两个 ArrayList/List 对象的排序

Question

例如，我将两个数组转换为 ArrayList，即 firstName 和 lastName。 我想使用名字对这两个列表进行排序，姓氏将跟随名字。

预期输出：

firstNameList = {Andrew, Johnson, William}
lastNameList = {Wiggins, Beru, Dasovich};

我的初始程序：

import java.util.Arrays;
import java.util.ArrayList;
import java.util.Collections;

String [] firstName = {William, Johnson, Andrew};
String [] lastName = {Dasovich, Beru, Wiggins};

//Will convert arrays above into list.
List <String> firstNameList= new ArrayList<String>();
List <String> lastNameList= new ArrayList<String>();

//Conversion
Collections.addAll(firstNameList, firstName);
Collections.addAll(lastNameList, lastName);

Answer 1

领域

正如我在评论中所述，我建议使用Person - POJO以语义方式绑定firstName和lastName ：

class Person {
    public static final String PERSON_TO_STRING_FORMAT = "{f: %s, l: %s}";

    private final String firstName;
    private final String lastName;

    private Person(final String firstName, final String lastName) {
        this.firstName = Objects.requireNonNull(firstName);
        this.lastName = Objects.requireNonNull(lastName);
    }

    public static Person of(final String firstName, final String lastName) {
        return new Person(firstName, lastName);
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    @Override
    public String toString() {
        return String.format(PERSON_TO_STRING_FORMAT, getFirstName(), getLastName());
    }
}

要将两个String[]的firstNames和lastNames转换为List<Person> ，可以提供一种方法：

    public static List<Person> constructPersons(
            final String[] firstNames,
            final String[] lastNames) {
        if (firstNames.length != lastNames.length) {
            throw new IllegalArgumentException("firstNames and lastNames must have same length");
        }
        return IntStream.range(0, firstNames.length)
                .mapToObj(index -> Person.of(firstNames[index], lastNames[index]))
                .collect(Collectors.toCollection(ArrayList::new));
    }

关于此方法的备注：在这里，我们使用collect(Collectors.toCollection(...))而不是collect(Collectors.toList())来控制列表可变性，因为我们要对列表进行排序。

从这里开始有两种一般路线：一种是通过public class Person implements Comparable<Person>使Person 具有可比性，另一种是编写Comparator<Person> 。 我们将讨论这两种可能性。

---

挑战

目标是对Person对象进行排序。 排序的主要标准是人的名字。 如果两个人的名字相同，则应按姓氏排序。 名字和姓氏都是String对象，应该按字典顺序排序，这是String的自然顺序。

---

解决方案 1：在Person上实现Comparable<Person>

实现比较的逻辑很简单：

1. 使用equals(...)比较两个人的firstName 。
2. 如果它们相等，则使用compareTo(...)比较lastName并返回结果。
3. 否则，将firstName与compareTo(...)进行比较并返回结果。

相应的方法将如下所示：

public class Person implements Comparable<Person> {
    ...
    @Override
    public final int compareTo(final Person that) {
        if (Objects.equals(getFirstName(), that.getFirstName())) {
            return getLastName().compareTo(that.getLastName());
        }
        return getFirstName().compareTo(that.getFirstName());
    }
    ...
}

虽然不是绝对必要的，但建议类的自然顺序（即Comparable实现）与其equals(...)一致。 由于现在情况并非如此，我建议覆盖equals(...)和hashCode() ：

public class Person implements Comparable<Person> {
    ...
    @Override
    public final boolean equals(Object thatObject) {
        if (this == thatObject) {
            return true;
        }
        if (thatObject == null || getClass() != thatObject.getClass()) {
            return false;
        }
        final Person that = (Person) thatObject;
        return Objects.equals(getFirstName(), that.getFirstName()) &&
                Objects.equals(getLastName(), that.getLastName());
    }

    @Override
    public final int hashCode() {
        return Objects.hash(getFirstName(), getLastName());
    }
    ...
}

以下代码演示了如何从两个String[]创建和排序List<Person> ：

final List<Person> persons = constructPersons(
        new String[]{"Clair", "Alice", "Bob", "Alice"},
        new String[]{"Clear", "Wonder", "Builder", "Ace"}
);
Collections.sort(persons);
System.out.println(persons);

