The Japanese wisely conserved their resources by copying technology when more advanced models were available. But it is not nearly as pervasive as us white folks might like to think (I say this because often these stories contain a tinge of 'those people could never be so clever as to do it on their own' attitude in them). Many stories of Japanese weapons of WWII being carbon copies of US/foreign products are in circulation, most are false. (The most famous and most false is that the Zero design was of US origin - it was completely indigenous, and incorporated many unique technologies ahead of its time - see Hiroshi's "Eagles of Mitsubishi"). Detailed examination shows that the Japanese prodcuts, while incorporating foreign ideas and technolgies, were mostly native, for better or worse.

Japanese subs fall into this same category.

After WWI, as a allied power, Japan recieved several U-boats, like all the other major allied naval powers did. They copied them. With the pacifist and depressed 20's upon Germany, Japan also hired many German U-boat experts. Together they made German based but native adapted hybrid designs. By the end of the '20s the Japanese had 'milked' these guys to their satisfaction, and gradually shut them out and sent them home. By the '30s Japanese subs had evolved to their own. Fire control, torpedo design, operational concept, size, all became unique. The Japanese tilted towards very large ocean going fleet boats designed for fleet action. They weree quite different than German u-boats. Quite a few carried midgets or aircraft, something the Germans never tried (with good reason). (See "The Times Book of Submarines")

During the war Germany tried to give technology on subs and aircraft to Japan, in return for rare supplies (quinine, rubber, etc.) The allied naval blockade thwarted this.

It should be noted that trading basic technology and adapting other countries machinery and techonlogy is quite common. The P-51 flew with a British engine, for example. Radar, cryptology machinery, shaped explosive warhead technology were all passed between Britian and America. American subs had German derived engine designs, and a carbon copy of a German electric torpedo. etc. etc.