Access Modifiers in Java

Access modifiers control the visibility of classes, fields, methods, and constructors. Java has four levels:

1. public

Accessible from anywhere — any class, in any package.

public class Car {
    public String model;
    public void drive() { }
}
2. protected
Accessible within the same package, plus subclasses in other packages (via inheritance).
class Animal {
    protected String name;
    protected void makeSound() { }
}

3. default (no modifier)
Accessible only within the same package. If you don't write any modifier, this is what you get.
class Utility {
    int helperValue;       // default access
    void helperMethod() { } // default access
}

4. private
Accessible only within the same class. Not visible to subclasses or anything outside.
class BankAccount {
    private double balance;
    private void logTransaction() { }
}

Where They Apply
Top-level classes/interfaces: only public or default (no private/protected)
Members (fields, methods, constructors, nested classes): all four modifiers apply