how to initialize arraylist of arraylist

Question

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.

Razvan · Accepted Answer · 2012-08-19 18:57:33Z

7

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));

answered Aug 19, 2012 at 18:57

Razvan

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2 Comments

rajya vardhan Over a year ago

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.

Razvan Over a year ago

So you're saying that timeTableList should be mutable. Than you need to use a for loop

Vikdor · Accepted Answer · 2012-08-19 18:54:14Z

4

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)));

answered Aug 19, 2012 at 18:54

Vikdor

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3 Comments

Paul Tomblin Over a year ago

No guarantee that nCopies returns an ArrayList if that was a requirement.

Vikdor Over a year ago

Completely agree, but I doubted if ArrayList was really a requirement and List wouldn't suffice here.

rajya vardhan Over a year ago

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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