This document contains the code for three Java classes: Stack, Queue, and Postfix. Stack and Queue define basic LIFO and FIFO data structures using a List to store elements. Postfix defines a class to convert infix arithmetic expressions to postfix notation by using Stacks and Queues to process operator precedence.

1. 1
Stack.java
importjava.util.*;
/**
* A LIFOstack of generic<CODE>Object</CODE>s.
*
* @author JimGlenn
* @version0.1 12/05/2005
*/
publicclassStack
{
/**
* The listthatholdsthe elementsonthisstack. The endof the
* lististhe topof the stack.
*/
private Listelements;
/**
* Createsan emptystack.
*/
publicStack()
{
elements=newArrayList();
}
/**
* Addsan itemto the top of thisstack.
*
* @param toAddthe itemto add
*/
publicvoidpush(ObjecttoAdd)
{
elements.add(toAdd);
}
/**
* Removesthe itematthe top of thisstack.
*
* @return the itemat the top of thisstack
*/
publicObjectpop()
{
returnelements.remove(elements.size() - 1);
}

2. 2
/**
* Returnsthe itemat the top of thisstack. That itemisnot
* removed.
*
* @return the itemat the top of thisstack
*/
publicObjectpeek()
{
returnelements.get(elements.size()- 1);
}
/**
* Returnsthe numberof elementsonthisstack.
*
* @return the size of thisstack
*/
publicintsize()
{
returnelements.size();
}
}

3. 3
Queue.java
importjava.util.*;
/**
* A FIFOqueue of generic<CODE>Object</CODE>s.
*
* @author JimGlenn
* @version0.1 12/05/2005
*/
publicclassQueue
{
/**
* The listthatholdsthe elementsonthisqueue. The frontof the
* lististhe frontof the queue.
*/
private Listelements;
/**
* Createsan emptyqueue.
*/
publicQueue()
{
elements=newLinkedList();
}
/**
* Addsan itemto the endof thisqueue.
*
* @param toAddthe itemto add
*/
publicvoidenqueue(ObjecttoAdd)
{
elements.add(toAdd);
}
/**
* Removesthe itematthe front of thisqueue.
*
* @return the itemat the frontof thisqueue
*/
publicObjectdequeue()
{
returnelements.remove(0);
}

4. 4
/**
* Returnsthe itemat the front of thisqueue. Thatitemisnot
* removed.
*
* @return the itemat the frontof thisqueue
*/
publicObjectfirst()
{
returnelements.get(0);
}
/**
* Returnsthe numberof elementsonthisqueue.
*
* @return the size of thisqueue
*/
publicintsize()
{
returnelements.size();
}
}

5. 5
Postfix.java
importjava.util.*;
importjava.io.*;
/**
* Anarithmeticexpressioninpostfix form.
*
* @author JimGlenn
* @version0.2 12/05/2005
* @version0.1 11/22/2004
*/
publicclassPostfix
{
private Stringexp;
/**
* Createsa postfix expressionfromthe givenprefix expression.
* Thisversiondoesnothandle errorsanddoesnot allow
* integerliterals.
*
* @param infix alegal infix expression
*/
publicPostfix(Stringinfix)
{
exp= "";
// create a queue containingthe tokensinthe infix expression;
// we do thisbecause StringTokenizerhasnoequivalentof peek
Queue queue =newQueue();
StringTokenizertok=newStringTokenizer(infix);
while (tok.hasMoreTokens())
queue.enqueue(tok.nextToken());
// create the operatorstack
Stack stack = newStack();
while (stack.size() +queue.size() >0)
{
if (queue.size()==0)
exp= exp+ stack.pop() +" ";
else if (isIdentifier((String)(queue.first())))
exp= exp+ queue.dequeue()+" ";
else if (queue.first().equals("("))
stack.push(queue.dequeue());
else if (stack.size()==0)

6. 6
stack.push(queue.dequeue());
else if (getPrecedence((String)(queue.first()))
> getPrecedence((String)(stack.peek())))
stack.push(queue.dequeue());
else if (stack.peek().equals("(")
&& queue.first().equals(")"))
{
stack.pop();
queue.dequeue();
}
else
exp= exp+ stack.pop() +" ";
}
}
/**
* Precedence levelsforarithmeticoperatorsandparentheses.
* Levelsare spacedoutto make it easiertoadd operatorswith
* in betweenprecedences.
*/
private staticfinal intADDITIVE= 0;
private staticfinal intMULTIPLICATIVE= 10;
private staticfinal intPARENTHESES= -10;
/**
* Returnsthe precedence of the givenoperator.
*
* @param op an operatoror parenthesis
* @return the precedence of the givenoperator.
*/
private staticintgetPrecedence(Stringop)
{
if (op.equals("+") ||op.equals("-"))
returnADDITIVE;
else if (op.equals("*") ||op.equals("/"))
returnMULTIPLICATIVE;
else if (op.equals("(") ||op.equals(")"))
returnPARENTHESES;
else
throw newIllegalArgumentException();
}
/**
* Determinesif the givenstringisanidentifier.
*
* @param id a string
* @return true iff the givenstringisan identifier
*/

7. 7
private staticbooleanisIdentifier(Stringid)
{
returnCharacter.isJavaIdentifierStart(id.charAt(0));
}
/**
* Returnsa stringrepresentationof thisexpression.
*
* @return a stringrepresentationof thisexpression.
*/
publicStringtoString()
{
returnexp;
}
publicstaticvoidmain(String[] args) throwsIOException
{
BufferedReaderin
= newBufferedReader(newInputStreamReader(System.in));
System.out.println("Enteralegal infix expression:");
System.out.println(newPostfix(in.readLine()));
}
}