reflection - Why I get java.lang.InstantiationException here? -


this question has answer here:

i'm learning reflection in java , write test code:

import java.lang.reflect.invocationtargetexception; import java.lang.reflect.method;  public class test  {     class base {         public base() {}         public void print(){             system.out.println("base");         }     }      class derived extends base {         @override         public void print() {             system.out.println("derived");         }     }      public static void main(string args[])     {         try {             class.forname(derived.class                     .gettypename())                     .getsuperclass()                     .getmethod("print", new class[0])                     .invoke(base.class.newinstance());// line 41         } catch (classnotfoundexception e) {             e.printstacktrace();         } catch (nosuchmethodexception e) {             e.printstacktrace();         } catch (invocationtargetexception e) {             e.printstacktrace();         } catch (illegalaccessexception e) {             e.printstacktrace();         } catch (instantiationexception e) {             e.printstacktrace();         }     } }

but when run code, get:

java.lang.instantiationexception: test$base     @ java.lang.class.newinstance(class.java:427)     @ test.main(test.java:41)     @ sun.reflect.nativemethodaccessorimpl.invoke0(native method)     @ sun.reflect.nativemethodaccessorimpl.invoke(nativemethodaccessorimpl.java:62)     @ sun.reflect.delegatingmethodaccessorimpl.invoke(delegatingmethodaccessorimpl.java:43)     @ java.lang.reflect.method.invoke(method.java:497)     @ com.intellij.rt.execution.application.appmain.main(appmain.java:140) caused by: java.lang.nosuchmethodexception: test$base.<init>()     @ java.lang.class.getconstructor0(class.java:3082)     @ java.lang.class.newinstance(class.java:412)     ... 6 more

could 1 please tell me why? constructor of class base public compiler still claims can't find constructor..

because base inner class , all (*) inner class constructors implicitly declare formal parameter of enclosing class @ index 0.

the constructor of (*) non-private inner member class implicitly declares, first formal parameter, variable representing enclosing instance of class (§15.9.2, §15.9.3).

in other words, it's not parameterless constructor. you'll need use class#getconstructor(class[]) appropriate constructor, invoke it.

base instance = base.class.getconstructor(test.class).newinstance(new test()); class.forname(derived.class.gettypename()).getsuperclass().getmethod("print", new class[0]).invoke(instance);

(all inner classes hard work with.)


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