当我们需要对集合的元素进行排序的时候，可以使用java.util.Comparator 创建一个比较器来进行排序。Comparator接口同样也是一个函数式接口，我们可以把使用lambda表达式。如下示例，

package com.common;import java.util.*;
import java.util.stream.Collectors;public class ComparatorTest {public static void main(String[] args) {Employee e1 = new Employee("John", 25, 3000, 9922001);Employee e2 = new Employee("Ace", 22, 2000, 5924001);Employee e3 = new Employee("Keith", 35, 4000, 3924401);List<Employee> employees = new ArrayList<>();employees.add(e1);employees.add(e2);employees.add(e3);/***     @SuppressWarnings({"unchecked", "rawtypes"})*     default void sort(Comparator<? super E> c) {*         Object[] a = this.toArray();*         Arrays.sort(a, (Comparator) c);*         ListIterator<E> i = this.listIterator();*         for (Object e : a) {*             i.next();*             i.set((E) e);*         }*     }**     sort 对象接收一个 Comparator 函数式接口，可以传入一个lambda表达式*/employees.sort((o1, o2) -> o1.getName().compareTo(o2.getName()));Collections.sort(employees, (o1, o2) -> o1.getName().compareTo(o2.getName()));employees.forEach(System.out::println);}
}/*** [Employee(name=John, age=25, salary=3000.0, mobile=9922001),* Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),* Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]*/
class Employee {String name;int age;double salary;long mobile;// constructors, getters & setterspublic Employee(String name, int age, double salary, long mobile) {this.name = name;this.age = age;this.salary = salary;this.mobile = mobile;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public long getMobile() {return mobile;}public void setMobile(long mobile) {this.mobile = mobile;}@Overridepublic String toString() {final StringBuilder sb = new StringBuilder("Employee{");sb.append("name='").append(name).append('\'');sb.append(", age=").append(age);sb.append(", salary=").append(salary);sb.append(", mobile=").append(mobile);sb.append('}');return sb.toString();}
}

使用 Comparator.comparing 进行排序

comparing 方法一

查看 Comparator 类内部实现，还有一个 comparing 方法，实现如下，

 public static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor){Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));}

其返回值是 (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2)); 一个lambda表达式，也就是一个Compator 。所以上面那个例子也可以改造成 如下，

package com.common;import java.util.*;public class ComparatorTest {public static void main(String[] args) {Employee e1 = new Employee("John", 25, 3000, 9922001);Employee e2 = new Employee("Ace", 22, 2000, 5924001);Employee e3 = new Employee("Keith", 35, 4000, 3924401);List<Employee> employees = new ArrayList<>();employees.add(e1);employees.add(e2);employees.add(e3);/***     @SuppressWarnings({"unchecked", "rawtypes"})*     default void sort(Comparator<? super E> c) {*         Object[] a = this.toArray();*         Arrays.sort(a, (Comparator) c);*         ListIterator<E> i = this.listIterator();*         for (Object e : a) {*             i.next();*             i.set((E) e);*         }*     }**     sort 对象接收一个 Comparator 函数式接口，可以传入一个lambda表达式*/employees.sort((o1, o2) -> o1.getName().compareTo(o2.getName()));Collections.sort(employees, (o1, o2) -> o1.getName().compareTo(o2.getName()));employees.forEach(System.out::println);/*** Comparator.comparing 方法的使用** comparing 方法接收一个 Function 函数式接口 ，通过一个 lambda 表达式传入**/employees.sort(Comparator.comparing(e -> e.getName()));/*** 该方法引用 Employee::getName 可以代替 lambda表达式*/employees.sort(Comparator.comparing(Employee::getName));}
}/*** [Employee(name=John, age=25, salary=3000.0, mobile=9922001),* Employee(name=Ace, age=22, salary=2000.0, mobile=5924001),* Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]*/
class Employee {String name;int age;double salary;long mobile;// constructors, getters & setterspublic Employee(String name, int age, double salary, long mobile) {this.name = name;this.age = age;this.salary = salary;this.mobile = mobile;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public long getMobile() {return mobile;}public void setMobile(long mobile) {this.mobile = mobile;}@Overridepublic String toString() {final StringBuilder sb = new StringBuilder("Employee{");sb.append("name='").append(name).append('\'');sb.append(", age=").append(age);sb.append(", salary=").append(salary);sb.append(", mobile=").append(mobile);sb.append('}');return sb.toString();}
}

comparing 方法二

 public static <T, U> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor,Comparator<? super U> keyComparator){Objects.requireNonNull(keyExtractor);Objects.requireNonNull(keyComparator);return (Comparator<T> & Serializable)(c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),keyExtractor.apply(c2));}

和comparing 方法一不同的是 该方法多了一个参数 keyComparator ，keyComparator 是创建一个自定义的比较器。

Collections.sort(employees, Comparator.comparing(Employee::getName, (s1, s2) -> {return s2.compareTo(s1);}));

使用 Comparator.reversed 进行排序

返回相反的排序规则，

/***  相反的排序规则*/
Collections.sort(employees, Comparator.comparing(Employee::getName).reversed());employees.forEach(System.out::println);

输出，

Employee{name='Keith', age=35, salary=4000.0, mobile=3924401}
Employee{name='John', age=25, salary=3000.0, mobile=9922001}
Employee{name='Ace', age=22, salary=2000.0, mobile=5924001}

使用 Comparator.nullsFirst进行排序

当集合中存在null元素时，可以使用针对null友好的比较器，null元素排在集合的最前面

employees.add(null);  //插入一个null元素
Collections.sort(employees, Comparator.nullsFirst(Comparator.comparing(Employee::getName)));
employees.forEach(System.out::println);Collections.sort(employees, Comparator.nullsLast(Comparator.comparing(Employee::getName)));
employees.forEach(System.out::println);

使用 Comparator.thenComparing 排序

首先使用 name 排序，紧接着在使用ege 排序，来看下使用效果

Collections.sort(employees, Comparator.comparing(Employee::getAge).thenComparing(Employee::getName));
employees.forEach(System.out::println);