Japanese submarines can dive much longer without having to surface. This is a significant technology. Japan’s Ministry of Defense has said it plans to replace conventional lead-acid batteries with more powerful lithium-ion cells, which will enable the vessels to cruise at high speeds underwater.
The Japanese submarine is as capable in combat as the German boats. Japan is also advanced in combat systems (which can pick out specific sounds of the enemy from surrounding noise and conduct operations based on this information). In addition, Japan has a well-developed supply chain for submarine building. There are companies that can custom-make even a single screw for a submarine.
Military ships require special steel plate processing. The inner shell of the boat should be as tough as it can be to resist the high pressure deep under the sea. The work requires skilled welding techniques that cannot be acquired overnight.