reclassify a list of strings JAVA -

i need java code

i have list saved in text file following :

a b c d c h b e b f b g g l e e j f k k m 1 2 1 3 2 4 

after reading file test.txt, lines added list ( named ch in code) composed of 2 parts, fist part source extracted variable src in code second part target extract trg in code

for instance, in first line : b source (src) et target (trg) b

now, want create new list (mylist in code) when there line target equal source of previous line. instance, let's stay fist line , see 5th line :

a b  b e  

the new list mylist must contain line : b e

so in general ch list must reclassified following :

a d c h b g l  b e  b e j  b f k m 1 2 4 1 3 

i obtain following exception :

line:a b exception in thread "main" java.lang.nullpointerexception @ pathcreator.test.main(test.java:48) picked _java_options: -xmx512m

this code not working basicly because building new list 3 elements mylist.add(src[a] +" "+trg[a]+" "+trg[b]); while it's more.

any help?

here code:

import java.io.bufferedreader; import java.io.filenotfoundexception; import java.io.filereader; import java.io.ioexception; import java.util.linkedlist;      public class test {          public static void main(string[] args) throws filenotfoundexception {                  filereader fis;                 fis = new filereader("c:\\test.txt");                  bufferedreader in = new bufferedreader(fis);                 string i=null;                 string line =null;                   string[] src = null;                  string[] trg = null;                   linkedlist<string> ch = new linkedlist<string>();                  linkedlist<string> mylist = new linkedlist<string>();                 try {                     while((i = in.readline()) != null)                     {                              line = i;                             ch.add(line);                              system.out.println(" line:"  + line);                             src = new string[ch.size()];                             trg =  new string[ch.size()];                               for(int j = 0; j<ch.size()-1;j++){                                    string pc = ch.get(j).tostring();                                   string[] pe = pc.split(" ");                                   src[j] = pe[0];                                   trg[j] = pe[1];                                   }                              for(int = 0; a<src.length; a++){                                 for(int b = a; b<trg.length; b++){                                     if(src[b].equals(trg[a]) && a!= b){                                         mylist.add(src[a] +" "+trg[a]+" "+trg[b]);                                          system.out.println(src[a] +" "+trg[a]+" "+trg[b]);                                     }                                 }                             }                             }                         }                   catch (ioexception e){                     system.out.println(e.getmessage());                     system.exit(1);                 }              }      }  

if understand problem:

the input list of edges of graph,
output list of roof-to-leaf paths of trees in graph.

i recommend fill graph structure input, search , use algorithm print roof-to-leaf paths (you can find 1 on google/stackoverflow).

your exception because of j<ch.size()-1, leaves last element of src , trg null. try j<ch.size().

finally, please don't use unintelligible abbreviations variable names, avoid trg, ch, pe, ...