What are the types of classes in Java?

Regular (concrete) classes: Can be instantiated, contain fields, methods, constructors.

Abstract classes: Cannot be instantiated directly. Can contain abstract methods (without implementation) and regular methods. Used to define common characteristics for subclasses.

abstract class AbstractClass {
    abstract void abstractMethod(); // Abstract method
    void regularMethod() {
        System.out.println("Regular method");
    }
}

Nested classes: Declared inside another class. Can be:

Static nested classes: Do not have access to non-static members of the outer class. Created without an instance of the outer class.
```
class OuterClass {
    static class StaticNestedClass {
        // ...
    }
}
```
Inner classes: Have access to all members of the outer class (including private). Are linked to an instance of the outer class.
```
class OuterClass {
    class InnerClass {
        // Has access to OuterClass members
    }
}
```

Local classes: Declared inside a method, loop, or if block. Access is limited to their scope.

class OuterClass {
    void someMethod() {
        class LocalClass {
            // Declared locally
        }
        LocalClass obj = new LocalClass();
    }
}

Anonymous classes: Without a name, created simultaneously with declaration or instantiation. Used for one-time use.

interface MyInterface {
    void myMethod();
}

MyInterface anon = new MyInterface() {
    @Override
    public void myMethod() {
        System.out.println("Anonymous class");
    }
};

Record classes (Java 14+): Concise syntax for declaring immutable classes intended for data storage (POJO).

record Person(String name, int age) {
    // Constructor, getters, equals(), hashCode(), toString() are auto-generated
}