Some people say that the Japanese government has borrowed too much money, and that its bonds may collapse soon just like the Greek bonds did.

But things are different. The Greek problem is that they spend far more than they produce, borrowing money from abroad.
The Japanese problems is that they spend far less than they produce. So the government borrows the money which the people keep in banks and spend that money to stimulate the economy and to uphold ever growing social welfare expenditure.

Thus, the Japanese government has not borrowed much from abroad, so the sovereign risk is not the issue.
Too large issuance of government bonds may eventually pressure the financial market and push up the interest rates, but there is no sign of it yet.

Japan can still go on. Its production basis is still very sound. It is the only country which enjoys trade surplus vis-a-vis China and South Korea. The more China grows, and the more they export, the more they have to import machine tools and high-tech materials and parts from Japan.

(You may ask whether I invest my money. The answer is; I am betting my money on the gold)