Help?? why am I getting an error?

import java.util.Stack;

import java.util.Scanner;

public class PostfixEvaluator

$\{$

private final static char ADD $=\text{'}+\text{'}$;

private final static char SUBTRACT $=\text{'}-\text{'}$;

private final static char MULTIPLY $=\text{'}*\text{'}$;

private final static char DIVIDE $=\text{'}/\text{'}$;

private final static char REMAINDER $=\text{'}\%\text{'}$;

private final static char MINUS $=\text{'}$m$\text{'}$;

private final static char EXCHANGE $=\text{'}$r$\text{'}$;

private final static char DUPLICATE $=\text{'}$d$\text{'}$;

private final static char PRINT $=\text{'}$p$\text{'}$;

private final static char PRINTNREMOVE $=\text{'}$n$\text{'}$;

private final static char ALL $=\text{'}$f$\text{'}$;

private final static char CLEAR $=\text{'}$c$\text{'}$;

private final static char HELP $=\text{'}$h$\text{'}$;

private Stack stack;

public PostfixEvaluator$()$

$\{$

stack $=$ new Stack$()$;

$\}$

public int evaluate$($String expr$)$

$\{$

int op$1,$ op$2,$ result $=0$;

String token;

Scanner scanner $=$ new Scanner$($expr$)$;

while $($scanner$.$hasNext$())$

$\{$

token $=$ scanner.next$()$;

if $($isOperator$($token$))$

$\{$

op$2=($stack$.$pop$()).$intValue$()$;

op$1=($stack$.$pop$()).$intValue$()$;

result $=$ evaluateSingleOperator$($token$.$charAt$(0),$ op$1,$ op$2)$;

stack.push$(($result$))$;

$\}$

else

stack.push$(($Integer$.$parseInt$($token$)))$;

$\}$

return result;

$\}$

private boolean isOperator$($String token$)$

$\{$

return $($ token.equals$("+")||$ token.equals$("-")||$

token.equals$("*")||$ token.equals$("/"))$;

$\}$

private int evaluateSingleOperator$($char operation, int op$1,$ int op$2)$

$\{$

int result $=0$;

switch $($operation$)$

$\{$

case ADD:

result $=$ op$1+$ op$2$;

break;

case SUBTRACT:

result $=$ op$1-$ op$2$;

break;

case MULTIPLY:

result $=$ op$1*$ op$2$;

break;

case DIVIDE:

result $=$ op$1/$ op$2$;

case REMAINDER:

result $=$ op$1/$ op$2$;

case MINUS:

stack.push$(-$stack.pop$())$;

break;

case EXCHANGE:

op$1=$ stack.pop$()$;

op$2=$ stack.pop$()$;

stack.push$($op$1)$;

stack.push$($op$2)$;

break;

case DUPLICATE:

int top $=$ stack.peek$()$;

stack.push$($top$)$;

break;

case PRINT:

System.out.println$($stack$.$peek$())$;

break;

case PRINTNREMOVE:

System.out.println$($stack$.$pop$())$;

break;

case ALL:

for $($int i $=0$; i $<$ stack.size$()$; i$++)$

$\{$

System.out.println$($stack$.$pop$())$;

$\}$

break;

case CLEAR:

stack.clear$()$;

break;

case HELP:

System.out.println$("$Enter the expression using the following integers:

$+,-,*,/,\%,$ m$,$ r$,$ d$,$ p$,$ f$,$ n$,$ c$,$ q$,$ h$")$;

break;

default :

System.out.println$("$Invalid input."$)$;

$\}$

return result;

$\}$

$\}$

import java.util.Scanner;

$/**$

$*$ Demonstrates the use of a stack to evaluate post fix expressions.

$*$

$*$

$*/$

public class PostFixTester

$\{$

public static void main$($String$[]$ args$)$

$\{$

String expression, again;

int result;

Scanner in $=$ new Scanner$($System$.$in$)$;

do

$\{$

PostfixEvaluator evaluator $=$ new PostfixEvaluator$()$;

System.out.println$("$Enter a valid post$-$fix expression one token $"+$

$"$at a time with a space between each token $($e$.$g$.54+321-+*)")$;

System.out.println$("$Each token must be an integer or an operator $(+,-,*,/)")$;

expression $=$ in$.$nextLine$()$;

result $=$ evaluator.evaluate$($expression$)$;

System.out.println$()$;

System.out.println$("$That expression equals $"+$ result$)$;

System.out.print$("$Evaluate another expression $[$Y$/$N$]?")$;

again $=$ in$.$nextLine$()$;

System.out.println$()$;

$\}$

while $($again$.$equalsIgnoreCase$("$y$"))$;

in$.$close$()$;

$\}$

$\}$

public class InStack

$\{$

private int$[]$ stack;

private int top;

public InStack$($int capacity$)$

$\{$

stack $=$ new int$[$capacity$]$;

top $=-1$;

$\}$

public void push$($int value$)$

$\{$

stack$[++$top$]=$ value;

$\}$

public int pop$()$

$\{$

return stack$[$top$--]$;

$\}$

public int peek$()$

$\{$

return stack$[$top$]$;

$\}$

public boolean isEmpty$()$

$\{$

return top $==-1$;

$\}$

public int size$()$

$\{$

return top $+1$;

$\}$

public void clear$()$

$\{$

top $=-1$;

$\}$

$\}$

Enter a valid post$-$fix expression one token at a time with a space between each token $($e$.$g$.54+321-+**)$

Each token must be an integer or an operator $(+,-,**,//)$

$23$ r $3+5$

ERROR