WifiInfo的频率仅自Lollipop(API 21)开始可用.

The frequency from WifiInfo is only available since Lollipop (API 21) http://developer.android.com/reference/android/net/wifi/WifiInfo.html#getFrequency()

android.net.wifi.WifiInfo

public int getFrequency ()

Returns the current frequency in FREQUENCY_UNITS.