How can I store HashMap inside a list?

问题

My hashmap stores the string as key and arraylist as the values. Now, I need to embed this into a list. That is, it will be of the following form:

List<HashMap<String, ArrayList<String>>>

These are the declarations I have used:

Map<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();
ArrayList<String> arraylist = new ArrayList<String>();
map.put(key,arraylist);
List<String> list = new ArrayList<String>();

Can anyone help me which method and how to use in the list to proceed storing my map into it?

回答1:

Always try to use interface reference in Collection, this adds more flexibility.
What is the problem with the below code?

List<Map<String,List<String>>> list = new ArrayList<Map<String,List<String>>>();//This is the final list you need
Map<String, List<String>> map1 = new HashMap<String, List<String>>();//This is one instance of the  map you want to store in the above list.
List<String> arraylist1 = new ArrayList<String>();
arraylist1.add("Text1");//And so on..
map1.put("key1",arraylist1);
//And so on...
list.add(map1);//In this way you can add.

You can easily do it like the above.

回答2:

First, let me fix a little bit your declaration:

List<Map<String, List<String>>> listOfMapOfList = 
    new HashList<Map<String, List<String>>>();

Please pay attention that I used concrete class (HashMap) only once. It is important to use interface where you can to be able to change the implementation later.

Now you want to add element to the list, don't you? But the element is a map, so you have to create it:

Map<String, List<String>> mapOfList = new HashMap<String, List<String>>();

Now you want to populate the map. Fortunately you can use utility that creates lists for you, otherwise you have to create list separately:

mapOfList.put("mykey", Arrays.asList("one", "two", "three"));

OK, now we are ready to add the map into the list:

listOfMapOfList.add(mapOfList);

BUT:

Stop creating complicated collections right now! Think about the future: you will probably have to change the internal map to something else or list to set etc. This will probably cause you to re-write significant parts of your code. Instead define class that contains you data and then add it to one-dimentional collection:

Let's call your class Student (just as example):

public Student {
    private String firstName;
    private String lastName;
    private int studentId;

    private Colectiuon<String> courseworks = Collections.emtpyList();

    //constructors, getters, setters etc
}

Now you can define simple collection:

Collection<Student> students = new ArrayList<Student>();

If in future you want to put your students into map where key is the studentId, do it:

Map<Integer, Student> students = new HashMap<Integer, Student>();

回答3:

Try the following:

List<Map<String, ArrayList<String>>> mapList = 
    new ArrayList<Map<String, ArrayList<String>>>();
mapList.add(map);

If your list must be of type List<HashMap<String, ArrayList<String>>>, then declare your map variable as a HashMap and not a Map.

回答4:

First you need to define the List as :

List<Map<String, ArrayList<String>>> list = new ArrayList<>();

To add the Map to the List , use add(E e) method :

list.add(map);

回答5:

class Student{
    //instance variable or data members.

    Map<Integer, List<Object>> mapp = new HashMap<Integer, List<Object>>();
    Scanner s1 = new Scanner(System.in);
    String name = s1.nextLine();
    int regno ;
    int mark1;
    int mark2;
    int total;
    List<Object> list = new ArrayList<Object>();
    mapp.put(regno,list); //what wrong in this part?
    list.add(mark1);
    list.add(mark2);**
    //String mark2=mapp.get(regno)[2];
}

来源：https://stackoverflow.com/questions/16516107/how-can-i-store-hashmapstring-arrayliststring-inside-a-list

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java