2020-06-25 • java • Stream API

java像使用SQL一样排序集合

字符串List排序

cities是一个字符串数组。注意london的首字母是小写的。

List<String> cities = Arrays.asList(
        "Milan",
        "london",
        "San Francisco",
        "Tokyo",
        "New Delhi"
);
System.out.println(cities);
//[Milan, london, San Francisco, Tokyo, New Delhi]

cities.sort(String.CASE_INSENSITIVE_ORDER);
System.out.println(cities);
//当使用sort方法，按照String.CASE_INSENSITIVE_ORDER（字母大小写不敏感）的规则排序，结果是：[london, Milan, New Delhi, San Francisco, Tokyo]

cities.sort(Comparator.naturalOrder());
System.out.println(cities);
//如果使用Comparator.naturalOrder()字母自然顺序排序，结果是：[Milan, New Delhi, San Francisco, Tokyo, london]

同样我们可以把排序器Comparator用在Stream管道流中:

cities.stream().sorted(Comparator.naturalOrder()).forEach(System.out::println);

结果：

Milan
New Delhi
San Francisco
Tokyo
london

整数类型List排序

List<Integer> numbers = Arrays.asList(6, 2, 1, 4, 9);
System.out.println(numbers); //[6, 2, 1, 4, 9]

//自然排序
numbers.sort(Comparator.naturalOrder()); 
System.out.println(numbers); //[1, 2, 4, 6, 9]

//倒序排序
numbers.sort(Comparator.reverseOrder());
System.out.println(numbers);  //[9, 6, 4, 2, 1]

按对象字段对对象集合排序

Employee员工类：

@Data
@AllArgsConstructor
public class Employee {
   private Integer id;
   private Integer age;   //年龄
   private String gender;  //性别
   private String firstName;  
   private String lastName;
}

Employee e1 = new Employee(1,23,"M","Rick","Beethovan");
Employee e2 = new Employee(2,13,"F","Martina","Hengis");
Employee e3 = new Employee(3,43,"M","Ricky","Martin");
Employee e4 = new Employee(4,26,"M","Jon","Lowman");
Employee e5 = new Employee(5,19,"F","Cristine","Maria");
Employee e6 = new Employee(6,15,"M","David","Feezor");
Employee e7 = new Employee(7,68,"F","Melissa","Roy");
Employee e8 = new Employee(8,79,"M","Alex","Gussin");
Employee e9 = new Employee(9,15,"F","Neetu","Singh");
Employee e10 = new Employee(10,45,"M","Naveen","Jain");
List<Employee> employees = Arrays.asList(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);

employees.sort(Comparator.comparing(Employee::getAge));
employees.forEach(System.out::println);

首先，我们创建了10个Employee对象，然后将它们转换为List
然后重点的的代码:使用了函数应用Employee::getAge作为对象的排序字段，即使用员工的年龄作为排序字段
然后调用List的forEach方法将List排序结果打印出来

结果：

Employee(id=2, age=13, gender=F, firstName=Martina, lastName=Hengis)
Employee(id=6, age=15, gender=M, firstName=David, lastName=Feezor)
Employee(id=9, age=15, gender=F, firstName=Neetu, lastName=Singh)
Employee(id=5, age=19, gender=F, firstName=Cristine, lastName=Maria)
Employee(id=1, age=23, gender=M, firstName=Rick, lastName=Beethovan)
Employee(id=4, age=26, gender=M, firstName=Jon, lastName=Lowman)
Employee(id=3, age=43, gender=M, firstName=Ricky, lastName=Martin)
Employee(id=10, age=45, gender=M, firstName=Naveen, lastName=Jain)
Employee(id=7, age=68, gender=F, firstName=Melissa, lastName=Roy)
Employee(id=8, age=79, gender=M, firstName=Alex, lastName=Gussin)

如果希望List按照年龄age的倒序排序，就使用reversed()方法：

employees.sort(Comparator.comparing(Employee::getAge).reversed());

Comparator链对对象集合排序

下面这段代码先是按性别的倒序排序，再按照年龄的倒序排序。

employees.sort(
        Comparator.comparing(Employee::getGender)
        .thenComparing(Employee::getAge)
        .reversed()
);
employees.forEach(System.out::println);

//都是正序 ，不加reversed
//都是倒序，最后面加一个reserved
//先是倒序（加reserved），然后正序
//先是正序（加reserved），然后倒序（加reserved）

结果：

Employee(id=8, age=79, gender=M, firstName=Alex, lastName=Gussin)
Employee(id=10, age=45, gender=M, firstName=Naveen, lastName=Jain)
Employee(id=3, age=43, gender=M, firstName=Ricky, lastName=Martin)
Employee(id=4, age=26, gender=M, firstName=Jon, lastName=Lowman)
Employee(id=1, age=23, gender=M, firstName=Rick, lastName=Beethovan)
Employee(id=6, age=15, gender=M, firstName=David, lastName=Feezor)
Employee(id=7, age=68, gender=F, firstName=Melissa, lastName=Roy)
Employee(id=5, age=19, gender=F, firstName=Cristine, lastName=Maria)
Employee(id=9, age=15, gender=F, firstName=Neetu, lastName=Singh)
Employee(id=2, age=13, gender=F, firstName=Martina, lastName=Hengis)