Get value String of ArrayList ArrayList String in Java

Question

I know it s simple question  but in ArrayList lt ArrayList lt String gt  gt  collection  ArrayList lt String gt  listOfSomething   collection  new ArrayList lt ArrayList lt String gt  gt     listOfSomething   new ArrayList lt String gt      listOfSomething Add  quot first quot    listOfSomething Add  quot second quot    collection Add listOfSomething   listOfSomething Clear    listOfSomething Add  quot first quot    collection Add listOfSomething    I want to take String from ArrayList of ArrayList  and I don t know how to do that  For example I go ArrayList lt String gt  myList   collection get 0    String s   myList get 0    and it works  but  Big update  private List lt S gt  valuesS  private List lt Z gt  valuesZ    ArrayList lt ArrayList lt String gt  gt  listOfS  ArrayList lt String gt  listOfZ    Override public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState           Zdatasource   new ZDataSource this           Zdatasource open            valuesZ   Zdatasource getAllZ                     Sdatasource   new SDataSource this           Sdatasource open            valuesS   Sdatasource getAllS             List lt Map lt String  String gt  gt  groupData                 new ArrayList lt Map lt String  String gt  gt             List lt List lt Map lt String  String gt  gt  gt  childData                 new ArrayList lt List lt Map lt String  String gt  gt  gt              listOfS   new ArrayList lt ArrayList lt String gt  gt             listOfZ   new ArrayList lt String gt             for  S i   valuesS       S is class             for  Z j   valuesZ       Z is class                 if j getNumerS   equals i getNumerS                           listOfZ add j getNumerZ                                       else                                         listOfZ add  quot nothing quot                                                    listOfS add listOfZ                   if  listOf isEmpty    listOfZ clear                  Override     public boolean onChildClick ExpandableListView parent  View v  int groupPosition              int childPosition  long id            try                              ArrayList lt String gt  myList   listOfS get groupPosition                String s   myList get childPosition            PrintToast  quot group  quot  Integer toString groupPosition   quot   child  quot  Integer toString childPosition     quot     quot   s                     catch Exception e                        Log e  quot FS quot   e toString                        return true         return me java lang IndexOutOfBoundsException  Invalid index 1  size is 0 when I click on item which really should exist  I didn t show code which generate ListView  but I can tell you that my listOfS contains 3 items  first is Null listOfZ  second listOfZ got 2 elements  third listOfZ got 1 element

User · Answer

Because the second element is null after you clear the list   Use    String s   myList get 0     And remember  index 0 is the first element

User · Answer

The right way to iterate on a list inside list is     iterate on the general list for int i   0   i  lt  collection size     i          ArrayList lt String gt  currentList   collection get i         now iterate on the current list     for  int j   0  j  lt  currentList size    j              String s   currentList get 1

User · Answer

listOfSomething Clear    listOfSomething Add  first    collection Add listOfSomething     You are clearing the list here and adding one element   first    the 1st reference of listOfSomething is updated as well sonce both reference the same object  so when you access the second element myList get 1   which does not exist anymore  you get the null   Notice both collection Add listOfSomething   save two references to the same arraylist object    You need to create two different instances for two elements   ArrayList lt ArrayList lt String gt  gt  collection   new ArrayList lt ArrayList lt String gt  gt      ArrayList lt String gt  listOfSomething1   new ArrayList lt String gt     listOfSomething1 Add  first    listOfSomething1 Add  second     ArrayList lt String gt  listOfSomething2   new ArrayList lt String gt     listOfSomething2 Add  first     collection Add listOfSomething1       collection Add listOfSomething2

User · Answer

A cleaner way of iterating the lists is      initialise the collection collection   new ArrayList lt ArrayList lt String gt  gt        iterate for  ArrayList lt String gt  innerList   collection        for  String string   innerList               do stuff with string

[java] Get value (String) of ArrayList<ArrayList<String>>(); in Java

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