example.java

if (null != mAdapterProvisions.getItem(position) &&
    null != ((NymiDevice) mAdapterProvisions.getItem(position)).getKeys() &&
    !((NymiDevice) mAdapterProvisions.getItem(position)).getKeys().isEmpty()) {
    ((NymiDevice) mAdapterProvisions.getItem(position)).sign("Message to be signed",
            ((NymiDevice) mAdapterProvisions.getItem(position)).getKeys().get(0), new NymiDevice.NymiSignCallback() {
                @Override
                public void onMessageSigned(int status, String signature) {
                    if (status == NymiDevice.NymiSignCallback.SIGN_LOCAL_SUCCESS) {
                        // Of course your code will want to make use of this value otherwise.
                        // This code intends to demonstrate flow.
                        Toast.makeText(MainActivity.this, "Sign (on a dummy message) returned: " + signature, Toast.LENGTH_SHORT).show();
                    } else {
                        Toast.makeText(MainActivity.this, "Sign failed", Toast.LENGTH_SHORT).show();
                    }
                }
            });

} else {
    Toast.makeText(MainActivity.this, "Error retrieving keys", Toast.LENGTH_SHORT).show();
}