As listed on the US Embassy site for Japan (but applicable to other western nations as well):
Travelers entering the country without a visa must have a return or onward ticket in their possession, proof of adequate funds for their stay, and be otherwise admissible under Japanese law.
Going to the airport with a one-way ticket and no visa is almost always denied boarding.
You might get through with a one-way and a promise, but someone with the identical situation could be refused entry by Japanese Immigration. Remember, there is no guarantee Japan will allow you into the country!
Airlines are fined megabucks for bringing in someone who then gets sent back. So they do function as a de facto gatekeeper. More than likely you will be denied boarding at your airport of departure unless you buy a return or onward ticket at the airport (very expensive). It would be wiser to get an air ticket with a cheap or free date change ability, or have an onward ticket by plane/ship/ferry etc to show. It'd be a real drag after a nightmarishly long airplane ride to be told your vacation plans are DOA.