List转换成Map，以List里元素的某个字段未key

List2Map.java

    
public class List2MapExample {
    public static void main(String[] args) {
        List<ClassRecordDTO> list = new ArrayList<>();
        list.add(new ClassRecordDTO(1, 2, "xxx", 0.0, 0.0));
        list.add(new ClassRecordDTO(1, 2, "yyy", 0.0, 0.0));
        list.add(new ClassRecordDTO(2, 3, "zzz", 0.0, 0.0));

        Map<String, ClassRecordDTO> result = list.stream().collect(Collectors.toMap(ClassRecordDTO::getClassName, classRecordDTO -> classRecordDTO));

        System.out.println(result.size());
    }
}