在開始之前，我先賣個關子提一個問題：我們現在有一個Employee員工類。 ~~~ @Data @AllArgsConstructor public class Employee { private Integer id; private Integer age; //年齡 private String gender; //性別 private String firstName; private String lastName; } ~~~ 你知道怎么對一個Employee對象組成的List集合，**先按照性別字段倒序排序，再按照年齡的倒序**進行排序么？如果您不知道4行代碼以內的解決方案（其實是1行代碼就可以實現，但筆者格式化為4行），我覺得您有必要一步步的看下去。  ## 一、字符串List排序 cities是一個字符串數組。**注意london的首字母是小寫的。** ~~~java 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); //[london, Milan, New Delhi, San Francisco, Tokyo] cities.sort(Comparator.naturalOrder()); System.out.println(cities); //[Milan, New Delhi, San Francisco, Tokyo, london] ~~~ * 當使用sort方法，按照String.CASE_INSENSITIVE_ORDER（字母大小寫不敏感）的規則排序，結果是：[london, Milan, New Delhi, San Francisco, Tokyo] * 如果使用Comparator.naturalOrder()字母自然順序排序，結果是：[Milan, New Delhi, San Francisco, Tokyo, london] 同樣我們可以把排序器Comparator用在Stream管道流中。 ~~~ cities.stream().sorted(Comparator.naturalOrder()).forEach(System.out::println); //Milan //New Delhi //San Francisco //Tokyo //london ~~~ 在java 7我們是使用Collections.sort()接受一個數組參數，對數組進行排序。**在java 8之后可以直接調用集合類的sort()方法進行排序**。sort()方法的參數是一個比較器Comparator接口的實現類，Comparator接口的我們下一節再給大家介紹一下。 ## 二、整數類型List排序 ~~~swift List<Integer> numbers = Arrays.asList(6, 2, 1, 4, 9); System.out.println(numbers); //[6, 2, 1, 4, 9] numbers.sort(Comparator.naturalOrder()); //自然排序 System.out.println(numbers); //[1, 2, 4, 6, 9] numbers.sort(Comparator.reverseOrder()); //倒序排序 System.out.println(numbers); //[9, 6, 4, 2, 1] ~~~ ## 三、按對象字段對`List<Object>`排序 這個功能就比較有意思了，舉個例子大家理解一下。 ~~~ 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，我們創建了10個Employee對象，然后將它們轉換為List * 然后重點的的代碼:使用了函數應用Employee::getAge作為對象的排序字段，即使用員工的年齡作為排序字段 * 然后調用List的forEach方法將List排序結果打印出來，如下(當然我們重寫了Employee的toString方法，不然打印結果沒有意義)： ~~~ 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) ~~~ * 如果我們希望List按照年齡age的倒序排序，就使用reversed()方法。如： ~~~ employees.sort(Comparator.comparing(Employee::getAge).reversed()); ~~~ ## 四、Comparator鏈對`List<Object>`排序 下面這段代碼先是按性別的倒序排序，再按照年齡的倒序排序。 ~~~ 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，最后面加一個reserved //先是倒序（加reserved），然后正序 //先是正序（加reserved），然后倒序（加reserved） ~~~ > 細心的朋友可能注意到：我們只用了一個reversed()倒序方法，這個和SQL的表述方式不太一樣。這個問題不太好用語言描述，建議大家去看一下視頻！ 排序結果如下： ~~~ 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) ~~~