code: public static String[] getHashKeys(Activity context) throws NoSuchAlgorithmException {
          PackageInfo packageInfo = null;
          String[] keys = null;
          try {
              packageInfo = context.getPackageManager().getPackageInfo(context.getPackageName(), PackageManager.GET_SIGNATURES);
              
              if(packageInfo != null && packageInfo.signatures != null && packageInfo.signatures.length > 0) {
              	
              	keys = new String[packageInfo.signatures.length];
              	for (int i = 0; i < packageInfo.signatures.length; i++) {
              		Signature signature = packageInfo.signatures[i];
              		MessageDigest md = MessageDigest.getInstance("SHA1");
              		md.update(signature.toByteArray());
              		String hash = new String(Base64.encode(md.digest(), 0));
              		keys[i] = hash;
  				}
              }
          } 
          catch (PackageManager.NameNotFoundException e) {
          	e.printStackTrace();
          }
          
          return keys;
  	}
docs: Returns the hash key used to authenticate this application with Facebook.  Useful for debugging.
  @param context
  @return
  @throws NoSuchAlgorithmException