如何在ArrayList的Arraylist中添加和检索值

Question

我知道如何创建 ArrayList的ArrayList ，但是知道如何添加新的ArrayList并为该特定ArrayList 添加值以及如何从该列表中检索数据。

ArrayList<ArrayList<Integer>> arrayList = new ArrayList<ArrayList<Integer>>();

Answer 1

You can try this

    ArrayList<ArrayList<Integer>> arrayList = new ArrayList<>();
    ArrayList<Integer> list1=new ArrayList<>();
    ArrayList<Integer> list2=new ArrayList<>();
    list1.add(1); // add to list
    list1.add(2);
    list2.add(3);
    list2.add(4);
    arrayList.add(list1); // add to list of list
    arrayList.add(list2);
    for(ArrayList<Integer> i:arrayList){ // iterate -list by list
        for(Integer integer:i){ //iterate element by element in a list

        }
    }

You can get element directly

  arrayList.get(0).get(0); // 0th list 0th value

adding value to 0th list

 arrayList.get(0).add(1);  // 1 will add to 0th index list-list1

Answer 2

how to add new ArrayLists and ADD value to that particular ArrayList and how to RETRIEVE the data form that list.

    ArrayList<ArrayList<Integer>> arrayList=new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> newAL= new ArrayList<Integer>();
    arrayList.add(newAL);
    newAL.add(1);
    newAL.add(2);
    newAL.clear();      
    System.out.println(arrayList.get(0));
    //Changes persist in your arraylist

So after adding ArrayList you can manipulate newAL as ArrayList stores reference you don't need to fetch and set element from main arrayList .

---

To retrive data you can Iterate (Use ForEach Loop ) over arrayList or you can do following

    Integer List0Item1=arrayList.get(0).get(1);//Get first element of list at 0
    arrayList.get(0).set(0, 10);//set 0th element of list at 0 as 10
    ArrayList<Integer> list10=arrayList.get(10);//get arraylist at position 10

Answer 3

It is a list of list where every object of the parent list is in turn a sublist.The code is something like below:

List<List<String>> mainList = new ArrayList<List<String>>();  

      List<String> sublist1 = new ArrayList<String>();  
      sublist1.add("State");
      sublist1.add("Country");
      sublist1.add("City");

      mainList .add(sublist1); 

      List<String> sublist2 = new ArrayList<String>();  
      sublist2.add("Sleep");
      sublist2.add("Suspend");
      sublist2.add("Wait");

      mainList .add(sublist2); 

      for(List<String> obj:mainList){ // iterate 
            for(String value:obj){

                System.out.println(value);
            }
        }

}
}
    Hope this will help to clear your doubt.