Abstract Classes

In the last section, we learned how to use inheritance to build new classes based on existing ones. But what if we want to force all subclasses to follow certain rules? That’s where abstract classes come in.

What Is an Abstract Class?

An abstract class is like a partially-built template. You can’t create an object from it directly, but other classes can extend it and fill in the missing pieces. Think of it like a contract: if a class extends an abstract class, it must follow the rules set by that abstract class.

In Java, you use the abstract keyword to create one:

public abstract class BankAccount {
    protected String name;
    protected int accountNumber;
    protected double balance;

    public BankAccount(String name, int accountNumber, double balance) {
        this.name = name;
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    public void deposit(double amount) {
        this.balance += amount;
    }

    public void withdraw(double amount) {
        this.balance -= amount;
    }

    // Abstract Methods
    public abstract void monthlyUpdate();
    public abstract String getAccountType();
}