代码：

package com.css.network_12_pre;

import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  
  try{
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_main);
   TextView tt = (TextView)findViewById(R.id.textView1);
   tt.setText("检查网络连接");
   checkNetworkInfo();
   checkNet();
  }catch(Exception e){
   e.printStackTrace();
  }
  
 }
 
 public boolean checkNet(){
  ConnectivityManager manager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
  NetworkInfo networkInfo = manager.getActiveNetworkInfo();
  if(networkInfo == null || !networkInfo.isAvailable()){
   new AlertDialog.Builder(this).setMessage("没有可用的网络") .setPositiveButton("ok", null).show();
   return false;
  }else{
   new AlertDialog.Builder(this).setMessage("网络正在使用中").setPositiveButton("ok", null).show();
   return true;
  }
 }
 
 private void checkNetworkInfo(){
  ConnectivityManager conman = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
  NetworkInfo mobile = conman.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);
  new AlertDialog.Builder(this).setMessage(mobile.getState().toString()).setPositiveButton("3G", null).show();
  NetworkInfo wifi = conman.getNetworkInfo(ConnectivityManager.TYPE_WIFI);
  new AlertDialog.Builder(this).setMessage(wifi.getState().toString()).setPositiveButton("wifi", null).show();
  
  if(mobile.getState().toString().equals("CONNECTED")){
   Toast.makeText(this, "当前连接的是3g", Toast.LENGTH_SHORT).show();
  }else if(wifi.getState().toString().equals("CONNECTED")){
   Toast.makeText(this, "当前连接的是wifi", Toast.LENGTH_SHORT).show();
  }else{
   Toast.makeText(this, "无网络服务", Toast.LENGTH_SHORT).show();
  }
 }

}

注册权限：

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