title: i have some error in fingerprint demo
body: <p>I was trying to make fingerprint demo. However, the following exception has occurred. I get <code>NullPointerException</code> on my button click</p>
  
  <pre><code>01-04 11:35:18.647 3295-3295/? E/AndroidRuntime: FATAL EXCEPTION: main
  Process: com.letsnurture.fingerprint, PID: 3295
  java.lang.NullPointerException: Attempt to invoke virtual method 'void javax.crypto.Cipher.init(int, java.security.Key)' on a null object reference
    at com.letsnurture.fingerprint.MainActivity.initCipher(MainActivity.java:179)
    at com.letsnurture.fingerprint.MainActivity.access$000(MainActivity.java:45)
    at com.letsnurture.fingerprint.MainActivity$1.onClick(MainActivity.java:131)
    at android.view.View.performClick(View.java:5198)
    at android.view.View$PerformClick.run(View.java:21147)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  
    01-04 12:08:55.382 1836-2383/? E/MDM: [83] b.run: Couldn't connect to Google API client: ConnectionResult{statusCode=API_UNAVAILABLE, resolution=null, message=null}
  </code></pre>
  
  <p>My code below:</p>
  
  <pre><code>import android.Manifest;
  import android.content.pm.PackageManager;
  import android.os.Bundle;
  import android.support.v4.app.ActivityCompat;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.View;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.app.KeyguardManager;
  import android.content.Intent;
  import android.content.SharedPreferences;
  import android.hardware.fingerprint.FingerprintManager;
  import android.security.keystore.KeyGenParameterSpec;
  import android.security.keystore.KeyPermanentlyInvalidatedException;
  import android.security.keystore.KeyProperties;
  import android.util.Base64;
  import android.util.Log;
  import android.widget.Button;
  import android.widget.TextView;
  import android.widget.Toast;
  
  
  import java.io.IOException;
  import java.security.InvalidAlgorithmParameterException;
  import java.security.InvalidKeyException;
  import java.security.KeyStore;
  import java.security.KeyStoreException;
  import java.security.NoSuchAlgorithmException;
  import java.security.NoSuchProviderException;
  import java.security.UnrecoverableKeyException;
  import java.security.cert.CertificateException;
  
  import javax.crypto.BadPaddingException;
  import javax.crypto.Cipher;
  import javax.crypto.IllegalBlockSizeException;
  import javax.crypto.KeyGenerator;
  import javax.crypto.NoSuchPaddingException;
  import javax.crypto.SecretKey;
  import javax.inject.Inject;
  
  
  public class MainActivity extends AppCompatActivity {
  
  
      private static final String TAG = MainActivity.class.getSimpleName();
  
      private static final String DIALOG_FRAGMENT_TAG = "myFragment";
      private static final String SECRET_MESSAGE = "Very secret message";
      /** Alias for our key in the Android Key Store */
      private static final String KEY_NAME = "my_key";
  
      @Inject
      KeyguardManager mKeyguardManager;
      @Inject
      FingerprintManager mFingerprintManager;
      @Inject
      FingerprintAuthenticationDialogFragment mFragment;
      @Inject
      KeyStore mKeyStore;
      @Inject
      KeyGenerator mKeyGenerator;
      @Inject
      Cipher mCipher;
      @Inject
      SharedPreferences mSharedPreferences;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
          setSupportActionBar(toolbar);
  
  
          Button purchaseButton = (Button) findViewById(R.id.purchase_button);
          mKeyguardManager = (KeyguardManager) getSystemService(KEYGUARD_SERVICE);
          if (!mKeyguardManager.isKeyguardSecure()) {
              // Show a message that the user hasn't set up a fingerprint or lock screen.
              Toast.makeText(this,
                      "Secure lock screen hasn't set up.\n"
                              + "Go to 'Settings -&gt; Security -&gt; Fingerprint' to set up a fingerprint",
                      Toast.LENGTH_LONG).show();
              purchaseButton.setEnabled(false);
              return;
          }
  
          //noinspection ResourceType
          mFingerprintManager = (FingerprintManager) getSystemService(FINGERPRINT_SERVICE);
          if (ActivityCompat.checkSelfPermission(this, Manifest.permission.USE_FINGERPRINT) != PackageManager.PERMISSION_GRANTED) {
              // TODO: Consider calling
              //    ActivityCompat#requestPermissions
              // here to request the missing permissions, and then overriding
              //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
              //                                          int[] grantResults)
              // to handle the case where the user grants the permission. See the documentation
              // for ActivityCompat#requestPermissions for more details.
              return;
          }
          if (!mFingerprintManager.hasEnrolledFingerprints()) {
              purchaseButton.setEnabled(false);
              // This happens when no fingerprints are registered.
              Toast.makeText(this,
                      "Go to 'Settings -&gt; Security -&gt; Fingerprint' and register at least one fingerprint",
                      Toast.LENGTH_LONG).show();
              return;
          }
          try {
              try {
                  createKey();
              } catch (NoSuchProviderException e) {
                  e.printStackTrace();
              }
          } catch (KeyStoreException e) {
              e.printStackTrace();
          }
          purchaseButton.setEnabled(true);
          purchaseButton.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  findViewById(R.id.confirmation_message).setVisibility(View.GONE);
                  findViewById(R.id.encrypted_message).setVisibility(View.GONE);
  
  
  
                  // Set up the crypto object for later. The object will be authenticated by use
                  // of the fingerprint.
                  try {
                      if (initCipher()) {
  
