java：存储HashMap <String, String> 在ArrayList中 <Object>

Question

ArrayList<Object> parameters = new ArrayList<Object>();

HashMap<String, String> parameter = new HashMap<String, String>();
parameter.put("key", "value");

parameters.add(parameter);
parameters.add((String) "additionalData"); //this line is here for a reason
destinationFunction(parameters);

....

destinationFunction(ArrayList<Object> data){
     HashMap<String, String> imported = (HashMap<String, String>) data.get(0);
     String value = imported.get("key");
}

How do i achieve this? When i try i receive no errors up until like 2 of destinationFunction where i receive null pointer exception

Answer 1

ArrayList<Object> parameters = new ArrayList<Object>(); 

relpace this line with what you want to store in array list like you want to store hash map then create arraylist of type hash map

 ArrayList<HashMap<String, String>> parameters = new ArrayList<HashMap<String, String>>();

hope this will help you

Answer 2

使用哈希图的数组列表

ArrayList<HashMap<String, String>>()

Answer 3

Try to put ArrayList<HashMap<String, String>> instead of ArrayList<Object>

create a HashMap and add into the ArrayList. For Example:-

ArrayList<HashMap<String, String>> list = new ArrayList<>();

HashMap<String, String> map = new HashMap<>();
map.put("KEY", "VALUE");

list.add(map); //Add the map into list

Answer 4

You did not specified what type are you storing in your arrayList. But you assume in your destinationFunction that this list contains only HashMap<String, String> elements. Besides terrible code style it is okay in java to write so. But you made a mistake when put in your arrayList an element of type String which is breaking your assumptions in destinationFunction .

PS Try to write more type safe code and avoid generic types with Object type parameter.

Answer 5

Alternative Solution would be using JsonObject and JsonArrays . they are perfect for such "scenarios" (working with String,Integer and etc with combination of List and Map).

they may also be useful for complex object(eg working with Images) but will be abit more difficult to implement. in such cases please refer to java_serialization

in your case use JSONArray instead of your ArrayList and JsonObject instead of HashMap .