java8 stream 集合笔记

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());

//list集合排序
List<SysMenuEntity> sysMenus;
//第一种升序
sysMenus.sort((o1, o2) -> o1.getSort().compareTo(o2.getSort()));

//第一种 降序
sysMenus.sort((o1, o2) -> o2.getSort().compareTo(o1.getSort()));

//第二种
Collections.sort(sysMenus,(s1,s2) -> s1.getSort().compareTo(s2.getSort()));

// 需要筛选的条件：从stuList中筛选出年龄为21和22的学生
List<Integer> ageList = new ArrayList<Integer>();
ageList.add(21);
ageList.add(22);

// JDK1.8提供了lambda表达式， 可以从stuList中过滤出符合条件的结果。
// 定义结果集
List<Student> result = null;
result = stuList.stream()
.filter((Student s) -> ageList.contains(s.getAge()))
.collect(Collectors.toList());

//取最大值

Optional<Student> userOp= result.stream().
        max(Comparator.comparingInt(Student ::getAge));
Student maxDay = userOp.get();

求和

BigDecimal:
BigDecimal bb =list.stream().map(Plan::getAmount).reduce(BigDecimal.ZERO,BigDecimal::add);
 

int、double、long:

double max = list.stream().mapToDouble(User::getHeight).sum();

二。
BigDecimal bb =list.stream().map(User::getAmount).reduce(BigDecimal.ZERO,(a,b)->{
if (a == null || b == null) {
//报错或定义兜底的值
// throw new Exception("");
return BigDecimal.ZERO;
}
return a.add(b);
});

三。

int ageSum = userList.stream().mapToInt(User::getAge).sum();

 拓展工具

//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;
    }