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Java : Comparator with Examples

May 2, 2024 March 9, 2022

Comparator (Java SE 21 & JDK 21) with Examples.
You will find code examples on most Comparator<T> methods.


Summary

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order.

Comparator (Java SE 21 & JDK 21)

Class diagram

See also : Comparable

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);

Methods

int compare (T o1, T o2)

Compares its two arguments for order.

Comparator (Java SE 21 & JDK 21) 
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);

static <T, U extends Comparable<? super U>> Comparator<T> comparing (Function<? super T,? extends U> keyExtractor)

Accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator<T> that compares by that sort key.

Comparator (Java SE 21 & JDK 21) 
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);
}

static <T, U> Comparator<T> comparing (Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)

Accepts a function that extracts a sort key from a type T, and returns a Comparator<T> that compares by that sort key using the specified Comparator.

Comparator (Java SE 21 & JDK 21) 
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);

{
    // Compares two strings lexicographically, ignoring case differences.
    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);
}

{
    // Compares two strings lexicographically.
    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);
}

static <T> Comparator<T> comparingDouble (ToDoubleFunction<? super T> keyExtractor)

Accepts a function that extracts a double sort key from a type T, and returns a Comparator<T> that compares by that sort key.

Comparator (Java SE 21 & JDK 21) 
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);

static <T> Comparator<T> comparingInt (ToIntFunction<? super T> keyExtractor)

Accepts a function that extracts an int sort key from a type T, and returns a Comparator<T> that compares by that sort key.

Comparator (Java SE 21 & JDK 21) 
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);

static <T> Comparator<T> comparingLong (ToLongFunction<? super T> keyExtractor)

Accepts a function that extracts a long sort key from a type T, and returns a Comparator<T> that compares by that sort key.

Comparator (Java SE 21 & JDK 21) 
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);

boolean equals (Object obj)

Indicates whether some other object is "equal to" this comparator.

Comparator (Java SE 21 & JDK 21) 
final var comparator1 = Comparator.<String>naturalOrder();
final var comparator2 = Comparator.<String>reverseOrder();

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

static <T extends Comparable<? super T>> Comparator<T> naturalOrder ()

Returns a comparator that compares Comparable objects in natural order.

Comparator (Java SE 21 & JDK 21) 
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]

static <T> Comparator<T> nullsFirst (Comparator<? super T> comparator)

Returns a null-friendly comparator that considers null to be less than non-null.

Comparator (Java SE 21 & JDK 21) 
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!");
    }

    // Result
    // ↓
    //NullPointerException!
}

static <T> Comparator<T> nullsLast (Comparator<? super T> comparator)

Returns a null-friendly comparator that considers null to be greater than non-null.

Comparator (Java SE 21 & JDK 21) 
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!");
    }

    // Result
    // ↓
    //NullPointerException!
}

default Comparator<T> reversed ()

Returns a comparator that imposes the reverse ordering of this comparator.

Comparator (Java SE 21 & JDK 21) 
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]

static <T extends Comparable<? super T>> Comparator<T> reverseOrder ()

Returns a comparator that imposes the reverse of the natural ordering.

Comparator (Java SE 21 & JDK 21) 
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]

default Comparator<T> thenComparing (Comparator<? super T> other)

Returns a lexicographic-order comparator with another comparator.

Comparator (Java SE 21 & JDK 21) 
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]

// Sorts the list by natural order.
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]

// Sorts the list by string length.
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]

// Sorts the list by string length and natural order.
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]

default <U extends Comparable<? super U>> Comparator<T> thenComparing (Function<? super T,? extends U> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

Comparator (Java SE 21 & JDK 21) 
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);

default <U> Comparator<T> thenComparing (Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)

Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.

Comparator (Java SE 21 & JDK 21) 
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);

default Comparator<T> thenComparingDouble (ToDoubleFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a double sort key.

Comparator (Java SE 21 & JDK 21) 
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);

default Comparator<T> thenComparingInt (ToIntFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts an int sort key.

Comparator (Java SE 21 & JDK 21) 
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);

default Comparator<T> thenComparingLong (ToLongFunction<? super T> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a long sort key.

Comparator (Java SE 21 & JDK 21) 
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);

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