dictionary - Java Arraylist and Map -

i don't have idea how search this:

random generator = new random(); map<integer, arraylist> mapofprevop = new hashmap<>(); arraylist<integer> listprev = new arraylist<>();   listprev = mapofprevop.get(operacja); system.out.println(listprev); // show []  int rnd = generator.nextint(op_cnt) + 1; listprev.add(rnd); system.out.println(mapofprevop.get(operacja)); // show value of listprev 

why second system.out print me on screen value of listprev?

it shouldn't still print [] ?

listprev = mapofprevop.get(operacja); 

this line works different expect?

this suggest @ first system.out.println invocation list empty.

if here https://docs.oracle.com/javase/7/docs/api/java/util/abstractcollection.html#tostring%28%29. can see tostring method list returns elements between square brackets. [] empty list.

at second call have added element why see it. need bare in mind in java pass objects reference meaning listprev references same list 1 contained in map.

if want value, suggest change

listprev = mapofprevop.get(operacja); 

to

listprev.addall(mapofprevop.get(operacja)); 

this add of elements mapofprevop.get(operacja) listprev without subsequent operations affecting map seems want.

also, map<integer, arraylist> mapofprevop = new hashmap<>(); better use interface types in delcarations have map. consider switching arraylist list.

the object use self can still arraylist, this:

map mapofprevop = new hashmap<>(); list listprev = new arraylist<>();

this means if wanted change linkedlist, change in 1 place rather 3. note exception of arrays.aslist lists lists can resized.