java - Why does android studio warn me about generic array creation for typed entry -

i have adapter in android project takes in reusable map , simplicity keeps array of data use.

private entry<string, string>[] mdata;  public myadapter(@nonnull map<string, string> data) {     mdata = (entry<string, string>[]) data.entryset().toarray(); } 

now wanted make little more efficient , rid of unchecked warning, changed using specific array type

mdata = data.entryset().toarray( new entry<string, string>[ data.size() ] ); 

however new entry<string, string>[ data.size() ] part flagged "generic array creation" no further information. why illegal?

i understand java has type erasure , generics result in different types (such explained here), above line of code still seems should legal me.

why illegal?

because arrays covariant. suppose able compile following code:

entry<string, string>[] array = new entry<string, string>[ isomap.size() ]; object[] objarray = array;  // not throw arraystoreexception. objarray[0] = new abstractmap.simpleentry<>(integer.valueof(0), integer.valueof(0)); string key = array[0].getkey(); 

at runtime, arraystoreexception not thrown assignment because abstractmap.simpleentry (the runtime type of abstractmap.simpleentry<integer, integer>) covariant map.entry (the runtime type of entry<string, string>) - fail classcastexception when try execute last line because key integer, not string.

however, have worry every element potentially being of wrong type - given reference entry<string, string>[], can't know how array initialized or updated, accessing given element in wrong way result in runtime exception.

(when "the wrong way", constrasting string key = array[0].getkey() invoking array[0].tostring() - objects have tostring() method, not attempt cast).

the safest way avoid such situation make creation of generic arrays illegal.

note different case of non-generic-but-covariant arrays:

string[] array = new string[1]; object[] objarray = array; // throws arraystoreexception. objarray[0] = integer.valueof(0); 

because assignment throw arraystoreexception, value of wrong type can never added array.

as such, although still have worry arraystoreexception when trying put value array, don't have worry classcastexceptions when reading value array.