@SpringBootTest(classes = DemoApplication.class)
@Slf4j
public class ListToMapTest {
@Test
public void test() {
List<Student> list = new ArrayList<>();
list.add(Student.builder().id(1).code(2).name("aa").build());
list.add(Student.builder().id(2).code(3).name("aa2").build());
list.add(Student.builder().id(3).code(4).name("aa3").build());
list.add(Student.builder().id(4).code(5).name("aa4").build());
list.add(Student.builder().id(5).code(5).name("aa5").build());
List<Student> list2 = new ArrayList<>();
list2.add(Student.builder().id(1).code(2).name("aa").classCode("一年级").build());
list2.add(Student.builder().id(2).code(3).name("aa").classCode("一年级").build());
list2.add(Student.builder().id(3).code(4).name("aa").classCode("一年级").build());
list2.add(Student.builder().id(4).code(5).name("aa4").classCode("二年级").build());
list2.add(Student.builder().id(5).code(5).name("aa5").classCode("二年级").build());
//id为key value为对象student
Map<Integer, Student> maps = list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
log.info(maps.toString());
//2022-04-21 09:17:37.724 INFO 150956 --- [ main] com.example.demo.test.ListToMapTest :
// {1=Student(id=1, name=aa, code=2),
// 2=Student(id=2, name=aa2, code=3),
// 3=Student(id=3, name=aa3, code=4),
// 4=Student(id=4, name=aa4, code=5),
// 5=Student(id=5, name=aa5, code=5)}
//id为key value为某一个属性学号
Map<Integer, Integer> codeMaps = list.stream().collect(Collectors.toMap(Student::getId, Student::getCode));
log.info(codeMaps.toString());
//{1=2, 2=3, 3=4, 4=5, 5=5}
//同理 value为某一个属性姓名
Map<Integer, String> nameMaps = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));
log.info(nameMaps.toString());
//{1=aa, 2=aa2, 3=aa3, 4=aa4, 5=aa5}
//如果姓名出现重复呢
Map<Integer, String> nameMaps2 = list2.stream().collect(Collectors.toMap(Student::getId, Student::getName));
log.info(nameMaps2.toString());
//{1=aa, 2=aa, 3=aa, 4=aa4, 5=aa5}
//年级为key value为学生姓名
//Map<String, String> classMaps = list2.stream().collect(Collectors.toMap(Student::getClassCode, Student::getName));
// log.info(classMaps.toString());
//报错java.lang.IllegalStateException: Duplicate key aa
//
// at java.util.stream.Collectors.lambda$throwingMerger$114(Collectors.java:133)
// at java.util.HashMap.merge(HashMap.java:1245)
// at java.util.stream.Collectors.lambda$toMap$172(Collectors.java:1320)
// at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
// at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
// at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
// at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
// at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
// at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
// at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
// at com.example.demo.test.ListToMapTest.test(ListToMapTest.java:73)
//班级名称为 key 学生名称为 value，但运行时出现了多个相同的key ，此时编译器就会抛出Duplicate key xxxx
//我们需要使用toMap的另外一个重载的方法！
//Collectors.toMap(keyMapper, valueMapper, mergeFunction)
/*
*第三个参数是当key 发生重复时处理的方法
*一种合并函数，用于解决两者之间的冲突与提供的相同键相关联的值到{@link Map#merge(Object, Object, BiFunction)}
* 该合并函数有两个参数，第一个参数为当前重复key 之前对应的值，第二个为当前重复key 现在数据的值。
* */
//1、重复时采用后面的value 覆盖前面的value
Map<String, String> map1 = list2.stream().collect(Collectors.toMap(Student :: getClassCode, Student :: getName,
(value1, value2 )->{
return value2;
}));
log.info(map1.toString());
//{一年级=aa, 二年级=aa5}
//重复时将之前的value 和现在的value拼接或相加起来；
Map<String, String> map2 = list2.stream().collect(Collectors.toMap(Student :: getClassCode, Student :: getName,
(key1 , key2)-> key1 + "," + key2 ));
log.info(map2.toString());
//{一年级=aa,aa,aa, 二年级=aa4,aa5}
//将重复key的数据变成一个集合
Map<String, List<String>> map3 = list2.stream().collect(Collectors.toMap(Student :: getClassCode,
// 此时的value 为集合，方便重复时操作
s -> {
List<String> studentNameList = new ArrayList<>();
studentNameList.add(s.getName());
return studentNameList;
},
// 重复时将现在的值全部加入到之前的值内
(List<String> value1, List<String> value2) -> {
value1.addAll(value2);
return value1;
}
));
log.info(map3.toString());
//{一年级=[aa, aa, aa], 二年级=[aa4, aa5]}
//统计各个年级的人数
Map<String, List<Student>> gmap = list2.parallelStream()
.collect(Collectors.groupingBy(o -> o.getClassCode()));
log.info("统计各个年级的人数"+gmap.toString());
Map<String, Integer> map = new HashMap();
gmap.forEach((k, v) -> {
/*BigDecimal qmkcje = v.stream().map(o -> o.getK0bnljcgje() != null ? o.getK0bnljcgje() : BigDecimal.ZERO).reduce(BigDecimal.ZERO, BigDecimal::add);
qmkcje = qmkcje==null ? new BigDecimal(0) : qmkcje;
BigDecimal lastk0bnljcgje = maps.get(k)==null ? new BigDecimal(0) : maps.get(k);
//每年1月份直接去Y_WLKXM_YB_JNCG-K0BNLJCGJE和Y_WLKXM_YB_JWCG-K0BNLJCGJE之和
BigDecimal k0bndycgje = isFirstMon(cgybcgzeldbQuery.getCalmonthEq())?qmkcje:qmkcje.subtract(lastk0bnljcgje);
String g0zgsdmtext = v.stream().map(o -> o.getG0zgsdmtext()).collect(Collectors.toList()).get(0);
CgybcgzeldbVo build = CgybcgzeldbVo.builder().g0gsdm(k).calmonth(cgybcgzeldbQuery.getCalmonthEq()).k0bndycgje(k0bndycgje).g0zgsdmtext(g0zgsdmtext).k0bnljcgje(qmkcje).build();
v.stream().map(o -> o.getK0bnljcgje() != null ? o.getK0bnljcgje() : BigDecimal.ZERO).reduce(BigDecimal.ZERO, BigDecimal::add);
list.add(build);*/
int size = v.size();
map.put(k,size);
});
log.info("统计各个年级的人数"+map.toString());
}
}
import lombok.Builder;
import lombok.Data;
/**
-
@author liu
-
@date 2022年04月21日 9:16
*/
@Data
@Builder
public class Student {//id
private int id;
//姓名
private String name;
//学号
private int code;
//班级
private String classCode;
}