Java raw type value assigned to generic type run time getClss() method error - 


public class box<t> {     private t t;     public box(t t){         this.t = t;     }     public void add(t t) {       this.t = t;     }     public t get() {       return t;     }     public static void main(string[] args) {       box<integer> b = new box(new string("may be"));       system.out.println(b.get()); // print out "may be"       system.out.println(b.get().getclass()); // error    } }

this code gives runtime error: 

exception in thread "main" java.lang.classcastexception: java.lang.string cannot cast java.lang.integer
  1. why b.get() not triggering runtime error?
  2. why runtime error occur when try class of class variable?

to more precise: why compiler inserting checkcast instruction bytecode only second get() (leading exception)?

please note:

  • in first case, result of get() used println(object): in other words: receiving side expects object, , "condition" true.
  • in second case, method invocation on returned object follow. , makes can make huge difference if returned type expected one. therefore compiler adding check guarantee following method call sound.

as background, 1 can java language specification, chapter 5.52:

the cast checked cast.

such cast requires run-time validity check. if value @ run time null, cast allowed. otherwise, let r class of object referred run-time reference value, , let t erasure (§4.6) of type named in cast operator. cast conversion must check, @ run time, class r assignment compatible type t, via algorithm in §5.5.3.

respectively chapter 5.53 checked casts @ run time.


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj...
Read more

performance - Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? -

i'm doing micro-optimization on performance critical part of code , came across sequence of instructions (in at&t syntax): add %rax, %rbx mov %rdx, %rax mov %rbx, %rdx i thought had use case xchg allow me shave instruction , write: add %rbx, %rax xchg %rax, %rdx however, dimay found agner fog's instruction tables , xchg 3 micro-op instruction 2 cycle latency on sandy bridge, ivy bridge, broadwell, haswell , skylake. 3 whole micro-ops , 2 cycles of latency! 3 micro-ops throws off 4-1-1-1 cadence , 2 cycle latency makes worse original in best case since last 2 instructions in original might execute in parallel. now... cpu might breaking instruction micro-ops equivalent to: mov %rax, %tmp mov %rdx, %rax mov %tmp, %rdx where tmp anonymous internal register , suppose last 2 micro-ops run in parallel latency 2 cycles. given register renaming occurs on these micro-architectures, though, doesn't make sense me done way. why wouldn't register renamer ...
Read more

c# - Asp.net web api : redirect unauthorized requst to forbidden page -

im trying redirect unauthorized request forbidden page instead i'm getting forbidden page in response body , how can fix ? here's startup class : app.createperowincontext(storecontext.create); app.createperowincontext<applicationusermanager>(applicationusermanager.create); app.createperowincontext<applicationsigninmanager>(applicationsigninmanager.create); app.usecookieauthentication(new cookieauthenticationoptions { authenticationtype = defaultauthenticationtypes.applicationcookie, expiretimespan = timespan.fromdays(30), }); app.useexternalsignincookie(defaultauthenticationtypes.externalcookie); app.usetwofactorsignincookie(defaultauthenticationtypes.twofactorcookie, timespan.fromminutes(5)); app.usetwofactorrememberbrowsercookie(defaultauthenticationtypes.twofactorrememberbrowsercookie); the method im trying reach : [httpget] [authorize(roles = "admin")] public string getcurrentusername() { return userman...
Read more