Java 8使用條件過濾並收集自定義Map

Question

我有一些集合List<Map<String, Object>>需要使用Java 8 lambda表達式進行過濾。 我將收到帶有標志的JSON對象，必須應用過濾條件。 如果未收到JSON對象，則不需要過濾。

protected List<Map<String, Object>> populate(List<SomeObject> someObjects, String string) {
    taskList.stream()
            // How to put condition here? Ho to skip filter if no filter oprions are received?
            .filter(someObject -> (if(string != null) someobject.getName == string))
           // The second problem is to collect custom map like
            .collect(Collectors.toMap("customField1"), someObject.getName()) ... // I need somehow put some additional custom fields here
}

現在我正在收集這樣的自定義地圖：

Map<String, Object> someMap = new LinkedHashMap<>();
someMap.put("someCustomField1", someObject.field1());
someMap.put("someCustomField2", someObject.field2());
someMap.put("someCustomField3", someObject.field3());

Answer 1

只需檢查是否需要應用過濾器，然后使用filter方法或不使用它：

protected List<Map<String, Object>> populate(List<SomeObject> someObjects, String string) {
    Stream<SomeObject> stream = someObjects.stream();
    if (string != null) {
         stream = stream.filter(s -> string.equals(s.getName()));
    }
    return stream.map(someObject -> {
        Map<String, Object> map = new LinkedHashMap<>();
        map.put("someCustomField1", someObject.Field1());
        map.put("someCustomField2", someObject.Field2());
        map.put("someCustomField3", someObject.Field3());
        return map;
    }).collect(Collectors.toList());
}

Answer 2

試試這個：

protected List<Map<String, Object>> populate(List<SomeObject> someObjects, String string) {
    return someObjects.stream()
            .filter(someObject -> string == null || string.equals(someObject.getName()))
            .map(someObject -> 
              new HashMap<String, Object>(){{
                    put("someCustomField1", someObject.Field1());
                    put("someCustomField2", someObject.Field2());
                    put("someCustomField3", someObject.Field3());
              }})
            .collect(Collectors.toList()) ;
}