java - Why is casting the class of a generic to Class<T> unsafe? -

i'm making methodpointer class in order simulate functionality of function pointers c++. @ first, doing objects, had thought -- why not make generic?

the problem came in constructor, attempted call constructor signature methodpointer(class<t> clazz, string methodname, class<?> ... paramclasses):

public methodpointer(t object, string methodname, class<?> ... paramclasses) {     this(object.getclass(), methodname, paramclasses);     this.object = object; } 

i assumed work fine, received following compiler error:

the constructor methodpointer<t>(class<capture#1-of ? extends object>, string, class<?>[]) undefined 

so, confused, did this:

public methodpointer(t object, string methodname, class<?> ... paramclasses) {     this((class<t>) object.getclass(), methodname, paramclasses);     this.object = object; } 

it compiles, receive following warning:

unchecked cast class<capture#1-of ? extends object> class<t> 

i guess problem don't understand class<capture#1-of ? extends object> means. thought since type of t inferred t object parameter necessary calling object.getclass() returns class object of type class<t>. apparently isn't case, though. can clear confusion?

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full class declaration , constructors:

public class methodpointer<t> {      //logger instance     private static final logger logger = logger.getlogger(methodpointer.class);      //object fields     private final method method;     private arraylist<object> args = new arraylist<object>();     private t object = null;       //constructors     public methodpointer(method method) {         this.method = method;     }      public methodpointer(class<t> clazz, string methodname, class<?> ... paramclasses) {         method themethod = null;         try {             themethod = clazz.getmethod(methodname, paramclasses);         }         catch(nosuchmethodexception nsme) {             logutil.log(logger, level.error, "unable find method " + methodname + " in " + clazz.getsimplename(), nsme);         }         method = themethod;     }      public methodpointer(t object, string methodname, class<?> ... paramclasses) {         this((class<t>) object.getclass(), methodname, paramclasses);         this.object = object;     } 

slaks' answer points out object.getclass() decays class<? extends object>, explain compile error. it's not safe cast class<t>.

getclass "returns runtime class" of object it's called on. example, if we're inside methodpointer<number>, object is-a number runtime type integer, double, etc. tells casting object.getclass() class<t> isn't safe because class<integer> , class<number> different objects, representing different classes - distinction seems relevant correctness of you're trying do.

so what's solution? well, don't cast. insist on taking class<t> caller.