I'm in Japan and I have a Mastercard, but I forgot my PIN. Do you know if it is possible to swipe and pay?
Japan is a bit behind the curve for chip & PIN, it's now possible in some places but swipe & sign remains universally accepted. Do remember that Japan remains a cash-heavy society and there are lots of smaller shops, restaurants and accommodations that operate on a cash-only basis.
The only time you really need a PIN is for cash advances from ATMs, although most Japanese ATMs don't accept any foreign credit cards period (see Wikivoyage for the gruesome, ever-changing details).