private static String getOperandText(EditText operandEditText) {
        String operandText = operandEditText.getText().toString();
        if (TextUtils.isEmpty(operandText)) {
            throw new NumberFormatException("operand cannot be empty!");
        }
        return operandText;
    }
@return the operand text which was entered in an {@link EditText}.
runner/AndroidJunitRunnerSample/app/src/main/java/com/example/android/testing/androidjunitrunnersample/CalculatorActivity.java