Java : Comparator (比較) - API使用例

更新日：2024年05月02日 公開日：2021年09月03日

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Comparator (Java SE 21 & JDK 21) の使い方まとめです。
だいたいのメソッドを網羅済みです。
API仕様書のおともにどうぞ。

概要

Comparator は、２つのオブジェクトの大小比較を実装するための関数型インタフェースです。
主に、Collections.sort や Arrays.sort、Stream.sorted などのソートに使われます。

似た API に、Comparable インタフェースがあります。
Comparable は実装したクラス自体が比較可能になるのに対して、Comparator は比較対象となるクラスに手を加える必要はありません。

以下は Comparator のコード例となります。

record Sample(String text, int num) {
    @Override
    public String toString() {
        return "(%s, %d)".formatted(text, num);
    }
}

final var comparator = new Comparator<Sample>() {
    @Override
    public int compare(Sample o1, Sample o2) {
        if (!o1.text().equals(o2.text())) {
            return o1.text().compareTo(o2.text());
        } else {
            return o1.num() - o2.num();
        }
    }
};

final var list = List.of(
        new Sample("bbb", 2), new Sample("bbb", 3), new Sample("bbb", 1),
        new Sample("aaa", 3), new Sample("aaa", 1), new Sample("aaa", 2)
);

// [(bbb, 2), (bbb, 3), (bbb, 1), (aaa, 3), (aaa, 1), (aaa, 2)]
System.out.println(list);

final var sorted = list.stream().sorted(comparator).toList();

// [(aaa, 1), (aaa, 2), (aaa, 3), (bbb, 1), (bbb, 2), (bbb, 3)]
System.out.println(sorted);

メソッド

record Sample(String text, int num) {
    @Override
    public String toString() {
        return "(%s, %d)".formatted(text, num);
    }
}

final var comparator = new Comparator<Sample>() {
    @Override
    public int compare(Sample o1, Sample o2) {
        if (!o1.text().equals(o2.text())) {
            return o1.text().compareTo(o2.text());
        } else {
            return o1.num() - o2.num();
        }
    }
};

final var list = List.of(
        new Sample("bbb", 2), new Sample("bbb", 3), new Sample("bbb", 1),
        new Sample("aaa", 3), new Sample("aaa", 1), new Sample("aaa", 2)
);

// [(bbb, 2), (bbb, 3), (bbb, 1), (aaa, 3), (aaa, 1), (aaa, 2)]
System.out.println(list);

final var sorted = list.stream().sorted(comparator).toList();

// [(aaa, 1), (aaa, 2), (aaa, 3), (bbb, 1), (bbb, 2), (bbb, 3)]
System.out.println(sorted);

record Sample(String textA, String textB) {
    @Override
    public String toString() {
        return "(%s, %s)".formatted(textA, textB);
    }
}

final var list = List.of(
        new Sample("ddd", "W"), new Sample("ccc", "X"),
        new Sample("bbb", "Z"), new Sample("aaa", "Y")
);

// [(ddd, W), (ccc, X), (bbb, Z), (aaa, Y)]
System.out.println(list);

{
    final var keyExtractor = new Function<Sample, String>() {
        @Override
        public String apply(Sample sample) {
            return sample.textA();
        }
    };

    final var comparator = Comparator.comparing(keyExtractor);
    final var sorted = list.stream().sorted(comparator).toList();

    // [(aaa, Y), (bbb, Z), (ccc, X), (ddd, W)]
    System.out.println(sorted);
}

{
    final var comparator = Comparator.comparing(Sample::textB);
    final var sorted = list.stream().sorted(comparator).toList();

    // [(ddd, W), (ccc, X), (aaa, Y), (bbb, Z)]
    System.out.println(sorted);
}

record Sample(String text, int num) {
    @Override
    public String toString() {
        return "(%s, %d)".formatted(text, num);
    }
}

final var list = List.of(
        new Sample("ccc", 2), new Sample("DDD", 1),
        new Sample("aaa", 4), new Sample("BBB", 3)
);

// [(ccc, 2), (DDD, 1), (aaa, 4), (BBB, 3)]
System.out.println(list);

{
    // 大文字・小文字区別なし。
    final var comparator = Comparator.comparing(
            Sample::text, String.CASE_INSENSITIVE_ORDER);
    final var sorted = list.stream().sorted(comparator).toList();

    // [(aaa, 4), (BBB, 3), (ccc, 2), (DDD, 1)]
    System.out.println(sorted);
}

{
    // 大文字・小文字区別あり。
    final var comparator = Comparator.comparing(Sample::text);
    final var sorted = list.stream().sorted(comparator).toList();

