankAccount
    public BankAccount(String owner, double initialBalance) {
        this.owner = owner;
        this.balance = initialBalance;
    }
    
    // Accessor (getter) — returns data without changing it
    public double getBalance() {
        return balance;
    }
    
    public String getOwner() {
        return owner;
    }
    
    // Mutator (setter) — changes the object's data
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
    
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
        }
    }
    
    // toString — provides a String representation
    public String toString() {
        return owner + ": $" + balance;
    }
}

BankAccount acct = new BankAccount("Alice", 1000.0);
acct.deposit(500.0);
acct.withdraw(200.0);
System.out.println(acct.getBalance());  // 1300.0
System.out.println(acct);               // Alice: $1300.0

public class Circle {
    private double radius;    // each Circle has its own radius
    private String color;     // each Circle has its own color
}

public class Student {
    private String name;     // only Student methods can access
    public int id;           // anyone can access (BAD practice!)
}

Student s = new Student();
// s.name = "Alice";  // COMPILE ERROR — name is private
s.id = 42;            // works — id is public (but you shouldn't do this)

// Without private — anyone can break your object:
public class BankAccount {
    public double balance;   // BAD!
}

BankAccount acct = new BankAccount();
acct.balance = -1000000;   // No validation! Negative balance!

// With private — you control access:
public class BankAccount {
    private double balance;  // GOOD!
    
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;   // validated!
        }
    }
}

BankAccount a = new BankAccount("Alice", 1000);
BankAccount b = new BankAccount("Bob", 500);

a.deposit(200);
// a.balance = 1200
// b.balance = 500 — unchanged!