java8 集合笔记

本是古典何须时尚 2021-12-03 07:59 209阅读 0赞

class User(){
    private String userCode;
    private String userName;
    }
    
    List<User> userList;
    
    //list 集合提取字段数组
    String [] productCodes = userList.stream().map(User::getUserCode).toArray(String[]::new);
    
    //list 集合提取某个字段 转换为新的list
    List<String> userCodes = userList.stream().map(user->user.getUserCode()).collect(Collectors.toList());
    
    //list 按照字段分组 依据字段等值的 放在同一个集合并指向KEY    
    Map<String, List<User>> userListMap = userList.stream().collect(Collectors.groupingBy(User::getUserCode));
    
    //list过滤筛选集合
    List<User> userList = userList .stream().filter(er -> er.getUserName().equals("张三")).collect(Collectors.toList());
    
    
    // 根据字段过滤去重
    List<String> rs = new ArrayList<>();
    List<User> checkUserList = userList.stream().filter(
    v -> {
    boolean flag = !rs.contains(v.getUserName());
    rs.add(v.getUserName());
    return flag;
    }).collect(Collectors.toList());
    
    //拓展工具  list转map
    public static <K,V> Map<K,V> listToMap(List<V> list, String keyField){
            Map<K,V> result = new HashMap<>();
            if(list != null && list.size() > 0 && StringUtils.isNotEmpty(keyField)){
                String firstLetter = keyField.substring(0, 1).toUpperCase();
                String getter = "get" + firstLetter + keyField.substring(1);
                try {
                    for(Object obj : list){
                        Method method = obj.getClass().getMethod(getter,null);
                        K key = (K) method.invoke(obj,null);
                        if(key != null && StringUtils.isNotEmpty(key.toString())){
                            result.put(key,(V) obj);
                        }
                    }
                } catch (NoSuchMethodException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }
            }
            return result;
        }

转载于:https://www.cnblogs.com/yanxioayu/p/10857999.html