在AndroidManifest.xml中加一個許可權

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

<intent-filter>  
    <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>

主程式碼中實現：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super
 
.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    
    checkNetwork();
    if (!checkNetwork()) {
        Toast.makeText(this, "沒有網路", Toast.LENGTH_LONG).show();
        Intent intent = new Intent("android.settings.WIRELESS_SETTINGS");
        startActivity(intent);
        return
 
;
    }

}
private boolean checkNetwork() {
    ConnectivityManager conn = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo net = conn.getActiveNetworkInfo();
    if (net != null && net.isConnected()) {
        return true;
    }
    return false;
}