You forgot the void before draw().
And you can put the content of the main() at the end of setup().

1. import java.io.*; import java.util.*;
2. void setup() {
     size(120,120);
     noLoop();
   }
3. void draw() {
   Sort1 s=new Sort1("z1.txt","z2.txt");
   s.sortfile();
   }
4. public class Sort1 {
   String infile;
   String outfile;
5. Sort1(String infile_, String outfile_) {
    infile=infile_;
    outfile=outfile_;
   }
6. void sortfile() throws Exception {    
   BufferedReader reader = new BufferedReader(new FileReader(infile));    
   Map<String, String> map=new TreeMap<String, String>();    
   String line="";    
   while((line=reader.readLine())!=null){            
   map.put(getField(line),line);    
   }    
   reader.close();    
   BufferedWriter writer = new BufferedWriter(new FileWriter(outfile));    
   for(String val : map.values()) {            
   writer.write(val);                  
   writer.newLine();    
   }    
   writer.close();
   }//sortfile()
   private  String getField(String line) {    
   return line.split(" ")[0]; //extract value you want to sort on
   }//getField()
   }