1  // Purpose.  Java calling C referencing Java attributes
 2  //           (Java passes object in, C gets/sets attributes of object)
   
   
 3  ////////////////////////////// JNIdemos.java \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
 4  class JNIdemos {
 5     private double balance  = 100;
 6     private double interest = .05;
 7     static {
 8        System.loadLibrary( "JNIdemos" );
 9     }
   
10     public double getBalance() {
11        return balance;
12     }
13     public native void compound();
   
14     public static void main( String[] args ) {
15        JNIdemos account = new JNIdemos();
16        System.out.println( "balance was " + account.getBalance() );
17        account.compound();
18        System.out.println( "balance is  " + account.getBalance() );
19     }
20  }
   
   
21  ////////////////////////////// JNIdemos.cpp \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
22  #include <iostream.h>
23  #include "JNIdemos.h"
   
24  extern "C" {
25  JNIEXPORT void JNICALL Java_JNIdemos_compound( JNIEnv* env,
26                                                 jobject this_ptr ) {
27     jclass   accountClass = env->GetObjectClass( this_ptr );
28     jfieldID balanceId    = env->GetFieldID( accountClass, "balance",  "D" );
29     jdouble  balance      = env->GetDoubleField( this_ptr, balanceId );
30     jfieldID interestId   = env->GetFieldID( accountClass, "interest", "D" );
31     jdouble  interest     = env->GetDoubleField( this_ptr, interestId );
32     balance += balance * interest;
33     env->SetDoubleField( this_ptr, balanceId, balance );
34  }
35  }
   
   
36  // C:> java JNIdemos
37  // balance was 100.0
38  // balance is  105.0