the constant pool lost #3 in class file

Question

I use "javap -verbose" to parse a class file and find the constant pool lost #3, anybody can tell me the reason?

 

  

Answer 1

Note that the preceding entry has the type Long and its documentation says:

All 8-byte constants take up two entries in the constant_pool table of the class file. If a CONSTANT_Long_info or CONSTANT_Double_info structure is the item in the constant_pool table at index n, then the next usable item in the pool is located at index n+2. The constant_pool index n+1 must be valid but is considered unusable.

and the neat addition

In retrospect, making 8-byte constants take two constant pool entries was a poor choice.

So javap decided to simply skip the unusable entry instead of printing something like “#3 unusable”…

Answer 2

Longs and Doubles take up two slots in the constant pool, so the slot after them is skipped. In this case slot 2 holds a Long, so slot 3 is skipped.