Static Nested Classes

class LinkedList<T> {
    private Node<T> head;

    // Static nested class — no outer instance reference
    private static class Node<T> {
        T data;
        Node<T> next;
        Node(T data) { this.data = data; }
    }

    public void addFirst(T item) {
        Node<T> node = new Node<>(item);
        node.next = head;
        head = node;
    }
}

class Builder {
    public static class Config {
        public final String host;
        public final int port;
        Config(String host, int port) {
            this.host = host; this.port = port;
        }
    }
}

Builder.Config config = new Builder.Config("localhost", 8080);
System.out.println(config.host + ":" + config.port); // localhost:8080

class Outer {
    private int x = 10;

    // Static nested: no access to x
    static class StaticNested {
        void doSomething() {
            // System.out.println(x); // compile error — no outer reference!
            System.out.println("Static nested");
        }
    }

    // Inner class: has access to x
    class Inner {
        void doSomething() {
            System.out.println(x); // 10 — can access outer fields
        }
    }
}

class Pizza {
    private final String size, crust;
    private final List<String> toppings;

    private Pizza(Builder b) {
        this.size = b.size; this.crust = b.crust; this.toppings = List.copyOf(b.toppings);
    }

    public static class Builder {
        private String size = "medium", crust = "thin";
        private List<String> toppings = new ArrayList<>();
        public Builder size(String s)    { this.size = s; return this; }
        public Builder crust(String c)   { this.crust = c; return this; }
        public Builder topping(String t) { this.toppings.add(t); return this; }
        public Pizza build()             { return new Pizza(this); }
    }
}

import java.util.*;

Map<String, Integer> scores = Map.of("Alice", 95, "Bob", 87, "Carol", 92);

for (Map.Entry<String, Integer> entry : scores.entrySet()) {
    System.out.printf("%-10s: %d%n", entry.getKey(), entry.getValue());
}

// Modern: using var
for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}

class UserServiceTest {
    // Static nested fake/stub — no outer instance needed
    static class FakeUserRepository implements UserRepository {
        private final Map<Long, User> store = new HashMap<>();

        public void save(User u) { store.put(u.id(), u); }

        public Optional<User> findById(long id) {
            return Optional.ofNullable(store.get(id));
        }
    }

    // @Test methods use FakeUserRepository
    void testFindUser() {
        FakeUserRepository repo = new FakeUserRepository();
        // ...
    }
}

class Order {
    private final long id;
    private final List<LineItem> items;

    // Static nested — logically belongs to Order, no outer reference needed
    public static class LineItem {
        public final String sku;
        public final int quantity;
        public final double unitPrice;

        public LineItem(String sku, int qty, double price) {
            this.sku = sku; this.quantity = qty; this.unitPrice = price;
        }

        public double total() { return quantity * unitPrice; }
    }

    // ...
}

class Pair<A, B> {
    // Static nested with its own type params
    public static class Mutable<X, Y> {
        public X first;
        public Y second;
        public Mutable(X first, Y second) {
            this.first = first; this.second = second;
        }
    }

    public static <X, Y> Mutable<X, Y> mutableOf(X x, Y y) {
        return new Mutable<>(x, y);
    }
}
Pair.Mutable<String, Integer> p = Pair.mutableOf("Alice", 30);
System.out.println(p.first + " is " + p.second); // Alice is 30