                          // Show the fingerprint dialog. The user has the option to use the fingerprint with
                          // crypto, or you can fall back to using a server-side verified password.
                          mFragment.setCryptoObject(new FingerprintManager.CryptoObject(mCipher));
                          boolean useFingerprintPreference = mSharedPreferences
                                  .getBoolean(getString(R.string.use_fingerprint_to_authenticate_key),
                                          true);
                          if (useFingerprintPreference) {
                              mFragment.setStage(
                                      FingerprintAuthenticationDialogFragment.Stage.FINGERPRINT);
                          } else {
                              mFragment.setStage(
                                      FingerprintAuthenticationDialogFragment.Stage.PASSWORD);
                          }
                          mFragment.show(getFragmentManager(), DIALOG_FRAGMENT_TAG);
                      } else {
                          // This happens if the lock screen has been disabled or or a fingerprint got
                          // enrolled. Thus show the dialog to authenticate with their password first
                          // and ask the user if they want to authenticate with fingerprints in the
                          // future
                          mFragment.setCryptoObject(new FingerprintManager.CryptoObject(mCipher));
                          mFragment.setStage(
                                  FingerprintAuthenticationDialogFragment.Stage.NEW_FINGERPRINT_ENROLLED);
                          mFragment.show(getFragmentManager(), DIALOG_FRAGMENT_TAG);
                      }
                  } catch (NoSuchPaddingException e) {
                      e.printStackTrace();
                  }
              }
          });
  
      }
  
      /**
       * Initialize the {@link Cipher} instance with the created key in the {@link #createKey()}
       * method.
       *
       * @return {@code true} if initialization is successful, {@code false} if the lock screen has
       * been disabled or reset after the key was generated, or if a fingerprint got enrolled after
       * the key was generated.
       */
  
      private boolean initCipher() throws NoSuchPaddingException {
          try {
              mKeyStore.load(null);
              SecretKey key = (SecretKey) mKeyStore.getKey(KEY_NAME, null);
             // mCipher = Cipher.getInstance("DES/CBC");
              mCipher.init(Cipher.ENCRYPT_MODE, key);
              return true;
          } catch (KeyPermanentlyInvalidatedException e) {
              return false;
          } catch (KeyStoreException | CertificateException | UnrecoverableKeyException | IOException
                  | NoSuchAlgorithmException | InvalidKeyException e) {
              throw new RuntimeException("Failed to init Cipher", e);
          }
      }
  
      public void onPurchased(boolean withFingerprint) {
          if (withFingerprint) {
              // If the user has authenticated with fingerprint, verify that using cryptography and
              // then show the confirmation message.
              tryEncrypt();
          } else {
              // Authentication happened with backup password. Just show the confirmation message.
              showConfirmation(null);
          }
      }
  
      // Show confirmation, if fingerprint was used show crypto information.
      private void showConfirmation(byte[] encrypted) {
          findViewById(R.id.confirmation_message).setVisibility(View.VISIBLE);
          if (encrypted != null) {
              TextView v = (TextView) findViewById(R.id.encrypted_message);
              v.setVisibility(View.VISIBLE);
              v.setText(Base64.encodeToString(encrypted, 0 /* flags */));
          }
      }
  
      /**
       * Tries to encrypt some data with the generated key in {@link #createKey} which is
       * only works if the user has just authenticated via fingerprint.
       */
      private void tryEncrypt() {
          try {
              byte[] encrypted = mCipher.doFinal(SECRET_MESSAGE.getBytes());
              showConfirmation(encrypted);
          } catch (BadPaddingException | IllegalBlockSizeException e) {
              Toast.makeText(this, "Failed to encrypt the data with the generated key. "
                      + "Retry the purchase", Toast.LENGTH_LONG).show();
              Log.e(TAG, "Failed to encrypt the data with the generated key." + e.getMessage());
          }
      }
  
      /**
       * Creates a symmetric key in the Android Key Store which can only be used after the user has
       * authenticated with fingerprint.
       */
      public void createKey() throws KeyStoreException, NoSuchProviderException {
          // The enrolling flow for fingerprint. This is where you ask the user to set up fingerprint
          // for your flow. Use of keys is necessary if you need to know if the set of
          // enrolled fingerprints has changed.
  
        //  mKeyStore=(KeyStore)getSystemService(Context);
          mKeyStore = KeyStore.getInstance(KeyStore.getDefaultType());
  
          try {
  
              mKeyStore.load(null);
              // Set the alias of the entry in Android KeyStore where the key will appear
              // and the constrains (purposes) in the constructor of the Builder
               mKeyGenerator = mKeyGenerator.getInstance("AES", "BC");
              mKeyGenerator.init(new KeyGenParameterSpec.Builder(KEY_NAME,
                      KeyProperties.PURPOSE_ENCRYPT |
                              KeyProperties.PURPOSE_DECRYPT)
                      .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
                      // Require the user to authenticate with a fingerprint to authorize every use
                      // of the key
                      .setUserAuthenticationRequired(true)
                      .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
                      .build());
              mKeyGenerator.generateKey();
          } catch (NoSuchAlgorithmException | InvalidAlgorithmParameterException
                  | CertificateException | IOException e) {
  
          }
      }
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          getMenuInflater().inflate(R.menu.menu_main, menu);
          return true;
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          int id = item.getItemId();
  
          if (id == R.id.action_settings) {
              Intent intent = new Intent(this, SettingsActivity.class);
              startActivity(intent);
              return true;
          }
          return super.onOptionsItemSelected(item);
      }
  }                                                                                                                         
  </code></pre>