  Many efforts have been made to reveal the nature of the overabundant resonant
structures observed by the worldwide experiments in the last two decades.
Hadronic molecules attract special attention because many of these seemingly
unconventional resonances are located close to the threshold of a pair of
hadrons. To give an overall feature of the spectrum of hadronic molecules
composed of a pair of heavy-antiheavy hadrons, namely, which pairs are possible
to form molecular states, we take charmed hadrons for example to investigate
the interaction between them and search for poles by solving the Bethe-Salpeter
equation. We consider all possible combinations of hadron pairs of the $S$-wave
singly-charmed mesons and baryons as well as the narrow $P$-wave charmed
mesons. The interactions, which are assumed to be meson-exchange saturated, are
described by constant contact terms which are resummed to generate poles. It
turns out that if a system is attractive near threshold by the light meson
exchange, there is a pole close to threshold corresponding to a bound state or
a virtual state, depending on the strength of interaction and the cutoff. In
total, 229 molecular states are predicted. The observed near-threshold
structures with hidden-charm, like the famous $X(3872)$ and $P_c$ states, fit
into the spectrum we obtain. We also highlight a $\Lambda_c\bar \Lambda_c$
bound state that has a pole consistent with the cross section of the
$e^+e^-\to\Lambda_c\bar \Lambda_c$ precisely measured by the BESIII
Collaboration.
