Inner and Nested Classes

A non-static member inner class carries an enclosing-instance reference and can access its members. A static nested class behaves like an ordinary class scoped inside another and is preferable when no outer instance is needed.

Four nested forms

class Cache {
    static final class Entry { String key; Object value; }

    final class Cursor {
        int position;
        int ownerSize() { return Cache.this.size(); }
    }

    int size() { return 0; }
}

Runnable task = new Runnable() {
    @Override public void run() { System.out.println("done"); }
};

Local classes are named inside a block; anonymous classes provide a one-off subtype expression. Both can capture final or effectively final locals. Lambdas are usually clearer for functional interfaces but differ in this, identity, and generated behavior. Avoid accidental retention: a long-lived inner object can keep its entire outer object alive. Use nesting to hide implementation details and express tight ownership, not merely to shorten names.