Now, where are these primitive data types when JDK instuled? Are these just for compiler is visible? I did not find file or folder about them.
One group of types,which you will use quite often in your programming, gets special treatment. You can think of these as primitive types. The reason for the special treatment is that to creat an object with new-especialy a small,simple variable-is not very efficient, because new places objects on the heap. For these types Java falls back on the approach taken by C and C++. That is, instead of creating the variable by using new, an “automatic” variable is created that is not a reference. The variable holds the value directly, and it is placed on the stack, so it is much more efficient.
Java determines the size of each primitive type. These sizes do not change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are more portable than programs in most other languages.