java - Why is it allowed to add primitive datatypes to an ArrayList? -

i understand possible add integer object arraylist of type integer. makes sense me. this:

arraylist<integer> list = new arraylist<integer>(); list.add(new integer(3));

but why possible add primitive datatype int instead of integer? this:

arraylist<integer> list = new arraylist<integer>(); list.add(3);

why allowed??

this called autoboxing. classes have corresponding primitives (e.g, long -> long, integer -> int), java handle conversion you.

it should noted behavior comes dark corners:

a performance penalty;
corner cases: when null unboxed primitive, nullpointerexception thrown, might unexpected programmer since looks primitive throwing exception.

Three