Java List和Map常用轉換處理

List轉Map

• list轉map（mapKey=id，mapValue = 對象本身）

// List<XxxObject> xxxList = new ArrayList<>(); //XxxObject中有id和name字段
Map<String, XxxObject> xxxMap0 = xxxList.stream().collect(Collectors.toMap(XxxObject::getId, o->o));

• list轉map（mapKey=id，mapValue = name）

 Map<String,String> xxxMap1 = xxxList.stream().collect(Collectors.toMap(XxxObject::getId,XxxObject::getName));

• list轉mapkey 沖突的解決辦法，這里選擇第二個key覆蓋第一個key。Function.identity()是簡潔寫法，也是返回對象本身

Map<String,XxxObject> xxxMap2 = xxxList.stream().collect(Collectors.toMap(XxxObject::getId, Function.identity(),(key1,key2)->key2));

List filter過濾

• 去掉xxxList中XxxObject的name=xxxx的數據

xxxList = xxxList.stream().filter(s->!s.getName().equals("xxxx")).collect(Collectors.toList());

取出List的ID用逗號隔開

  String ids = list.stream().map(XxxObject::getId).collect(Collectors.joining(","));

Map遍歷

• Map.Entry遍歷得到key和value

for (Map.Entry<String, XxxObject> entry : xxxMap0.entrySet()) {
  String key = entry.getKey();
  XxxObject val = entry.getValue();
}