What are access modifiers?

Access modifiers control the visibility and accessibility of classes, methods, and fields. They are the primary mechanism for enforcing encapsulation. Java's four access levels (most to least restrictive): (1) private: accessible only within the same class. Not accessible from subclasses or other classes: private double balance; // Only BankAccount can access private void updateLedger() {}; (2) package-private (default, no keyword): accessible within the same package — the default when no modifier is specified: double interestRate; // Any class in same package void calculateInterest() {}; (3) protected: accessible within the same package AND by subclasses (even in different packages): protected String ownerName; // Accessible by subclasses protected void sendAlert() {}; (4) public: accessible from anywhere — any class in any package: public String getAccountNumber() { return accountNumber; } public void deposit(double amount) {}. Visibility matrix: private < default (package) < protected < public. Best practices: fields should almost always be private; getters/setters can be public if needed; helper methods should be private; constructor visibility depends on design (private for Singleton, public for normal, protected for abstract). Other OOP languages: C# has internal (assembly-wide), protected internal; Swift has fileprivate, internal (module); Python uses naming convention (_single = protected, __double = private via name mangling — not enforced).