Parsing Tree Structure In Java

I have an excel file with data listed as the following, i m trying to parse down using POI
 
A
     B
          C
               D1
               D2
          F
               G1
               G2
               G3
          M
               S1
     R
          T             
     U
L
  X
     Y
         Z
 
is it possible to generate an output like the following
 
A
A-->B
A-->B-->C
A-->B-->C-->D1
A-->B-->C-->D2
A-->B-->F
A-->B-->F-->G1
A-->B-->F-->G2
A-->B-->F-->G3
A-->B-->M
A-->B-->M-->S1
A-->R
A-->R-->T
A-->U
L
L-->X
L-->X-->Y
L-->X-->Y-->Z
 
I have been trying from quite some time but havent figured out the logic ...