depends on how much time they needed to call it "boom".
for one day 5% it is boom
japanese stocks are expected to go up this year(not just sony)
since japan has announced a US-style quantative easing(=virtual money printing=more debts)
it is likely that japanese stocks will perform pretty much the same way like the us-stocks did since the first QE.
the difference:japan is hoping for an inflation/devaluation of the yen while the usa have to keep the dollar strong in a relative way as a weak dollar is supposed to lose its status as world reserve currency