Japan was a net importer of food. Starve they would.
Not so much.
I can't find direct numbers, but a little bit of math can still get us there...
In 1943, Japan proper had about 73 million citizens, and imported the equivalent of 2 million tons of rice.
That sounds like a lot, right?
But 73 million active people would eat the equivalent of nearly 50 million tons per year, meaning imports supplied only 4% of their needs.
The rest must come from Japan's home grown production.
The loss of two million tons of imported rice would equate to roughly 100 calories per person per day.
So the Japanese would not starve, period.
They had to be defeated, and the only choice other than A-bomb was invasion.
Given their fanatical resistance, invasion would cost hundreds of thousands of Americans plus millions of Japanese.
So our A-bombs over Hiroshima and Nagasaki were simply the most merciful way possible to end the war, quickly.