how to get the default HTTP USER AGENT from the android device?

When you use web view to access the user-agent, make sure you run the

new WebView(this).getSettings().getUserAgentString();

on the UI thread.

If you want access the user agent on background thread. use

System.getProperty("http.agent")

To check whether a user-agent is valid or not use this https://deviceatlas.com/device-data/user-agent-tester

An alternative

if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.JELLY_BEAN_MR1) {
    String userAgent = WebSettings.getDefaultUserAgent(context);
}