Understanding Oracle's ALL_TAB_COLUMNS

Question

I'm new to Oracle and database administration in general.

As context, I want to create a Java class that will give me the information I would get using DESC SOME_TABLE.

I cannot find any specific way to do that in Java, however I found that ALL_TAB_COLUMNS could give me similar info. I tried it in SQL Developer to see how different the output was. It turns out the results are a lot more different that I was expecting.

I was hoping someone could walk me through how to interpret the following:

desc SOME_TABLE;

select 
       COLUMN_NAME
     , DATA_TYPE
     , DATA_LENGTH
     , NULLABLE
  from ALL_TAB_COLUMNS
 where TABLE_NAME='SOME_TABLE'
 order by column_id;

Gives the output:

Name            Null    Type
--------------- ------- ----------------------
UIDPK                   NUMBER(20)
NAME                    VARCHAR2(255)

2 rows selected

COLUMN_NAME      DATA_TYPE     DATA_LENGTH    NULLABLE
--------------- -------------- -------------- -------- 
UIDPK            NUMBER        22              N
UIDPK            NUMBER        22              N
UIDPK            NUMBER        22              Y
NAME             VARCHAR2      255             N
NAME             VARCHAR2      255             Y
NAME             VARCHAR2      255             N

6 rows selected

Why is each column repeated 3 times? Why is the datatype and length different on UIDPK and why is NULLABLE not the same?