    // [(BBB, 3), (DDD, 1), (aaa, 4), (ccc, 2)]
    System.out.println(sorted);
}

record Sample(double num, String text) {
    @Override
    public String toString() {
        return "(%.1f, %s)".formatted(num, text);
    }
}

final var list = List.of(
        new Sample(2.0, "ccc"), new Sample(3.0, "bbb"),
        new Sample(4.0, "aaa"), new Sample(1.0, "ddd")
);

// [(2.0, ccc), (3.0, bbb), (4.0, aaa), (1.0, ddd)]
System.out.println(list);

final var comparator = Comparator.comparingDouble(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(1.0, ddd), (2.0, ccc), (3.0, bbb), (4.0, aaa)]
System.out.println(sorted);

record Sample(int num, String text) {
    @Override
    public String toString() {
        return "(%d, %s)".formatted(num, text);
    }
}

final var list = List.of(
        new Sample(2, "ccc"), new Sample(3, "bbb"),
        new Sample(4, "aaa"), new Sample(1, "ddd")
);

// [(2, ccc), (3, bbb), (4, aaa), (1, ddd)]
System.out.println(list);

final var comparator = Comparator.comparingInt(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(1, ddd), (2, ccc), (3, bbb), (4, aaa)]
System.out.println(sorted);

record Sample(long num, String text) {
    @Override
    public String toString() {
        return "(%d, %s)".formatted(num, text);
    }
}

final var list = List.of(
        new Sample(20000000000L, "ccc"), new Sample(30000000000L, "bbb"),
        new Sample(40000000000L, "aaa"), new Sample(10000000000L, "ddd")
);

// [(20000000000, ccc), (30000000000, bbb), (40000000000, aaa), (10000000000, ddd)]
System.out.println(list);

final var comparator = Comparator.comparingLong(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(10000000000, ddd), (20000000000, ccc), (30000000000, bbb), (40000000000, aaa)]
System.out.println(sorted);

final var comparator1 = Comparator.<String>naturalOrder();
final var comparator2 = Comparator.<String>reverseOrder();

System.out.println(comparator1.equals(comparator2)); // false

final var comparator = Comparator.<String>naturalOrder();

final var list = List.of("ccc", "ddd", "aaa", "bbb");
final var sorted = list.stream().sorted(comparator).toList();

System.out.println(list); // [ccc, ddd, aaa, bbb]
System.out.println(sorted); // [aaa, bbb, ccc, ddd]

final var list = new ArrayList<String>();
list.add("ccc");
list.add("ddd");
list.add(null);
list.add("aaa");
list.add("bbb");

// [ccc, ddd, null, aaa, bbb]
System.out.println(list);

final var comparator = Comparator.<String>naturalOrder();

{
    final var nullsFirstComparator = Comparator.nullsFirst(comparator);
    final var sorted = list.stream().sorted(nullsFirstComparator).toList();

    // [null, aaa, bbb, ccc, ddd]
    System.out.println(sorted);
}

{
    final var nullsLastComparator = Comparator.nullsLast(comparator);
    final var sorted = list.stream().sorted(nullsLastComparator).toList();

    // [aaa, bbb, ccc, ddd, null]
    System.out.println(sorted);
}

{
    try {
        final var sorted = list.stream().sorted(comparator).toList();
    } catch (NullPointerException e) {
        System.out.println("NullPointerException!");
    }

    // 結果
    // ↓
    //NullPointerException!
}

final var list = new ArrayList<String>();
list.add("ccc");
list.add("ddd");
list.add(null);
list.add("aaa");
list.add("bbb");

// [ccc, ddd, null, aaa, bbb]
System.out.println(list);

final var comparator = Comparator.<String>naturalOrder();

{
    final var nullsFirstComparator = Comparator.nullsFirst(comparator);
    final var sorted = list.stream().sorted(nullsFirstComparator).toList();

    // [null, aaa, bbb, ccc, ddd]
    System.out.println(sorted);
}

{
    final var nullsLastComparator = Comparator.nullsLast(comparator);
    final var sorted = list.stream().sorted(nullsLastComparator).toList();

    // [aaa, bbb, ccc, ddd, null]
    System.out.println(sorted);
}

{
    try {
        final var sorted = list.stream().sorted(comparator).toList();
    } catch (NullPointerException e) {
        System.out.println("NullPointerException!");
    }

    // 結果
    // ↓
    //NullPointerException!
}

final var comparator = Comparator.<String>naturalOrder();
final var reversedComparator = comparator.reversed();

final var list = List.of("ccc", "ddd", "aaa", "bbb");
System.out.println(list); // [ccc, ddd, aaa, bbb]

final var sorted = list.stream().sorted(comparator).toList();
final var reversedSorted = list.stream().sorted(reversedComparator).toList();

