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Following line will initialize arraylist with 9 elements with value true.

public ArrayList<Boolean> timeTable = new ArrayList<Boolean>(Collections.nCopies(9, true));

But how can i initialize arraylist of arraylist? 

public ArrayList<ArrayList<Boolean>> timeTable = new ArrayList<ArrayList<Boolean>>(Collections.nCopies(9, true));

It should mean that outer arraylist has 9 inner arraylist and each inner arraylist has 9 elements with true value.

Similar to How can I initialize an ArrayList with all zeroes in Java? But not exactly same...

Scenario is that i need to maintain a monthly list of daily timetables. Now daily timetable will have only 9 entries, so immutable is fine. But monthly list needs to be appended each month. So it can't be an arraylist.

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rajya vardhan
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2 Answers2

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Given this line form java docs: "Returns an immutable list consisting of n copies of the specified object"

public ArrayList<Boolean> timeTable = new ArrayList<Boolean>(Collections.nCopies(9, true));

public ArrayList<ArrayList<Boolean>> timeTableLists = new ArrayList<ArrayList<Boolean>>(Collections.nCopies(9, timeTable));
Razvan
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  • Scenario is that i need to maintain a monthly list of daily timetables. Now daily timetable will have only 9 entries, so immutable is fine. But monthly list needs to be appended each month. So it can't be an arraylist. – rajya vardhan Aug 19 '12 at 19:12
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    So you're saying that timeTableList should be mutable. Than you need to use a for loop – Razvan Aug 19 '12 at 19:16
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Firstly, it is recommended to use interface types wherever possible. That would make your 

ArrayList<ArrayList<Boolean>> -> List<List<Boolean>>.

Then, the initialization statement would become 

public List<List<Boolean>> timeTable = Collections.nCopies(9, (Collections.nCopies(9, true)));
Vikdor
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    No guarantee that nCopies returns an ArrayList if that was a requirement. – Paul Tomblin Aug 19 '12 at 18:57
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    Completely agree, but I doubted if ArrayList was really a requirement and List wouldn't suffice here. – Vikdor Aug 19 '12 at 18:59
  • Scenario is that i need to maintain a monthly list of daily timetables. Now daily timetable will have only 9 entries, so immutable is fine. But monthly list needs to be appended each month. So it can't be an arraylist. – rajya vardhan Aug 19 '12 at 19:13