无法获取本地inetAddress

Question

I'm trying to get the local InetAddress of my wlan0 interface on my Android phone

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import java.net.InetAddress;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        InetAddress ia = Utils.getLocalInetIP4Address("wlan0");

        setContentView(R.layout.activity_main);
    }
}

and this is the content of Utils.java :

import java.net.Inet4Address;
import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.InterfaceAddress;
import java.net.SocketException;
import java.util.Enumeration;

public class Utils {
    public static InetAddress getLocalInetIP4Address(String nw_if) {
        try {
            Enumeration<NetworkInterface> interfs = NetworkInterface.getNetworkInterfaces();

            for(NetworkInterface interf; interfs.hasMoreElements();) {
            interf = interfs.nextElement();

                if(interf.getName().equals(nw_if)) {
                    Enumeration<InetAddress> inetaddrs = interf.getInetAddresses(); //Get all IPs (IPv4 or IPv6) associated to this interface in an InetAddress object
                    for(InetAddress addr; inetaddrs.hasMoreElements();) {
                        addr = inetaddrs.nextElement();
                        if(addr instanceof Inet4Address) {
                            return addr;
                        }
                    }
                }
            }
        } catch(SocketException ex) {
            System.out.println(ex.getMessage());
        }
        return null;
    }
}

when I launch the application it doesn't start, on the screen I get the message that "the application stopped in a strang way" (or something like that in english).

Those are the errors I get:

10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{example.antonio.broadcast_adhoc/example.antonio.broadcast_adhoc.MainActivity}: java.lang.NullPointerException
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2306)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.access$600(ActivityThread.java:156)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1340)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:99)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:153)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5297)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:511)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:  Caused by: java.lang.NullPointerException
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at com.android.internal.os.LoggingPrintStream.println(LoggingPrintStream.java:298)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at example.antonio.broadcast_adhoc.Utils.getLocalInetIP4Address(Utils.java:29)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at example.antonio.broadcast_adhoc.MainActivity.onCreate(MainActivity.java:12)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:5122)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2270)
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.access$600(ActivityThread.java:156) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1340) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:99) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:153) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5297) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:511) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833) 
10-18 19:28:09.371 14961-14961/example.antonio.broadcast_adhoc E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 

Answer 1

You have omitted the permission required from your AndroidManifest.xml

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />