一点一滴记录 Java 8 stream 的使用

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String 转 List

String str = "1,2,3,4";
List lists = Arrays.stream(str.split(",")).map(s -> Long.parseLong(s.trim())).collect(Collectors.toList());

List 转 String

list.stream().map(String::valueOf).collect(Collectors.joining(","))

List<> 对象获取某个值 转为 Long[] , String[] , Integer[]

基本类型：
List list = new ArrayList();  
list.add(1L);
list.add(2L);
list.add(3L);

Long[] l = list.stream().toArray(Long[]::new)
String[] s = list.stream().toArray(String[]::new)
Integer[] in = list.stream().toArray(Integer[]::new)

对象形式：
List list = new ArrayList();

ScheduleJobEntity sc = new ScheduleJobEntity()
sc.setId(1)
list.add(sc);

sc = new ScheduleJobEntity()
sc.setId(2)
list.add(sc);

sc = new ScheduleJobEntity()
sc.setId(3)
list.add(sc);

list.stream().map(ScheduleJobEntity::getId).toArray(Long[]::new)
list.stream().map(ScheduleJobEntity::getId).toArray(Long[]::new)
list.stream().map(ScheduleJobEntity::getId).toArray(Long[]::new)

Long[] l = list.stream().toArray(Long[]::new)
String[] s = list.stream().toArray(String[]::new)
Integer[] in = list.stream().toArray(Integer[]::new)

List<> 对象获取某个值 转为 list

List = new ArrayList();
List items = itemsBeans.stream().map(ItemsBean::getItemid).collect(Collectors.toList());

Set 转 List

Set sets = xxx
List hids =  sets.stream().collect(Collectors.toList());

List 转 Map

List hosts = xxxx
Map hostMap = hosts.stream().collect(Collectors.toMap(Hosts::getHostid,Hosts::getName,(oldValue, newValue) -> newValue));

 List 转 Map

List hosts = xxxx
Map appsMap= apps.stream().collect(Collectors.toMap(ItemTriggerGroupByName::getHostId, e -> e , (oldValue, newValue) -> newValue));

 List 分组 转 Map>

Map> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));

List> 转 Map

List> ma = xxx

Map mapResource = ma.stream().map(Map::entrySet).flatMap(Set::stream).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> newValue));

取map的字段
ma.stream().collect(Collectors.toMap(p -> { return p.get("code")+"";}, p -> { return p.get("value")+"";}

转化 再改变 key 
ma.stream().map(Map::entrySet).flatMap(Set::stream).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> { return v1 + "," + v2; }))

List 求和，最小，最大值，平均值获取

单数组类型

List strs = Arrays.asList(34,5,1,76,23);
最小值
strs .stream().min(Comparator.comparing(Function.identity())).get();
最大值
strs .stream().max(Comparator.comparing(Function.identity())).get();


对象数组类型
int sum = empList.stream().mapToInt(Employee::getAge()).sum();
int max = empList.stream().mapToInt(Employee::getAge()).max().getAsInt();
int min = empList.stream().mapToInt(Employee::getAge()).min().getAsInt();
double avg = empList.stream().mapToInt(Employee::getAge()).average().getAsDouble();
System.out.println("最大值："+max+"\n最小值："+min+"\n总和："+sum+"\n平均值："+avg);