How can I get a IP of the local interface that directly connects to a default GW

Question

-Environment-

1. VPN connection(OpenVPN)
   -Default gateway is changed into VPN server IP
   -Virtual interface(tunXX) for VPN connection directly connects to a default gateway

2. Interfaces & default gateway

   interfaces :
     lo        inet addr:127.0.0.1
     wlan0     inet addr:150.149.131.5
     tun0      inet addr:10.8.0.14
   
   default gateway : 10.8.0.2
   

In this environment, I want to know "10.8.0.14" regardless of the interface name using Android API.

I cound find only a way to get default route IP address below

mSocket = new Socket(dstAddress, peerPortNum); 
mStrMyIPAddr = mSocket.getLocalAddress().getHostAddress();

But, I don not want to use Socket