How to convert CLOB to VARCHAR2 in oracle to java?

Question

In pl/sql,I write the code and it runs well.But when I using java,it tells me wrong.How to fix it?

select clobcontent,to_char(clobcontent),DBMS_LOB.SUBSTR(clobcontent,4000,1),
CAST(clobcontent AS VARCHAR2) from t_student 

If I use to_char in the java,it is wrong.

ORA-64203:Destination buffer too small to hold CLOB data after character set conversion.

So I try to cut out the colbcontent.I use DBMS_LOB.SUBSTR,but it is still wrong.

ora-06502 pl/sql numeric or value error character string buffer too small.

What's wrong?My sql statement runs well in pl/sql developer but runs wrong in java.