Comparator.comparing比较排序

使用外部比较器Comparator进行排序

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 employees = new ArrayList<>(); employees.add(e1); employees.add(e2); employees.add(e3); /** * @SuppressWarnings({"unchecked", "rawtypes"}) * default void sort(Comparator c) { * Object[] a = this.toArray(); * Arrays.sort(a, (Comparator) c); * ListIterator 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 & setters public 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; } @Override public 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 super U>> Comparator comparing( Functionsuper T, ? extends U> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator & 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 employees = new ArrayList<>(); employees.add(e1); employees.add(e2); employees.add(e3); /** * @SuppressWarnings({"unchecked", "rawtypes"}) * default void sort(Comparator c) { * Object[] a = this.toArray(); * Arrays.sort(a, (Comparator) c); * ListIterator 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 & setters public 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; } @Override public 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  Comparator comparing( Functionsuper T, ? extends U> keyExtractor, Comparatorsuper U> keyComparator) { Objects.requireNonNull(keyExtractor); Objects.requireNonNull(keyComparator); return (Comparator & 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); 

 

 

转载：https://my.oschina.net/xinxingegeya/blog/2046405