System.out.println(sorted); // [aaa, bbb, ccc, ddd]
System.out.println(reversedSorted); // [ddd, ccc, bbb, aaa]

final var comparator = Comparator.<String>reverseOrder();

final var list = List.of("ccc", "ddd", "aaa", "bbb");
final var sorted = list.stream().sorted(comparator).toList();

System.out.println(list); // [ccc, ddd, aaa, bbb]
System.out.println(sorted); // [ddd, ccc, bbb, aaa]

final var list = List.of("aaa", "bbb", "ccc", "b", "c", "a", "cc", "aa", "bb");
System.out.println(list); // [aaa, bbb, ccc, b, c, a, cc, aa, bb]

// 自然順序によるソート
final var naturalComparator = Comparator.<String>naturalOrder();
final var naturalSorted = list.stream().sorted(naturalComparator).toList();
System.out.println(naturalSorted); // [a, aa, aaa, b, bb, bbb, c, cc, ccc]

// 文字列の長さでソート
final var lengthComparator = Comparator.comparingInt(String::length);
final var lengthSorted = list.stream().sorted(lengthComparator).toList();
System.out.println(lengthSorted); // [b, c, a, cc, aa, bb, aaa, bbb, ccc]

// 文字列の長さ -> 自然順序でソート
final var comparator = lengthComparator.thenComparing(naturalComparator);
final var sorted = list.stream().sorted(comparator).toList();
System.out.println(sorted); // [a, b, c, aa, bb, cc, aaa, bbb, ccc]

record Sample(int num, String text) {
    @Override
    public String toString() {
        return "(%d, %s)".formatted(num, text);
    }
}

final var list = List.of(
        new Sample(2, "ccc"), new Sample(2, "bbb"), new Sample(2, "aaa"),
        new Sample(1, "bbb"), new Sample(1, "ccc"), new Sample(1, "aaa")
);

// [(2, ccc), (2, bbb), (2, aaa), (1, bbb), (1, ccc), (1, aaa)]
System.out.println(list);

final var numComparator = Comparator.comparing(Sample::num);
final var numSorted = list.stream().sorted(numComparator).toList();

// [(1, bbb), (1, ccc), (1, aaa), (2, ccc), (2, bbb), (2, aaa)]
System.out.println(numSorted);

final var comparator = numComparator.thenComparing(Sample::text);
final var sorted = list.stream().sorted(comparator).toList();

// [(1, aaa), (1, bbb), (1, ccc), (2, aaa), (2, bbb), (2, ccc)]
System.out.println(sorted);

record Sample(int num, String text) {
    @Override
    public String toString() {
        return "(%d, %s)".formatted(num, text);
    }
}

final var list = List.of(
        new Sample(2, "ccc"), new Sample(2, "BBB"), new Sample(2, "aaa"),
        new Sample(1, "bbb"), new Sample(1, "CCC"), new Sample(1, "AAA")
);

// [(2, ccc), (2, BBB), (2, aaa), (1, bbb), (1, CCC), (1, AAA)]
System.out.println(list);

final var numComparator = Comparator.comparing(Sample::num);
final var numSorted = list.stream().sorted(numComparator).toList();

// [(1, bbb), (1, CCC), (1, AAA), (2, ccc), (2, BBB), (2, aaa)]
System.out.println(numSorted);

final var comparator = numComparator.thenComparing(
        Sample::text, String.CASE_INSENSITIVE_ORDER);
final var sorted = list.stream().sorted(comparator).toList();

// [(1, AAA), (1, bbb), (1, CCC), (2, aaa), (2, BBB), (2, ccc)]
System.out.println(sorted);

record Sample(String text, double num) {
    @Override
    public String toString() {
        return "(%s, %.1f)".formatted(text, num);
    }
}

final var list = List.of(
        new Sample("bbb", 2.0), new Sample("bbb", 3.0), new Sample("bbb", 1.0),
        new Sample("aaa", 3.0), new Sample("aaa", 1.0), new Sample("aaa", 2.0)
);

// [(bbb, 2.0), (bbb, 3.0), (bbb, 1.0), (aaa, 3.0), (aaa, 1.0), (aaa, 2.0)]
System.out.println(list);

final var textComparator = Comparator.comparing(Sample::text);
final var textSorted = list.stream().sorted(textComparator).toList();

