Writing Classes

By the end of this lesson, you will be able to design and implement a complete Java class with fields, constructors, accessors, mutators, and toString.

Class Design Checklist

A well-designed class has:

1. Private instance variables (fields) for the object's state
2. One or more constructors
3. Public getter methods (accessors)
4. Public setter methods (mutators) where appropriate
5. Override of toString() for meaningful output
6. Override of equals() for content comparison (optional but good practice)

Complete Example: BankAccount

public class BankAccount {
    // 1. Private fields
    private String owner;
    private double balance;
    private int accountNumber;
    private static int nextAccountNumber = 1001;  // shared counter

    // 2. Constructor
    public BankAccount(String owner, double initialDeposit) {
        this.owner = owner;
        this.balance = initialDeposit;
        this.accountNumber = nextAccountNumber;
        nextAccountNumber++;
    }

    // 3. Accessors (getters)
    public String getOwner() { return owner; }
    public double getBalance() { return balance; }
    public int getAccountNumber() { return accountNumber; }

    // 4. Mutators (setters and other modifiers)
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public boolean withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            return true;  // success
        }
        return false;  // failed (insufficient funds or invalid amount)
    }

    // 5. toString
    public String toString() {
        return "Account #" + accountNumber + " (" + owner + "): $" + balance;
    }
}

Using the Class

BankAccount alice = new BankAccount("Alice", 1000.0);
BankAccount bob = new BankAccount("Bob", 500.0);

alice.deposit(250.0);
boolean success = alice.withdraw(200.0);

System.out.println(alice);   // calls toString automatically
// → Account #1001 (Alice): $1050.0

System.out.println(BankAccount.nextAccountNumber);  // ERROR! nextAccountNumber is private

Design Decisions

Why private fields? Protects integrity — prevents code outside the class from setting balance to -$1,000,000. All changes go through methods that can validate.

Why validate in setters? The withdraw method returns false if there aren't sufficient funds, instead of allowing a negative balance. The class enforces its own invariants.

Why static nextAccountNumber? Every account needs a unique number; the counter is shared across ALL instances of the class, not per-object.