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.

Answer 1

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));