adloader.java

public class AdLoader {

    public InterstitialAd mInterstitialAd;
    public InterstitialAd wInterstitialAd;
    public static AdLoader adLoader;

    public static AdLoader getAds() {

        if (adLoader == null) {
            adLoader = new AdLoader();
        }
        return adLoader;
    }

    public void loadFullAdmob(Context context) {
        mInterstitialAd = new InterstitialAd(context);
        mInterstitialAd.setAdUnitId(context.getString(R.string.admob_unit_id));

        mInterstitialAd.loadAd(new AdRequest.Builder().build());
    }

    public void showAdmob(Activity activity) {
        if (mInterstitialAd.isLoaded()) {
            mInterstitialAd.show();
        } else {
            Log.d("TAG", "The interstitial wasn't loaded yet.");
        }
        loadFullAdmob(activity);
    }

}

我在这行收到错误警报：

mInterstitialAd = new InterstitialAd(context);

可能是上下文的问题，我不知道如何解决。
我在mainactivity.java中也有这个

AdLoader.getAds().loadFullAdmob(this);