---

解决方案 2：实现Comparator<Person>

实现挑战部分中给出的排序比较的比较器的传统实现可能如下所示：

class PersonByFirstNameThenByLastNameComparator implements Comparator<Person> {
    public static final PersonByFirstNameThenByLastNameComparator INSTANCE =
            new PersonByFirstNameThenByLastNameComparator();

    private PersonByFirstNameThenByLastNameComparator() {}

    @Override
    public int compare(final Person lhs, final Person rhs) {
        if (Objects.equals(lhs.getFirstName(), rhs.getFirstName())) {
            return lhs.getLastName().compareTo(rhs.getLastName());
        }
        return lhs.getFirstName().compareTo(rhs.getFirstName());
    }
}

示例调用可能如下所示：

final List<Person> persons = constructPersons(
        new String[]{"Clair", "Alice", "Bob", "Alice"},
        new String[]{"Clear", "Wonder", "Builder", "Ace"}
);
persons.sort(PersonByFirstNameThenByLastNameComparator.INSTANCE);
System.out.println(persons);

在 Java 8 中，通过Comparator.comparing -API简化了Comparator的构造。 要使用Comparator -API 定义实现挑战部分中给出的顺序的Comparator.comparing ，我们只需要一行代码：

Comparator.comparing(Person::getFirstName)
    .thenComparing(Person::getLastName)

以下代码演示了如何使用此Comparator对List<Person>进行排序：

final List<Person> persons = constructPersons(
        new String[]{"Clair", "Alice", "Bob", "Alice"},
        new String[]{"Clear", "Wonder", "Builder", "Ace"}
);
persons.sort(Comparator.comparing(Person::getFirstName)
    .thenComparing(Person::getLastName));
System.out.println(persons);

---

结束语

一个MRE可以用Ideone 。

我会质疑将名字和姓氏拆分为两个单独数组的初始设计决定。 我选择不包括该方法List<Person> constructPersons(String[] firstNames, String[] lastNames)类Person因为这仅仅是适配器代码。 它应该包含在某个映射器中，但不是Person存在的功能。

Answer 2

您可以通过将两个数组合并到一个 Names 流中来实现这一点，使用名字和姓氏，对该流进行排序，然后重新创建两个列表。

    String[] firstName = {"William", "Johnson", "Andrew"};
    String[] lastName = {"Dasovich", "Beru", "Wiggins"};

    final var sortedNames = IntStream.range(0, firstName.length)
            .mapToObj(i -> new Name(firstName[i], lastName[i]))
            .sorted(Comparator.comparing(n -> n.firstName))
            .collect(Collectors.toList());

    final var sortedFirstNames = sortedNames.stream()
            .map(n -> n.firstName)
            .collect(Collectors.toList());
    final var sortedLastNames = sortedNames.stream()
            .map(n -> n.lastName)
            .collect(Collectors.toList()); 

Answer 3

正如评论所强调的那样，您的问题是您对姓名和姓氏使用了两个不同的列表，因此这两种类型的数据的排序过程完全无关。 一个可能的解决方案是创建一个包含两个字段name和surname的新类Person并实现Comparable接口，如下所示：

public class Person implements Comparable<Person> {
    public String firstName;
    public String lastName;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "Person [firstName=" + firstName + ", lastName=" + lastName + "]";
    }

    @Override
    public int compareTo(Person o) {
        return this.firstName.compareTo(o.firstName);
    }

    public static void main(String[] args) {
        Person[] persons = { new Person("William", "Dasovich"),
                             new Person("Johnson", "Beru"),
                             new Person("Andrew", "Wiggins") };

        Collections.sort(Arrays.asList(persons));
        for (Person person : persons) {
            System.out.println(person);
        }
    }
}

Collections.sort方法按firstName提供Person数组的顺序。

Answer 4

因为firstName和lastName相互连接，所以您应该创建一个类来为它们建模。 让我们称这个类为Person ：

class Person {
    private final String firstName;
    private final String lastName;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    // Add toString, equals and hashCode as well.
}

现在，创建一个人员列表：

List<Person> persons = Arrays.asList(
    new Person("Andrew", "Wiggins"),
    new Person("Johnson", "Beru"),
    new Person("William", "Dasovich"));

现在，要对其进行排序，您可以在带有比较器的流上使用sorted方法。 这将创建一个新的List<Person>将被排序。 Comparator.comparing函数将让您选择要排序的Person类的哪个属性。 像这样的东西：

List<Person> sortedPersons = persons.stream()
        .sorted(Comparator.comparing(Person::getFirstName))
        .collect(Collectors.toList());

