List集合的排序方式

本文主要讲述Lsit集合的排序方式：

  1 /**
  2  * 集合的排序方式：2种方式
  3  */
  4 public class GenericWork {
  5     public static void main(String[] args) {
  6         ArrayList<Employee> employeeArrayList = new ArrayList<>();
  7         employeeArrayList.add(new Employee("tom",20000,2000,12,4));
  8         employeeArrayList.add(new Employee("jack",18000,2002,2,10));
  9         employeeArrayList.add(new Employee("white",8000,2001,8,5));
 10         employeeArrayList.add(new Employee("jack",18000,2002,1,5));
 11         employeeArrayList.add(null);
 12 
 13         // 排序方式1：
 14 //        Collections.sort(employeeArrayList, new Comparator<Employee>() {
 15 //            @Override
 16 //            public int compare(Employee o1, Employee o2) {
 17 //                int res = o1.getName().compareTo(o2.getName());
 18 //                if(res == 0){
 19 //                    res = o1.getBirthday().compareTo(o2.getBirthday());
 20 //                }
 21 //                return res;
 22 //            }
 23 //        });
 24 
 25         // 排序方式2：
 26         employeeArrayList.sort(new Comparator<Employee>() {
 27             @Override
 28             public int compare(Employee o1, Employee o2) {
 29                 if(o1 == null || o2 == null){
 30                     return 0;
 31                 }
 32                 int res = o1.getName().compareTo(o2.getName());
 33                 if(res == 0){
 34                     res = o1.getBirthday().compareTo(o2.getBirthday());
 35                 }
 36                 return res;
 37             }
 38         });
 39 
 40         // 遍历
 41         Iterator<Employee> iterator = employeeArrayList.iterator();
 42         while (iterator.hasNext()) {
 43             Employee employee = iterator.next();
 44             if(employee != null){
 45                 System.out.println(employee);
 46             }
 47         }
 48     }
 49 }
 50 // 注意这里实现Comparable接口泛型的使用
 51 class MyDate implements Comparable<MyDate>{
 52     private int year;
 53     private int mon;
 54     private int day;
 55 
 56     public MyDate(int year, int mon, int day) {
 57         this.year = year;
 58         this.mon = mon;
 59         this.day = day;
 60     }
 61 
 62     @Override
 63     public int compareTo(MyDate date) {
 64         int res = this.year - date.year;
 65         if(res == 0){
 66             res = this.mon - date.mon;
 67         }
 68         if(res == 0){
 69             res = this.day - date.day;
 70         }
 71         return res;
 72     }
 73 
 74     @Override
 75     public String toString() {
 76         return "year=" + year +
 77                 ", mon=" + mon +
 78                 ", day=" + day ;
 79     }
 80 }
 81 
 82 class Employee {
 83     private String name;
 84     private double sal;
 85     private MyDate birthday;
 86 
 87     public Employee(String name, double sal, int year,int mon,int day) {
 88         this.name = name;
 89         this.sal = sal;
 90         this.birthday = new MyDate(year,mon,day);
 91     }
 92 
 93     public String getName() {
 94         return name;
 95     }
 96 
 97     public void setName(String name) {
 98         this.name = name;
 99     }
100 
101     public MyDate getBirthday() {
102         return birthday;
103     }
104 
105     @Override
106     public String toString() {
107         return "Employee{" +
108                 "name='" + name + '\'' +
109                 ", sal=" + sal +
110                 ", birthday: " + birthday +
111                 '}';
112     }
113 }

总结：

方式1：Collections工具类的sort()方法，使用匿名内部类，实现Comparator接口，实现接口的compare()方法，并且要求比较的对象类实现Comparable接口，实现接口的compareTo()方法。

方式2：List类的sort()方法，使用匿名内部类，实现Comparator接口，实现接口的compare()方法，并且要求比较的对象类实现Comparable接口，实现接口的compareTo()方法。

注意：使用泛型的好处，使得List集合只能添加泛型指定的类的对象，否则编译报错。但是null仍然能够添加【list.add(null)】，因此需要在compare(Employee o1，Employee o2)中，判断传入的o1和o2是否为空，若为空，则不进行比较，直接返回0，否则有空指针异常【运行时异常】。