"Write the program that prints its own source code"
See the catalog at Puzzles/Quine
In J that would be "write an expression which results in a string that is composed of same characters in same sequence as original expression.
Another way to say it is to define string s such that (-: ".)s
It can be debated whether empty string counts as J expression and therefore is allowable entry, so to reduce on trivial discussion let's assume non-empty s.
Here is a solutions
(,~,{:,{:)'(,~,{:,{:)'''And another one:
(,{:)^:2,~'(,{:)^:2,~'''And this one has same length (24 characters) now but may be reduced to 22 in (future) J6
(,2:#{:),~'(,2:#{:),~'''Can there be shorter expressions with this property?
Is it possible to construct expression with this property that has odd number of characters?