JavaComparator教程：手把手教你自定义排序

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Comparator在Java中用于定制排序规则，其核心方法是compare(T o1, T o2)，允许开发者自定义对象的比较逻辑。1. 可通过实现Comparator接口并重写compare方法定义排序规则；2. 使用匿名内部类、Lambda表达式或Comparator.comparing()简化创建过程；3. 对List进行排序可使用Collections.sort(List list, Comparator c)或List接口的默认方法list.sort(Comparator c)；4. 链式排序可通过thenComparing()方法实现，例如先按年龄再按姓名排序；5. 处理空指针异常时，可使用Comparator.nullsFirst()或Comparator.nullsLast()指定null值的排序位置。

Comparator在Java中用于定制排序规则，它允许你根据自己的逻辑来比较对象，而不是依赖对象自身的compareTo方法。简单来说，就是你想怎么排，就怎么排。

解决方案

Comparator接口的核心在于compare(T o1, T o2)方法。你需要实现这个方法，定义两个对象o1和o2的比较逻辑。

import java.util.Comparator;

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public static void main(String[] args) {
        Person p1 = new Person("Alice", 30);
        Person p2 = new Person("Bob", 25);
        Person p3 = new Person("Charlie", 30);

        // 使用匿名内部类创建Comparator，按年龄升序排序
        Comparator ageComparator = new Comparator() {
            @Override
            public int compare(Person o1, Person o2) {
                return o1.getAge() - o2.getAge();
            }
        };

        // 使用Lambda表达式简化Comparator，按姓名排序
        Comparator nameComparator = (o1, o2) -> o1.getName().compareTo(o2.getName());

        // 使用Comparator.comparing，按年龄排序
        Comparator ageComparator2 = Comparator.comparing(Person::getAge);


        // 可以看到，即使年龄相同，排序结果也会因为输入顺序而不同
        System.out.println("年龄升序(匿名内部类): " + (ageComparator.compare(p1, p2)));
        System.out.println("姓名升序(Lambda): " + (nameComparator.compare(p1, p2)));
        System.out.println("年龄升序(Comparator.comparing): " + (ageComparator2.compare(p1, p2)));
    }
}

如何使用Comparator对List进行排序？

使用Collections.sort(List list, Comparator c)方法或list.sort(Comparator c)方法。前者是Collections类的静态方法，后者是List接口的默认方法（Java 8+）。

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

public class ComparatorListSort {

    public static void main(String[] args) {
        List people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 30));

        // 使用Collections.sort
        Collections.sort(people, (p1, p2) -> p1.getName().compareTo(p2.getName()));
        System.out.println("Collections.sort: " + people);

        people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 30));


        // 使用list.sort
        people.sort((p1, p2) -> p1.getAge() - p2.getAge());
        System.out.println("list.sort: " + people);
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }
}

Comparator.comparing方法有什么用？

Comparator.comparing()方法简化了Comparator的创建，特别是当排序逻辑基于对象的某个属性时。它接受一个Function作为参数，该Function用于提取用于比较的键。

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

public class ComparatorComparing {

    public static void main(String[] args) {
        List people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 30));

        // 使用Comparator.comparing按年龄排序
        people.sort(Comparator.comparing(Person::getAge));
        System.out.println("Comparator.comparing (Age): " + people);

        people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 30));

        // 使用Comparator.comparing按姓名排序
        people.sort(Comparator.comparing(Person::getName));
        System.out.println("Comparator.comparing (Name): " + people);
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }
}

如何实现Comparator的链式排序？

可以使用thenComparing()方法实现链式排序。例如，先按年龄排序，年龄相同再按姓名排序。

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

public class ComparatorThenComparing {

    public static void main(String[] args) {
        List people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 30));
        people.add(new Person("David", 30));

        // 先按年龄排序，年龄相同再按姓名排序
        people.sort(Comparator.comparing(Person::getAge).thenComparing(Person::getName));
        System.out.println("thenComparing: " + people);
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }
}

如何处理Comparator中的空指针异常？

当用于比较的属性可能为空时，需要小心处理空指针异常。可以使用Comparator.nullsFirst()或Comparator.nullsLast()来指定null值的排序位置。

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

public class ComparatorNulls {

    public static void main(String[] args) {
        List people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person(null, 25));
        people.add(new Person("Charlie", 30));

        // null值排在最前
        people.sort(Comparator.comparing(Person::getName, Comparator.nullsFirst(String::compareTo)));
        System.out.println("nullsFirst: " + people);

        people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person(null, 25));
        people.add(new Person("Charlie", 30));

        // null值排在最后
        people.sort(Comparator.comparing(Person::getName, Comparator.nullsLast(String::compareTo)));
        System.out.println("nullsLast: " + people);
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }
}

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