本文整理了,在平时的开发过程中尝试使用Java 8简化代码量的几个示例,不断学习补充中。
1.List<Map>
转为List<String>
/**
* 转换:List<Map>转List<String>
*
* @return
*/
public void convertToList() {
List<Map> mapList = new ArrayList<>();
Map<String, Object> map;
for (int i = 0; i < 5; i++) {
map = new HashMap<>();
map.put("BLOG_NAME", i + "个空格");
mapList.add(map);
}
List<String> blogNameList = mapList.stream().map(s -> (String) s.get("BLOG_NAME")).collect(Collectors.toList());
blogNameList.forEach(System.out::println);
}
2.List<Object>
获取Object(filter)
/**
* 转换:List<Map>转Map
*
* @return
*/
public void convertToList() {
List<Map> mapList = new ArrayList<>();
Map<String, Object> map;
for (int i = 0; i < 5; i++) {
map = new HashMap<>();
map.put("BLOG_NAME", i + "个空格");
mapList.add(map);
}
Map<String, Object> fourSpaces = mapList.stream() // Convert to steam
.filter(x -> "4个空格".equals(x.get("BLOG_NAME"))) // we want "4个空格" only
.findAny() // If 'findAny' then return found
.orElse(null);
System.out.println(fourSpaces.get("BLOG_NAME"));
}
3.获取Map的第一个元素key值
//if order is important, e.g. with a TreeMap/LinkedHashMap
map.entrySet().stream().findFirst();
//if order is not important or with unordered maps (HashMap...)
map.entrySet().stream().findAny();
4.Collectors.toMap
用法
用法示例1:将Java List转为Map
Map<String, Long> result = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites));
用法示例2:将Java List转为Map
List<GraphNode> graphNodeList = new ArrayList<>();
Map<String, Object> nodesIdMap = graphNodeList.stream().collect(Collectors.toMap(GraphNode::getId, s -> s));
用法示例3:将Java List转为Map
List<PubOrganStaff> allStaffs = selectByExample(example);
Map identityInfoMap = allStaffs.stream().collect(Collectors.toMap(s -> s.getOrganId() + "#" + s.getExtType(), PubOrganStaff::getExtValue));
5.List<String>
转String
String country;
List<String> cities = new ArrayList<>();
cities.add("Delhi");
cities.add("New York");
cities.add("Beijing");
cities.add("1kjh1231");
country = cities.stream()
.filter( c -> c.equals("Beijing"))
.findAny()
.map(v -> "China")
.orElse(null);
System.out.println(country);
6.平均值(average)
https://stackoverflow.com/questions/10791568/calculating-average-of-an-array-list
OptionalDouble average = marks
.stream()
.mapToDouble(a -> a)
.average();
return average.isPresent() ? average.getAsDouble() : 0;
7.排序(sorted)
示例1:按照字段排序
List<Map<String,String>> list = new ArrayList<>();
Map<String,String> map = new HashMap<>();
map.put("userid","JJ05");
list.add(map);
map = new HashMap<>();
map.put("userid","JJ02");
list.add(map);
map = new HashMap<>();
map.put("userid","JJ04");
list.add(map);
map = new HashMap<>();
map.put("userid","JJ01");
list.add(map);
list = list.stream().sorted(Comparator.comparing(o->o.get("userid"))).collect(Collectors.toList());
for(Map<String,String> m :list){
System.out.println(m.get("userid"));
}
示例2: 按照时间逆序排列
Collections.sort(resultlist, Comparator.comparing(o -> {
try {
return DateTool.parseDatetime((String) ((Map) o).get("procEndTime"), "yyyyMMdd HH:mm:ss");
} catch (ParseException e) {
e.printStackTrace();
}
return null;
}).reversed());
8.过滤(filter)
过滤符合条件的元素
List<SrItemsStandard> srItemsStandardList = new ArrayList<>();
SrItemsStandard standard = new SrItemsStandard();
standard.setCycles("1,2,3");
srItemsStandardList.add(standard);
standard = new SrItemsStandard();
standard.setCycles("2,3");
srItemsStandardList.add(standard);
standard = new SrItemsStandard();
standard.setCycles("1,3");
srItemsStandardList.add(standard);
srItemsStandardList = srItemsStandardList.stream().filter(s-> StringUtils.contains(s.getCycles(),"2")).collect(Collectors.toList());
System.out.println(srItemsStandardList.size()); // 2
9.分组(groupingBy)
Map<String, List<Student>> result = list.stream().collect(Collectors.groupingBy(Student::getAge));
10.Collectors.toSet()
用法
List<Map> applyDsInfoList = queryApplyDsInfoService.queryApplyDsInfoForApplyObject(parameters);
Set<String> dsIdSet = applyDsInfoList.stream().map(s -> (String) s.get("DS_ID")).collect(Collectors.toSet());
11.Collectors.joining()
用法
List<String> list = Arrays.asList("1","2","3");
String s = list.stream().collect(Collectors.joining(",")); // 1,2,3
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