Java Class calling another class?

i have a class called infix and a separate class called postfix. what i am trying to do is make the infix class create a string of postfix notation and then call the postfix class to do the computation.
i have the string all ready to be passed to the postfix class for computation..my question is...how do i get it to that class?

HERE ARE THE TWO CLASSES. THE POSTFIX WORKS FOR SURE AND I AM TRYING TO GET THE INFIX TO WORK. THE QUESTION IS IN THE RETURN OF THE INFIX. DONT TRY TO CORRECT THE INFIX CODE. I AM WORKING ON THAT BY MYSELF. I JUST NEED HELP ON CONNECTING THEM IF POSSIBLE. CHECK THE RETURN OF INFIX. THANKS ALOT
import java.util.*;
public class PostFix
     private Set<String> ops = new HashSet<String>();
     private List<String> lst = new LinkedList<String>();
     private static Map<String, String> m = new HashMap<String, String>();
     public List<String> getLst()
          return lst;
     public Map<String, String> getMap()
          return m;
     public PostFix( String s )
          ops.add("+");
          ops.add("-");
          ops.add("*");
          ops.add("/");
          ops.add("%");
          StringTokenizer sTok = new StringTokenizer(s);
          while( sTok.hasMoreTokens() )
               lst.add( sTok.nextToken() );
          m.put( "a" , "26" );
          m.put( "b" , "25" );
          m.put( "c" , "24" );
          m.put( "d" , "23" );
          m.put( "e" , "22" );
          m.put( "f" , "21" );
          m.put( "g" , "20" );
          m.put( "h" , "19" );
          m.put( "i" , "18" );
          m.put( "j" , "17" );
          m.put( "k" , "16" );
          m.put( "l" , "15" );
          m.put( "m" , "14" );
          m.put( "n" , "13" );
          m.put( "o" , "12" );
          m.put( "p" , "11" );
          m.put( "q" , "10" );
          m.put( "r" , "9" );
          m.put( "s" , "8" );
          m.put( "t" , "7" );
          m.put( "u" , "6" );
          m.put( "v" , "5" );
          m.put( "w" , "4" );
          m.put( "x" , "3" );
          m.put( "y" , "2" );
          m.put( "z" , "1" );
     public int Value()
          Stack<Integer>operandStack = new Stack<Integer>();
          Iterator<String> iterator = lst.iterator();
          String tmp;
          int val1 = 0;
          int val2 = 0;
          int tmpInt = 0;
          while( iterator.hasNext() )
               tmp = iterator.next();
               if( !ops.contains( tmp ) )
                    operandStack.push(Integer.parseInt(tmp));
               else
                    val1 = operandStack.pop();
                    val2 = operandStack.pop();
                    Character c = new Character( tmp.charAt(0) );
                    switch( c )
                         case '+':
                              operandStack.push(val1 + val2);
                              break;
                         case '-':
                              operandStack.push(val2 - val1);
                              break;
                         case '*':
                              operandStack.push(val1 * val2);
                              break;
                         case '/':
                              operandStack.push(val2 / val1);
                              break;
                         case '%':
                              operandStack.push(val2 % val1);
                              break;
          return operandStack.peek();
     public int Value( Map m )
          Stack<Integer> operandStack = new Stack<Integer>();
          Iterator<String> iterator = lst.iterator();
          String tmp;
          String x;
          int val1 = 0;
          int val2 = 0;
          int tmpInt = 0;
          while( iterator.hasNext() )
               tmp = iterator.next();
               if( !ops.contains( tmp ) && !m.containsKey( tmp ) )
                    operandStack.push(Integer.parseInt(tmp));
               else if ( !ops.contains(tmp) && m.containsKey(tmp) )
                    x = m.get(tmp).toString();
                    operandStack.push(Integer.parseInt(x));
               else{
                    val1 = operandStack.pop();
                    val2 = operandStack.pop();
                    Character c = new Character( tmp.charAt(0) );
                    switch( c )
                         case '+':
                              operandStack.push(val1 + val2);
                              break;
                         case '-':
                              operandStack.push(val2 - val1);
                              break;
                         case '*':
                              operandStack.push(val1 * val2);
                              break;
                         case '/':
                              operandStack.push(val2 / val1);
                              break;
                         case '%':
                              operandStack.push(val2 % val1);
                              break;
          return operandStack.peek();
     and the infix is
import java.util.*;
public class InFix
     PostFix pFix;
     private Set<String> LowOps = new HashSet<String>();
     //private Set<String> parenthesis = new HashSet<String>();
     private Set<String> HighOps = new HashSet<String>();
     private List<String> lst = new LinkedList<String>();
     public List<String> getLst()
          return lst;
     public InFix( String s )
          LowOps.add("+");
          LowOps.add("-");
          HighOps.add("*");
          HighOps.add("/");
          //ops.add("%");
          StringTokenizer sTok = new StringTokenizer(s);
          while( sTok.hasMoreTokens() )
               lst.add( sTok.nextToken() );
     public int Value()
          Stack <String> operatorStack = new Stack<String>();
          Iterator<String> iterator = lst.iterator();
          String tmp = "";
          String pFixStr = "";
          while( iterator.hasNext() )
               tmp = iterator.next();
               if( tmp == "(" )
                    operatorStack.push(tmp);
               else if( tmp == ")" )
                    String peektmp = operatorStack.peek();
                    while( peektmp != "(" )
                         pFixStr.concat(operatorStack.pop() );
               else if( !LowOps.contains(tmp) && !HighOps.contains(tmp) )//pushes if its a number
                    pFixStr.concat( tmp );
               else if( LowOps.contains( tmp ) )
                    operatorStack.push(tmp);
               else if( HighOps.contains( tmp ) )
                    operatorStack.push(tmp);
          while ( !operatorStack.empty() )
               pFixStr.concat(operatorStack.pop());
          System.out.println( pFixStr );
     return <THIS IS WHERE I WANT TO RETURN THE COMPUTATION FROM THE POSTFIX CLASS. IS THIS CORRECT TO TRY AND RETURN A POSTFIX(PFIXSTR) OR SOMETHING LIKE THAT>
     }THANKS

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