July and august are hot. Sometime really hot. I was in Japan in the summer of 2008, 2011 and 2012, and there was some pretty hot periods. I do remember seing temp of 40C around Tokyo on TV in 2012. After, it also depend where in Japan.
I can tell that you should always have something to drink with you.
But the nice thing about summer is that there is so many festival, if you do not want to go to any festival, avoir summer.
It really depends on where you go. North of Tokyo is quit nice. Gunma prefecture and up.
Chiba and Tokyo are crazy hot! Japan actaully has really nice beaches though, so I wouldn't shy away from it. Just be ready to whip out the sunscreen and even umbrella for shade.