使用 ComparableT 或 ComparatorT 对列表进行排序

假设我们正在使用它们的名字和姓氏来代表一个 Person。我们已经创建了一个基本类来实现这一点并实现了正确的 equals 和 hashCode 方法。

public class Person {

    private final String lastName; //invariant - nonnull
    private final String firstName; //invariant - nonnull

    public Person(String firstName, String lastName){
        this.firstName = firstName != null ? firstName : "";
        this.lastName = lastName != null ? lastName : "";
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public String toString() {
        return lastName + ", " + firstName;
    }

    @Override
    public boolean equals(Object o) {
        if (! (o instanceof Person)) return false;
        Person p = (Person)o;
        return firstName.equals(p.firstName) && lastName.equals(p.lastName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(firstName, lastName);
    }
}

现在我们想按名称对 Person 对象列表进行排序，例如在以下场景中：

public static void main(String[] args) {
    List<Person> people = Arrays.asList(new Person("John", "Doe"),
                                     new Person("Bob", "Dole"),
                                     new Person("Ronald", "McDonald"),
                                     new Person("Alice", "McDonald"),
                                     new Person("Jill", "Doe"));
    Collections.sort(people); //This currently won't work.
}

不幸的是，如上所述，上面目前不会编译。Collections.sort(..) 只知道如果列表中的元素具有可比性，或者给出自定义比较方法，如何对列表进行排序。

如果你被要求排序以下列表：1,3,5,4,2，你可以毫无疑问地说答案是 1,2,3,4,5。这是因为 Integers（在 Java 和数学上都有）具有自然排序，标准的默认比较基本排序。为了给我们的 Person 类一个自然的顺序，我们实现 Comparable<Person>，这需要实现 compareTo(Person p):的方法

public class Person implements Comparable<Person> {

    private final String lastName; //invariant - nonnull
    private final String firstName; //invariant - nonnull

    public Person(String firstName, String lastName) {
        this.firstName = firstName != null ? firstName : "";
        this.lastName = lastName != null ? lastName : "";
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public String toString() {
        return lastName + ", " + firstName;
    }

    @Override
    public boolean equals(Object o) {
        if (! (o instanceof Person)) return false;
        Person p = (Person)o;
        return firstName.equals(p.firstName) && lastName.equals(p.lastName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(firstName, lastName);
    }

    @Override
    public int compareTo(Person other) {
        // If this' lastName and other's lastName are not comparably equivalent,
        // Compare this to other by comparing their last names.
        // Otherwise, compare this to other by comparing their first names
        int lastNameCompare = lastName.compareTo(other.lastName);
        if (lastNameCompare != 0) {
            return lastNameCompare;
        } else {
            return firstName.compareTo(other.firstName);
        }
    }
}

现在，给出的主要方法将正常运行

public static void main(String[] args) {
    List<Person> people = Arrays.asList(new Person("John", "Doe"),
                                     new Person("Bob", "Dole"),
                                     new Person("Ronald", "McDonald"),
                                     new Person("Alice", "McDonald"),
                                     new Person("Jill", "Doe"));
    Collections.sort(people); //Now functions correctly

    //people is now sorted by last name, then first name:
    // --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald
}

但是，如果你不想要或者不能修改类 Person，你可以提供一个自定义 Comparator<T> 来处理任何两个 Person 对象的比较。如果你被要求对以下列表进行排序：circle, square, rectangle, triangle, hexagon 你不能，但如果你被要求根据角落的数量对该列表进行排序，你可以。就这样，提供比较器指示 Java 如何比较两个通常不可比较的对象。

public class PersonComparator implements Comparator<Person> {

    public int compare(Person p1, Person p2) {
        // If p1's lastName and p2's lastName are not comparably equivalent,
        // Compare p1 to p2 by comparing their last names.
        // Otherwise, compare p1 to p2 by comparing their first names
        if (p1.getLastName().compareTo(p2.getLastName()) != 0) {
            return p1.getLastName().compareTo(p2.getLastName());
        } else {
            return p1.getFirstName().compareTo(p2.getFirstName());
        }
    }
}

//Assume the first version of Person (that does not implement Comparable) is used here
public static void main(String[] args) {
    List<Person> people = Arrays.asList(new Person("John", "Doe"),
                                     new Person("Bob", "Dole"),
                                     new Person("Ronald", "McDonald"),
                                     new Person("Alice", "McDonald"),
                                     new Person("Jill", "Doe"));
    Collections.sort(people); //Illegal, Person doesn't implement Comparable.
    Collections.sort(people, new PersonComparator()); //Legal

    //people is now sorted by last name, then first name:
    // --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald
}

比较器也可以创建/用作匿名内部类

//Assume the first version of Person (that does not implement Comparable) is used here
public static void main(String[] args) {
    List<Person> people = Arrays.asList(new Person("John", "Doe"),
                                     new Person("Bob", "Dole"),
                                     new Person("Ronald", "McDonald"),
                                     new Person("Alice", "McDonald"),
                                     new Person("Jill", "Doe"));
    Collections.sort(people); //Illegal, Person doesn't implement Comparable.

    Collections.sort(people, new PersonComparator()); //Legal

    //people is now sorted by last name, then first name:
    // --> Jill Doe, John Doe, Bob Dole, Alice McDonald, Ronald McDonald

    //Anonymous Class
    Collections.sort(people, new Comparator<Person>() { //Legal
        public int compare(Person p1, Person p2) {
            //Method code...
        }
    });
}

Version >= Java SE 8

基于 Lambda 表达式的比较器

从 Java 8 开始，比较器也可以表示为 lambda 表达式

    //Lambda
    Collections.sort(people, (p1, p2) -> { //Legal
        //Method code....
    });

比较器默认方法

此外，Comparator 接口上有一些有趣的默认方法用于构建比较器：下面构建一个比较器，通过 lastName 和 firstName 进行比较。

Collections.sort(people, Comparator.comparing(Person::getLastName)
                                .thenComparing(Person::getFirstName));

颠倒比较器的顺序

任何比较器也可以使用 reversedMethod 轻松反转，reversedMethod 将升序降序为降序。