適切なネスト方法の選択

// Top-level: used across many packages
public class Product {
    // ...
}

public interface Repository<T, ID> {
    Optional<T> findById(ID id);
    void save(T entity);
}

// These belong in separate .java files and are widely imported

class Graph {
    // Edge only makes sense in the context of a Graph
    public static class Edge {
        public final int from, to, weight;
        public Edge(int from, int to, int weight) {
            this.from = from; this.to = to; this.weight = weight;
        }
    }

    private List<Edge> edges = new ArrayList<>();

    public void addEdge(int from, int to, int weight) {
        edges.add(new Edge(from, to, weight));
    }
}

class DataStore<T> {
    private final List<T> data;

    DataStore(List<T> data) { this.data = data; }

    // Must access data — inner class makes sense
    class Snapshot {
        private final List<T> copy;

        Snapshot() { this.copy = List.copyOf(data); } // accesses outer

        T get(int idx) { return copy.get(idx); }
        int size()     { return copy.size(); }
    }

    Snapshot snapshot() { return new Snapshot(); }
}

static int countWords(String text, boolean caseSensitive) {
    class WordCounter {
        Map<String, Integer> counts = new HashMap<>();

        void count(String word) {
            String key = caseSensitive ? word : word.toLowerCase();
            counts.merge(key, 1, Integer::sum);
        }

        int uniqueCount() { return counts.size(); }
    }

    WordCounter wc = new WordCounter();
    for (String w : text.split("\\s+")) wc.count(w);
    return wc.uniqueCount();
}
System.out.println(countWords("the cat sat on the mat", false)); // 5

// Lambda: clean for SAM
list.sort((a, b) -> a.compareTo(b));

// Anonymous: multi-method, or extending abstract class
java.util.TimerTask task = new java.util.TimerTask() {
    @Override public void run() { System.out.println("Tick!"); }
};
// TimerTask is abstract class — can't use lambda directly

class HttpClient {
    // Static nested: request config has no outer reference
    public static class RequestConfig {
        public final int timeoutMs;
        public final String userAgent;
        RequestConfig(int timeout, String ua) { this.timeoutMs = timeout; this.userAgent = ua; }
    }

    // Static nested: response is standalone data carrier
    public static class Response {
        public final int statusCode;
        public final String body;
        Response(int code, String body) { this.statusCode = code; this.body = body; }
    }

    private final RequestConfig config;
    HttpClient(RequestConfig config) { this.config = config; }

    public Response get(String url) {
        // Implementation using this.config
        return new Response(200, "body");
    }
}

// In package com.example.order:

// Package-private: visible in package, no nesting
class OrderLine {  // no 'public' modifier
    final String sku;
    final int qty;
    OrderLine(String sku, int qty) { this.sku = sku; this.qty = qty; }
}

// Public class in same package can use OrderLine freely
public class Order {
    private List<OrderLine> lines = new ArrayList<>();
    // ...
}

// Instead of nested inner class hierarchy:
sealed interface Shape permits Circle, Rectangle, Triangle {}
record Circle(double r) implements Shape {}
record Rectangle(double w, double h) implements Shape {}
record Triangle(double b, double h) implements Shape {}

static double area(Shape s) {
    return switch (s) {
        case Circle c      -> Math.PI * c.r() * c.r();
        case Rectangle r   -> r.w() * r.h();
        case Triangle t    -> 0.5 * t.b() * t.h();
    };
}