java - Are we allowed to use wildcard during instantiation -


why these declaration invalid in java?

  1. list<number> test = new arraylist<? extends number>();

  2. list test = new arraylist<? extends number>();

are not allowed use wildcard during instantiation. , if wildcards useful passing them methods?

and list<object> test = new arraylist<integer>(); illegal because generics not covariant correct?

the ? wildcard character means "unknown" not "any". doesn't make sense instantiate new container of unknown contents, put in there? can't used anything!

so declaraion new arraylist<? extends number>() means "some specific thing extends number, don't know what." does not mean "anything extends number."
list<number> assigned allow both double , integer added it, actual contents of list<? extends number> might float! (or whatever else.) consider happen in code if wildcard worked "any":

    list<integer> listinteger = new arraylist<integer>();     listinteger.add(integer.valueof(1));     list<? extends number> listwildcard = listinteger;     listwildcard.add(double.valueof(1.0)); //this not compile     integer integer = listinteger.get(1);//because throw classcastexception

footnote regarding second example: declaring paramaterized type no type parameter called using raw type. considered programming error. syntax legal code written before java 5 still compiles. don't if scenario isn't backward compatability pre-java 5.


Comments

Post a Comment

Popular posts from this blog

How to remove text and logo OR add Overflow on Android ActionBar using AppCompat on API 8? -

have researched death , cannot find answer. i have actionbar using appcompat. supporting api v7 (api v8 do). actionbar works api v11. api < v11 has problem of removing icon (and feature) actionbar without supplying overflow. big picture have app logo, app title, , 3 icons squidging same actionbar on low end phone. trying make room! i happy 1 of 2 solutions. either: a method of getting overflow functionality can retrieved pull down. (preferred) method of removing icon , text actionbar below current xml api 11+: <resources> <!-- base application theme api 11+. theme replaces appbasetheme res/values/styles.xml on api 11+ devices. --> <style name="appbasetheme" parent="theme.appcompat.light.darkactionbar"> <item name="android:actionbarstyle">@style/liteactionbarstyle</item> <item name="actionbarstyle">@style/liteactionbarstyle</item> </st...
Read more

html - How to style widget with post count different than without post count -

i have border below li elements in wordpress widget types appropriate. element inside set block gives them nice easy click full width. the problem, when choose option "show post count" when adding widget such categories or archives displays post count plain text after tag , because tag block pushes post number text line below. whether choose show post count or not show post count still gives exact same element identifyer example "widget_categories". how style post count not on next line still keep links non post count list items full width? here's example on sidebar www.bbmthemes.com/themes/modular/blog-standard/ in order solve problem need remove display:block; for ul.widgets ul a , add in it's place line-height: 33px; what going achieve have same visual effect. what going lose doing links going work on text , not on "box". the other way out of find post count coming , make changes this. example change text count i...
Read more

url rewriting - How to redirect a http POST with urlrewritefilter -

i have question urlrewritefilter , until not find in net. i want redirect http post in tomcat7. here example... the call http post ulr http://localhost:8080/oldapplication/example?a=123&b=2 this call contains content either xml or json. filter configured works , urlrewrite.xml contains: <?xml version="1.0" encoding="utf-8"?> <!doctype urlrewrite public "-//tuckey.org//dtd urlrewrite 4.0//en" "http://www.tuckey.org/res/dtds/urlrewrite4.0.dtd"> <urlrewrite use-query-string="true"> <rule> <condition type="method">post</condition> <from>^(.*)$</from> <to type="redirect">/newapplication$1</to> </rule> </urlrewrite> in access log can see call http://localhost:8080/oldapplication/example?a=123&b=2 gets redirected http://localhost:8080/newapplication/example?a=123&b=2 fine until now. pr...
Read more