Programming Fundamentals › Object-Oriented Programming

Access Modifiers

public, private and protected: who may see a member.

Also known as: access modifiers, public private protected, visibility modifiers

Access modifiers control which code may see or use a member of a class (a field or method). The usual three:

class Account {
    private double balance;                 // hidden
    public void deposit(double amount) {    // the allowed way in
        if (amount > 0) balance += amount;
    }
}

Code outside can call deposit, but can’t set balance directly. That keeps the rules in one place (encapsulation).

Languages differ

• Java, C#, TypeScript, C++: enforced by the compiler, with keywords.
• Python: by convention. A leading underscore (_balance) means “internal, please don’t touch”. A double underscore triggers name mangling, but is still not true privacy.
• JavaScript: #balance makes a truly private field in classes. Before that, underscores were a convention.
• Go: capitalized names are exported, lowercase ones aren’t.

Guidelines

• Make things private by default, and expose only what callers need. You can open up later, but taking things away breaks users.
• Public members are a promise. Once other code depends on them, changing them is a breaking change.
• Don’t expose fields just to read them. Offer a method or property (getters and setters).
• Protected is easy to overuse. Subclasses often get tightly coupled to a parent’s internals (inheritance).
• Private is not security. It protects against accidental misuse, not attackers.