MainActivity.java

...
public final String TAG = this.getClass().getSimpleName();

protected void onCreate(Bundle savedInstanceState) {
    ...

    printKeyHash();
}

private void printKeyHash() {
    try {
        PackageInfo info = getPackageManager()
                .getPackageInfo("io.github.ziginsider.facebooksdkdemo",
                        PackageManager.GET_SIGNATURES);
        for (Signature signature:info.signatures) {
            MessageDigest md = MessageDigest.getInstance("SHA");
            md.update(signature.toByteArray());
            Log.d(TAG, "KeyHash: " + Base64.encodeToString(md.digest(),Base64.DEFAULT));
        }
    } catch (PackageManager.NameNotFoundException e) {
        e.printStackTrace();
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
}
...