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It's a question which you really need to address to Java's designers.

The only answer I can offer is that Java (at least originally) was intended to be a simple language and they deliberately cut out a lot of stuff that's in C/C++ including unsigned types.

They've subsequently added some stuff back (such as enums) though not, unfortunately, unsigned types which can make life difficult if you need to interoperate with native code.

However, if experience of C# is anything to go by, unsigned types are not used much outside of interop anyway, so it's not as serious an omission as you might think.