Java list转map，list转list

public class ListToListOrMapUtils {
    public static List list2list(List list, String fieldName4Key,Class<?> c) {
        /**
         *@param
         * list:输入list
         * fieldName4Key:字段名
         * c:类
         */
        List<Object> objectArrayList = new ArrayList<>();
        if (list != null) {
            try {
                PropertyDescriptor propDesc = new PropertyDescriptor(fieldName4Key, c);
                Method methodGetKey = propDesc.getReadMethod();
                for (int i = 0; i < list.size(); i++) {
                    Object o = list.get(i);
                    Object invoke = methodGetKey.invoke(o);
                    if(null != invoke){
                        objectArrayList.add(invoke);
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
                throw new IllegalArgumentException("field can't match the key!");
            }
        }
        return objectArrayList;
    }

    public static  <K, V> Map<K, V> list2Map(List<V> list, String fieldName4Key,Class<V> c) {
        Map<K, V> map = new HashMap<K, V>();
        if (list != null) {
            try {
                PropertyDescriptor propDesc = new PropertyDescriptor(fieldName4Key, c);
                Method methodGetKey = propDesc.getReadMethod();
                for (int i = 0; i < list.size(); i++) {
                    V value = list.get(i);
                    @SuppressWarnings("unchecked")
                    K key = (K) methodGetKey.invoke(value);
                    map.put(key, value);
                }
            } catch (Exception e) {
                throw new IllegalArgumentException("field can't match the key!");
            }
        }
        return map;
    }

用法如下：

Map<Object, User> userMap = ListToListOrMapUtils.list2Map(userList, "id", User.class);
List uidList = ListToListOrMapUtils.list2list(userList, "uid", User.class);

lambda 表达式实现

list转map:
List<Student> students= new ArrayList<>();
Map<Long, Student> mapOfStudent = students.stream().collect(Collectors.toMap(s -> s.getTaskCode(), s -> s,(v1,v2)->v1);

list转set:
List<Student> students= new ArrayList<>();
Set<String> sdfSet = students.stream()
                        .map(students::getStudentCode)
                        .collect(Collectors.toSet());
listתlist:
List<Student> students= students.stream().filter((Student student) ->
                studentIds.contains(student.getId())).collect(Collectors.toList());
分组
 Map<Long, List<ObjectRuleRef>> collect = objectRuleRefs.stream().collect(Collectors.groupingBy(item -> item.getObjectId()));