Setting the Value for Arraylist of Arraylist

Question

Hello I want to Fill Up my Arraylist of Arraylist named as QuestionIdList_Section
and for that i have make the temp Arraylist named QUESTION_ID_Of_SectionId_Temp that will be clear after adding into QuestionIdList_Section

My Code is as Below so that you can understand how i have codded :

public static ArrayList<String> QUESTION_ID_Of_SectionId_Temp = new ArrayList<String>();
public static ArrayList<ArrayList<String>> QuestionIdList_Section = new ArrayList<ArrayList<String>>();

QUESTION_ID_Of_SectionId_Temp.add("Hello");
QUESTION_ID_Of_SectionId_Temp.add("Hiii");

QuestionIdList_Section.add(0,QUESTION_ID_Of_SectionId_Temp);

Log.i(TAG, "******Before " + QuestionIdList_Section);
Log.i(TAG, "******Before "+ QUESTION_ID_Of_SectionId_Temp);

QUESTION_ID_Of_SectionId_Temp.clear();

Log.i(TAG, "******After  " + QuestionIdList_Section);
Log.i(TAG, "******After " + QUESTION_ID_Of_SectionId_Temp);

After Executing the Code i am getting Different Result for both Variable.

as Below :

******Before [[Hello, Hiiii]]
******Before [Hello, Hiiii]
******After [[]]
******After []

can some one Please Help me to Understand where i am lacking here. i want to clear temp arraylist so that i can put the different Values for UESTION_ID_Of_SectionId_Temp so for second index of QuestionIdList_Section i will set the Value different.

Thanks in Advance.

use QUESTION_ID_Of_SectionId_Temp.remove(item) instead of QUESTION_ID_Of_SectionId_Temp.clear(); — Deepzz
– Deepzz, Commented Feb 2, 2013 at 8:29
just change QuestionIdList_Section.add(0,QUESTION_ID_Of_SectionId_Temp); to QuestionIdList_Section.add(0,new ArrayList<String>(QUESTION_ID_Of_SectionId_Temp)); and your code will work because you will need to copy ArrayList instead of passing reference before cleaning it — ρяσѕρєя K
– ρяσѕρєя K, Commented Feb 2, 2013 at 8:32
Hi, just as a hint, look at the naming conventions of java: oracle.com/technetwork/java/codeconv-138413.html — boskop
– boskop, Commented Feb 2, 2013 at 9:05

pktangyue · Accepted Answer · 2013-02-02 08:28:32Z

3

QUESTION_ID_Of_SectionId_Temp is just a reference.

So if you clear it, the value in QuestionIdList_Section will also be clear.

What you should do is

QUESTION_ID_Of_SectionId_Temp = new ArrayList<String>();

instead of

QUESTION_ID_Of_SectionId_Temp.clear();

edited Feb 2, 2013 at 8:28

answered Feb 2, 2013 at 8:23

pktangyue

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amit · Accepted Answer · 2013-02-02 08:24:39Z

2

The ArrayList associated with QuestionIdList_Section holds a reference of the ArrayList associated with QUESTION_ID_Of_SectionId_Temp.
Thus, when clearing the temp ArrayList, it will also reflect on the ArrayList which is in QuestionIdList_Section.

You might want to create a new instance of temp array and add it to the main array list as follows:

QUESTION_ID_Of_SectionId_Temp = new ArrayList<String>();
QuestionIdList_Section.add(QUESTION_ID_Of_SectionId_Temp);

After doing so, every element you add to QUESTION_ID_Of_SectionId_Temp will be shown in thew second index of QuestionIdList_Section.

answered Feb 2, 2013 at 8:24

amit

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Dan D. · Accepted Answer · 2013-02-02 08:24:17Z

1

You are clearing the QUESTION_ID_Of_SectionId_Temp array which means it has no elements. The second "After" print displays it empty.

The first "After" displays the contents of the QuestionIdList_Section which still contains the ArrayList above, which is empty now.

answered Feb 2, 2013 at 8:24

Dan D.

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