For what design reason is there no sizeof operator in Java? Knowing that it is very useful in C++ and C#, how can you get the size of a certain type if needed?
Because the size of primitive types is explicitly mandated by the Java language. There is no variance between JVM implementations.
Moreover, since allocation is done by the new operator depending on its argument there is no need to specify the amount of memory needed.
It would sure be convenient sometimes to know how much memory an object will take so you could estimate things like max heap size requirements but I suppose the Java Language/Platform designers did not think it was a critical aspect.