java - ArrayList with unlimited predefined values -


i know how initialize arraylist has unlimited amount of predefined values, access element of arbitrary index , update value.

for example, arraylist testarraylist initiated int value 0, use testarraylist.set(123, 15) set 124th element in arraylist 15 instead of getting out of bound exception. when queries testarraylist.get(99999), predefined value 0.

i've been trying initialize arraylist first use loop add 0s it, have decide how big initial arraylist should be. if made arraylist add 100 0s it, couldn't .get(200) on before adding amount of 0s.

is i'm looking possible arraylist? if not, recommend alternative solutions?

thank you.

edit: reason wanted list wanted able iterate through in order. i'm making simple calendar-like program, add/retrieve/modify "notes" of arbitrary "date", able print out notes of every "day" given specific period.

you don't want create arraylist, since in order populate 99999'th index, you'll have populate indices 0 99998 first, wasteful.

a hashmap<integer,integer> better option, since populate indices care about.

a hashmap integer key can thought of if mapping indices of "sparse arraylist" small array, reduces storage requirements while keeping expected insertion , lookup times constant.

for example:

map<integer,integer> map = new hashmap<>(); int value = map.getordefault(99999,0); // return 0 if map has no value                                         // key 99999 map.put (99999, value + 1);

edit: seeing requirement iteration order, can use treemap<integer,integer> instead of hashmap<integer,integer>. way iterating on keys according numeric order.


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