해시 코드와 배열 인덱스

public final class DirectIndexFailure {
    public static void main(String[] args) {
        String[] values = new String[10];
        values[3] = "hash";
        values[-7] = "collision";
        System.out.println(values[3]);
    }
}

java.lang.ArrayIndexOutOfBoundsException: Index -7 out of bounds for length 10

import java.util.ArrayList;
import java.util.List;

public final class IntegerBucketTable {
    public static void main(String[] args) {
        IntTable table = new IntTable(8);
        table.put(1, "array");
        table.put(9, "hash");
        table.put(-7, "negative");
        System.out.println(table.get(1));
        System.out.println(table.get(9));
        System.out.println(table.get(-7));
        System.out.println("sizes=" + table.bucketSizes());
    }

    private static final class IntTable {
        private final List<List<Entry>> buckets = new ArrayList<>();

        IntTable(int bucketCount) {
            if (bucketCount <= 0) throw new IllegalArgumentException("bucketCount");
            for (int i = 0; i < bucketCount; i++) {
                buckets.add(new ArrayList<>());
            }
        }

        void put(int key, String value) {
            List<Entry> bucket = buckets.get(index(key));
            for (int i = 0; i < bucket.size(); i++) {
                if (bucket.get(i).key() == key) {
                    bucket.set(i, new Entry(key, value));
                    return;
                }
            }
            bucket.add(new Entry(key, value));
        }

        String get(int key) {
            for (Entry entry : buckets.get(index(key))) {
                if (entry.key() == key) {
                    return entry.value();
                }
            }
            return null;
        }

        private int index(int key) {
            return Math.floorMod(Integer.hashCode(key), buckets.size());
        }

        String bucketSizes() {
            StringBuilder out = new StringBuilder();
            for (List<Entry> bucket : buckets) {
                out.append(bucket.size());
            }
            return out.toString();
        }
    }

    private static final class Entry {
        private final int key;
        private final String value;

        private Entry(int key, String value) {
            this.key = key;
            this.value = value;
        }

        private int key() { return key; }
        private String value() { return value; }
    }
}

import java.util.HashMap;
import java.util.Map;

public final class HashedBoardPostCli {
    public static void main(String[] args) {
        Map<Integer, Post> posts = new HashMap<>();
        add(posts, new Post(101, "hash-index", 45));
        add(posts, new Post(205, "collision", 55));
        int requested = 205;
        Post found = posts.get(requested);
        System.out.println("found=" + found.title());
        int total = 0;
        for (Post post : posts.values()) {
            total += post.viewCount();
        }
        System.out.println("total=" + total);
    }

    private static void add(Map<Integer, Post> posts, Post post) {
        if (posts.putIfAbsent(post.id(), post) != null) {
            throw new IllegalArgumentException("duplicate id=" + post.id());
        }
    }

    private static final class Post {
        private final int id;
        private final String title;
        private final int viewCount;

        private Post(int id, String title, int viewCount) {
            if (id <= 0 || title == null || title.isBlank() || viewCount <= 0) {
                throw new IllegalArgumentException("invalid post");
            }
            this.id = id;
            this.title = title;
            this.viewCount = viewCount;
        }

        private int id() { return id; }
        private String title() { return title; }
        private int viewCount() { return viewCount; }
    }
}