Answer 5

TreeSet 可以做到：
（使用 Turing85 建议的 Person 类）

import java.util.Set;
import java.util.TreeSet;

public class PersonTest {

    private static class Person implements Comparable<Person> {

        private final String firstName;
        private final String lastName;

        public Person(final String firstName, final String lastName) {
            this.firstName = firstName;
            this.lastName  = lastName;
        }

        @Override
        public int compareTo(final Person otherPerson) {
            return this.firstName.compareTo(otherPerson.firstName);
        }

        @Override
        public String toString() {
            return this.firstName + " " + this.lastName;
        }
    }

    public static void main(final String[] args) {

        final Set<Person> people = new TreeSet<>();
        /**/              people.add(new Person("William", "Dasovich"));
        /**/              people.add(new Person("Johnson", "Beru"));
        /**/              people.add(new Person("Andrew",  "Wiggins"));

        people.forEach(System.out::println);
    }
}

但是 Streams 和一个稍微简单的 Person 类也可以这样做：

import java.util.stream.Stream;

public class PersonTest {

    private static class Person {

        private final String firstName;
        private final String lastName;

        public Person(final String firstName, final String lastName) {
            this.firstName = firstName;
            this.lastName  = lastName;
        }

        @Override
        public String toString() {
            return this.firstName + " " + this.lastName;
        }
    }

    public static void main(final String[] args) {

        Stream.of(
                new Person("William", "Dasovich"),
                new Person("Johnson", "Beru"    ),
                new Person("Andrew",  "Wiggins" ) )

            .sorted ((p1,p2) -> p1.firstName.compareTo(p2.firstName))
            .peek   (System.out::println)

            .sorted ((p1,p2) -> p1.lastName .compareTo(p2.lastName))
            .forEach(System.out::println);
    }
}

Answer 6

    String[] firstName = {"William", "Johnson", "Andrew"};
    String[] lastName = {"Dasovich", "Beru", "Wiggins"};

    // combine the 2 arrays and add the full name to an Array List 
    // here using a special character to combine, so we can use the same to split them later
    // Eg. "William # Dasovich"
    List<String> combinedList = new ArrayList<String>();
    String combineChar = " # ";        
    for (int i = 0; i < firstName.length; i++) {
        combinedList.add(firstName[i] + combineChar + lastName[i]);
    }
    // Sort the list 
    Collections.sort(combinedList);

    // create 2 empty lists
    List<String> firstNameList = new ArrayList<String>();
    List<String> lastNameList = new ArrayList<String>();

    // iterate the combined array and split the sorted names to two lists
    for (String s : combinedList) {
        String[] arr = s.split(combineChar);
        firstNameList.add(arr[0]);
        lastNameList.add(arr[1]);
    }
    System.out.println(firstNameList);
    System.out.println(lastNameList);

Answer 7

如果你不想创建 DTO 来保持名字和姓氏在一起，你可以使用一种基于 java 流的函数方式：

1. 用列表创建对来绑定这两个值
2. 根据名字对它们进行排序
3. 把这对夫妇弄平，以便有一个一维的列表

 String[] firstName = {"William", "Johnson", "Andrew"};
 String[] lastName = {"Dasovich", "Beru", "Wiggins"};

//Will convert arrays above into list.
        List<String> firstNameList = new ArrayList<String>();
        List<String> lastNameList = new ArrayList<String>();

//Conversion
        Collections.addAll(firstNameList, firstName);
        Collections.addAll(lastNameList, lastName);

        List<String> collect = firstNameList
                .stream()
                .map(name -> {
                    List<String> couple = List.of(name, lastNameList.get(0));
                    lastNameList.remove(0);
                    return couple;
                })
                .sorted(Comparator.comparing(l -> l.get(0)))
                .flatMap(Collection::stream)
                .collect(Collectors.toList());

Answer 8

    String[] firstNames = {William, Johnson, Andrew};
    String[] lastNames = {Dasovich, Beru, Wiggins};

    //Will convert arrays above into list.
    List<String> firstNameList = new ArrayList<String>();
    List<String> lastNameList = new ArrayList<String>();


    Map<String, String> lastNameByFirstName = new HashMap<>();
    for (int i = 0; i < firstNames.length; i++) {
        lastNameByFirstName.put(firstNames[i], lastNames[i]);
    }

    //Conversion
    Collections.addAll(firstNameList, firstNames);
    Collections.sort(firstNameList);
    for (String firstName : firstNameList) {
        lastNameList.add(lastNameByFirstName.get(firstName));
    }