读取二维码；错误：找不到符号变量 AppConstants

Question

我复制了一个代码，通过相机读取二维码并获取文本；但我得到编译错误-> 错误：找不到符号变量 AppConstants。我也不明白为什么在 handleResult() 中需要意图以及如何取回二维码的结果文本。我想 result.getText() 给出了结果文本。但是怎么用呢？有没有更好的例子？

错误->

 error: cannot find symbol
        intent.putExtra(AppConstants.KEY_QR_CODE, result.getText());
                        ^
  symbol:   variable AppConstants
  location: class QRScannerActivity

代码：

public class QRScannerActivity extends AppCompatActivity implements ZXingScannerView.ResultHandler {
    private ZXingScannerView mScannerView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_q_r_scanner);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        mScannerView = new ZXingScannerView(this);
        setContentView(mScannerView);

    }

    @Override
    public void onResume() {
        super.onResume();
        mScannerView.setResultHandler(this);
        mScannerView.startCamera();
    }

    @Override
    public void onPause() {
        super.onPause();
        mScannerView.stopCamera();

    }
    @Override
    public void handleResult(Result result) {
        Intent intent = new Intent();
        intent.putExtra(AppConstants.KEY_QR_CODE, result.getText());
        setResult(RESULT_OK, intent);
        finish();
    }
}