Java8 list从小到大排序 null值放到最后 Last

浅浅的花香味﹌ 2024-03-17 09:58 61阅读 0赞

## Java8 list从小到大排序 null值放到最后 Last ##

这里介绍【Collections】中的【sort】来进行排序。

当list中的对象是null，或者对象中的排序项目是null时遇到的【java.lang.NullPointerException】。话不多说，直接上代码

### 创建对象： ###

*      @Data
        @Builder
        @NoArgsConstructor
        @AllArgsConstructor
        public class EarlyReservedKey {
         
          private String id;
          private String name;
          private String age;
        }

### 测试代码： ###

*      @Test
          public void wwwwwww() throws Exception {
         
            List<EarlyReservedKey> earlyKeyList = new ArrayList<>();
            earlyKeyList.add(new EarlyReservedKey("3","3","3"));
            earlyKeyList.add(new EarlyReservedKey("1","1","1"));
            earlyKeyList.add(new EarlyReservedKey("2",null,"2"));
            earlyKeyList.add(new EarlyReservedKey(null,null,null));
            earlyKeyList.add(new EarlyReservedKey("4","4","4"));
         
            System.out.println("************sort**before***************************");
            earlyKeyList.forEach(System.out::println);
            Collections.sort(
                    earlyKeyList,
                    Comparator.nullsLast(
                            Comparator.comparing(
                                    EarlyReservedKey::getName
                                    ,Comparator.nullsLast(Comparable::compareTo))
                    )
            );
         
            System.out.println("************sort**after***************************");
            earlyKeyList.forEach(System.out::println);
          }

### 输出结果 ###

*  通过【nullsLast】，可将null的情况排在最后，当然也有首位的方法【nullsfirst】
 *      ************sort**before***************************
        EarlyReservedKey(id=3, name=3, age=3)
        EarlyReservedKey(id=1, name=1, age=1)
        EarlyReservedKey(id=2, name=null, age=2)
        EarlyReservedKey(id=null, name=null, age=null)
        EarlyReservedKey(id=4, name=4, age=4)
        ************sort**after***************************
        EarlyReservedKey(id=1, name=1, age=1)
        EarlyReservedKey(id=3, name=3, age=3)
        EarlyReservedKey(id=4, name=4, age=4)
        EarlyReservedKey(id=2, name=null, age=2)
        EarlyReservedKey(id=null, name=null, age=null)
 *  如果list中没有null对象或者age不是null的时候，【,Comparator.nullsLast(Comparable::compareTo)】该函数则可不要。
 *  但是如不确定有无null的话，则此处加上最好。
 *  对于list排序的另外一种方法
 *      @Test
          public void test() throws Exception {
         
            List<EarlyReservedKey> earlyKeyList = new ArrayList<>();
            earlyKeyList.add(new EarlyReservedKey("3","3","3"));
            earlyKeyList.add(new EarlyReservedKey("1","1","1"));
            earlyKeyList.add(new EarlyReservedKey("2",null,"2"));
            earlyKeyList.add(new EarlyReservedKey(null,null,null));
            earlyKeyList.add(new EarlyReservedKey("4","4","4"));
         
            System.out.println("************sort**before***************************");
            earlyKeyList.forEach(System.out::println);
            earlyKeyList.sort(Comparator.nullsLast(
                    Comparator.comparing(
                            EarlyReservedKey::getName
                            , Comparator.nullsLast(Comparable::compareTo))
            ));
         
            System.out.println("************sort**after***************************");
            earlyKeyList.forEach(System.out::println);
          }
 *  输出结果和上面的一样。

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