// [(aaa, 3.0), (aaa, 1.0), (aaa, 2.0), (bbb, 2.0), (bbb, 3.0), (bbb, 1.0)]
System.out.println(textSorted);

final var comparator = textComparator.thenComparingDouble(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(aaa, 1.0), (aaa, 2.0), (aaa, 3.0), (bbb, 1.0), (bbb, 2.0), (bbb, 3.0)]
System.out.println(sorted);

record Sample(String text, int num) {
    @Override
    public String toString() {
        return "(%s, %d)".formatted(text, num);
    }
}

final var list = List.of(
        new Sample("bbb", 2), new Sample("bbb", 3), new Sample("bbb", 1),
        new Sample("aaa", 3), new Sample("aaa", 1), new Sample("aaa", 2)
);

// [(bbb, 2), (bbb, 3), (bbb, 1), (aaa, 3), (aaa, 1), (aaa, 2)]
System.out.println(list);

final var textComparator = Comparator.comparing(Sample::text);
final var textSorted = list.stream().sorted(textComparator).toList();

// [(aaa, 3), (aaa, 1), (aaa, 2), (bbb, 2), (bbb, 3), (bbb, 1)]
System.out.println(textSorted);

final var comparator = textComparator.thenComparingInt(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(aaa, 1), (aaa, 2), (aaa, 3), (bbb, 1), (bbb, 2), (bbb, 3)]
System.out.println(sorted);

record Sample(String text, long num) {
    @Override
    public String toString() {
        return "(%s, %d)".formatted(text, num);
    }
}

final var list = List.of(
        new Sample("bbb", 2), new Sample("bbb", 3), new Sample("bbb", 1),
        new Sample("aaa", 3), new Sample("aaa", 1), new Sample("aaa", 2)
);

// [(bbb, 2), (bbb, 3), (bbb, 1), (aaa, 3), (aaa, 1), (aaa, 2)]
System.out.println(list);

final var textComparator = Comparator.comparing(Sample::text);
final var textSorted = list.stream().sorted(textComparator).toList();

// [(aaa, 3), (aaa, 1), (aaa, 2), (bbb, 2), (bbb, 3), (bbb, 1)]
System.out.println(textSorted);

final var comparator = textComparator.thenComparingLong(Sample::num);
final var sorted = list.stream().sorted(comparator).toList();

// [(aaa, 1), (aaa, 2), (aaa, 3), (bbb, 1), (bbb, 2), (bbb, 3)]
System.out.println(sorted);

関連記事

• List(リスト)の基本
• Map(マップ)の基本
• Set(セット)の基本
• 配列 vs. List

• API 使用例
  • Collection (コレクション)
    • AbstractCollection
    • SequencedCollection
  • Collections (コレクション操作)
  • Comparable
  • Iterator
    • ListIterator
  • List (リスト)
    • AbstractList
    • AbstractSequentialList
    • ArrayList
    • CopyOnWriteArrayList (並列処理用リスト)
    • LinkedList (二重リンク・リスト)
  • Map (マップ)
    • AbstractMap
    • ConcurrentHashMap (並列処理用マップ)
    • ConcurrentMap (並列処理用マップ)
    • ConcurrentSkipListMap (並列処理用マップ)
    • EnumMap (列挙型のマップ)
    • HashMap (ハッシュマップ)
    • LinkedHashMap (順序を保持するマップ)
    • NavigableMap
    • SequencedMap
    • SortedMap
    • TreeMap (ソートされたマップ)
  • Map.Entry (キーと値のペア)
    • AbstractMap.SimpleEntry
    • AbstractMap.SimpleImmutableEntry
  • Queue (キュー)
    • AbstractQueue
    • ArrayBlockingQueue (ブロッキング・配列キュー)
    • ArrayDeque (両端キュー)
    • BlockingDeque (ブロッキング・両端キュー)
    • BlockingQueue (ブロッキング・キュー)
    • ConcurrentLinkedDeque (並列処理用・両端キュー)
    • ConcurrentLinkedQueue (並列処理用キュー)
    • Deque (両端キュー)
    • LinkedBlockingDeque (ブロッキング・リンク両端キュー)
    • LinkedBlockingQueue (ブロッキング・リンクキュー)
    • LinkedList (二重リンク・リスト)
    • PriorityBlockingQueue (ブロッキング・優先度付きキュー)
    • PriorityQueue (優先度付きキュー)
  • RandomAccess
  • Set (セット)
    • AbstractSet
    • ConcurrentHashMap.KeySetView (並列処理用セット)
    • ConcurrentSkipListSet (並列処理用セット)
    • CopyOnWriteArraySet (並列処理用セット)
    • HashSet (ハッシュセット)
    • LinkedHashSet (順序を保持するセット)
    • NavigableSet
    • SequencedSet
    • SortedSet
    • TreeSet (ソートされたセット)
  • Spliterator