  A comparative study between the L\"uscher's finite volume method and the
time-dependent HAL QCD method is given for the $\Xi\Xi$($^1\mathrm{S}_0$)
interaction as an illustrative example. By employing the smeared source and the
wall source for the interpolating operators, we show that the effective energy
shifts $\Delta E_{\rm eff} (t)$ in L\"uscher's method do not agree between
different sources, yet both exhibit fake plateaux. On the other hand, the
interaction kernels $V(\vec{r})$ obtained from the two sources in the HAL QCD
method agree with each other already for modest values of $t$. We show that the
energy eigenvalues $\Delta E(L)$ in finite lattice volumes ($L^3$) calculated
by $V(\vec{r})$ indicate that there is no bound state in the
$\Xi\Xi(^1\mathrm{S}_0)$ channel at $m_{\pi}=0.51$ GeV in 2+1 flavor QCD.
