generics - Why do we use "? super T" in java? -


this question has answer here:

for example, using when using generic comparator - comparator<? super t> c

i understand means comparator compare t , super classes, don't understand logic behind it. why not use "? extends t" ? or, example, have found similar question of asked "list<? super number>" - mean list gonna receive number objects , super classes? number's superclass object, mean list can receive object?

it doesn't mean t , super-classes. means t , sub-classes. no, list cannot receive object, can receive sub-class of number.

list<? super number> numbers = new arraylist<>(); integer = 23; double d = 3.14; object o = new object(); numbers.add(i); numbers.add(d); numbers.add(o); // compilation error

integer , double both extend number. sub-classes of number. can added list of numbers.

number sub-class of object. if @ source code, not explicitly extend (although implement serializable). means super-class object. cannot add object numbers list.

object o2 = 300; numbers.add(o2);    // compilation error numbers.add((number) o2);   // no compilation error once cast

in fact, can add object if number. of course, numbers objects, although not objects numbers. however, if declared object, first have cast number.


Comments

Post a Comment

Popular posts from this blog

get url and add instance to a model with prefilled foreign key :django admin -

Image
i have page in each contest shown in row. each row contains contest id , link other model names have foreign key contest. news model: class news(models.model): title = models.charfield(max_length=50, null=true) description = models.textfield() time = models.timefield( default= datetime.datetime.now().time()) contest = models.foreignkey(contest) i want use admin adding , changing these models. want when user clicks on 'news' has foreign key contest id=1, shows list of news value foreign key. use {% instance in instances %} <tr class="{% cycle 'row1' 'row2' %}"> <td>{{ instance.title }}</td> <td>{{ instance.starttime }}</td> <td>{{ instance.timespan }}</td> <td>{{ instance.date }}</td> <td><a href="/adm...
Read more

css - Make div keyboard-scrollable in jQuery Mobile? -

from can tell, although jquery-mobile-powered pages can contain divs overflow set scroll or auto, , these divs can scrolled one-screen bar or mouse wheel, cannot scrolled using arrow keys, page-up/page-down, or home/end. instead, official "page" div (with data-role="page") absorbs input. perhaps other divs can't acquire focus, i'm not sure. is there way around this? edit: jsfiddle of simple example: https://jsfiddle.net/qogz0shx/ <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script> <script src="https://ajax.googleapis.com/ajax/libs/jquerymobile/1.4.5/jquery.mobile.min.js"> </script> <link rel="stylesheet" href="https://code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.css"/> <style> #outer { overflow:scroll; height: 50vh; width: 50vw; } #inner { height: 500vh; width: 500vw; } ...
Read more

ruby on rails - Seeing duplicate requests handled with Unicorn -

recently upgraded our ec2 instances c3.large in order boost more unicorn workers handle increased traffic our site. (6xworkers per machine - 2 ec2 instances). when did this, started seeing duplicate records being created! looks somehow, maybe 2 unicorn workers attempting process same request? in nginx logs see 1 [post] request particular rails controller - yet 2 records being created in database. i'm hitting kind of race condition issue - have no idea how debug this. doesn't happen time, when it's frustrating well. noticed 2 database records within 5 seconds of each other. (and yes disable button when user clicks submit button). unicorn 4.8.2 ruby 1.9.3 rails 3.2.14 any great! thanks! although may not fix problem, upgrade ruby 2.0.0. there blog mentioning idea upgrade ruby 2.0.0 if using unicorn. for example: https://www.digitalocean.com/community/articles/how-to-optimize-unicorn-workers-in-a-ruby-on-rails-app specifically: if using ruby 1.9, ...
Read more