在编写程序时，不可避免的一个问题就是排序。在Java代码中如何对一个集合进行排序？我们今天就整理一个Java中用于排序的工具。

1.原子类型数组排序

对于原子类型数组排序，Arrays.sort。且只能由小到大的排序。无法使用Comparator

Arrays.sort(array);

2.引用类型数组排序

引用类型数组或者集合排序，就可以使用Comparator

        Integer[] array = {3,2,6,1,7,2,78,11};
        Arrays.sort(array, (x,y)->y-x);
        //结果：[78, 11, 7, 6, 3, 2, 2, 1]
        System.out.println(Arrays.toString(array));

Comparator需要重写一个，2个入参和返回1个整型的结果
整数的正、负代表2个数是否调换位置，y是x后面的数
- +：说明需要调换，y，x
- -：说明不需要调换 x,y
- 0: 也不用调换
  3.引用类型集合排序
```
   List<Integer> integers = Arrays.asList(3, 2, 6, 1, 7, 2, 78, 11);
   Collections.sort(integers); 
   //[1, 2, 2, 3, 6, 7, 11, 78]
   System.out.println(integers);
   Collections.sort(integers,(x,y)->y-x);
   //[78, 11, 7, 6, 3, 2, 2, 1]
   System.out.println(integers);
```
  4.Comparator的用法
  前面的一些演示，主要目的是说，Comparator一般放在哪些地方。下面具体演示一些用法

手写lamda表达式法

     List<Integer> integers = Arrays.asList(3, 2, 6, 1, 7, 2, 78, 11);
     Collections.sort(integers,(x,y)->y-x);
     //[78, 11, 7, 6, 3, 2, 2, 1]
     System.out.println(integers);

提取可比较字段类型比较

 class People {
     private Integer id;
     private String name;
     private Integer age;
     public People(Integer id, String name, Integer age) {
         this.id = id;
         this.name = name;
         this.age = age;
     }
     public Integer getAge() {
         return age;
     }
     @Override
     public String toString() {
         return "People{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + '}';
     }
 }
 @Test
 public void testComparing() {
     List<People> people = Arrays.asList(new People(1, "高溪", 23),
         new People(2, "James", 37),
         new People(3, "Kobe", 32),
         new People(4, "WY", 25));
     Collections.sort(people,Comparator.comparing(People::getAge));
     System.out.println(people);
 }

按照People类的age字段进行排序
Comparator.comaring(),提供一个排序的字段

int、double等确定排序字段类型

 class People {
     private Integer id;
     private String name;
     private Integer age;
     public People(Integer id, String name, Integer age) {
         this.id = id;
         this.name = name;
         this.age = age;
     }
     public Integer getAge() {
         return age;
     }
     public Integer getId() {
         return id;
     }
     public String getName() {
         return name;
     }
     @Override
     public String toString() {
         return "People{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + '}';
     }
 }
 @Test
 public void testComparing() {
     List<People> people = Arrays.asList(new People(1, "高溪", 23),
         new People(3, "James", 37),
         new People(2, "Kobe", 37),
         new People(4, "WY", 25));
     Collections.sort(people,Comparator.comparingInt(People::getAge));
     System.out.println(people);
 }

排序字段age是Integer
直接可以指定按照int排序

按照多个字段排序

     List<People> people = Arrays.asList(new People(1, "高溪", 23),
         new People(3, "James", 37),
         new People(2, "Kobe", 37),
         new People(4, "WY", 25));
     Collections.sort(people,Comparator.comparingInt(People::getAge).thenComparing(Comparator.comparingInt(People::getId)));
     System.out.println(people);

age如果相同按照id排序

null字段处理

     List<People> people = Arrays.asList(new People(1, "高溪", 23),
         new People(3, "James", 37),
         new People(2, "Kobe", null),
         new People(4, "WY", 25));
     //fixme: java.lang.NullPointerException
     Collections.sort(people,Comparator.comparing(People::getAge));
     System.out.println(people);

如果字段有null值，会报错

List<People> people = Arrays.asList(new People(1, "高溪", 23),
       new People(3, "James", 37),
       new People(2, "Kobe", null),
       new People(4, "WY", 25));
   Collections.sort(people,Comparator.comparing(People::getAge,Comparator.nullsFirst(Comparator.naturalOrder())));
   System.out.println(people);

先指定排序字段

nullFisrt中指定的是对所提取字段的排序方式。

   List<People> people = Arrays.asList(new People(1, "高溪", 23),
       new People(3, "James", 37),
       new People(2, "Kobe", null),
       new People(4, "WY", 25));
   Collections.sort(people,Comparator.comparing(People::getAge,Comparator.nullsFirst(Comparator.comparingInt(x->-x))));
   System.out.println(people);

nullFirst中的比较器是提取字段的值再提取

自然排序和逆序排序

     List<People> people = Arrays.asList(new People(1, "高溪", 23),
         new People(3, "James", 37),
         new People(2, "Kobe", 34),
         new People(4, "WY", 25));
     Collections.sort(people,Comparator.comparing(People::getAge,Comparator.reverseOrder()));
     System.out.println(people);
     Collections.sort(people,Comparator.comparing(People::getAge,Comparator.naturalOrder()));
     System.out.println(people);

提取字段一定是可排序字段，实现了Comparable
默认是自然排序
可以通过调用Comparator.reverseOrder()实现逆序。

雨润工作笔记

Comparator的使用

1.原子类型数组排序

2.引用类型数组排序

3.引用类型集合排序

4.Comparator的用法