NullPointerException
Hello! I am writing a java app to convert Infix to Postfix, and I am getting a NullPointerException that I cannot trace. Any suggestions?
toPostfixTester.java
publicclass toPostfixTester{
publicstaticvoid main (String[] args){
toPostfix convertor =new toPostfix("1+2+3");
convertor.convert();
System.out.println(convertor.returnOutput());
}
}
toPostfix.java
publicclass toPostfix{
String input;
lifoStack output;
lifoStack stack;
public toPostfix (String i){
input = i;
stack =new lifoStack();
}
// Driver for conversion from infix to postfix
publicvoid convert(){
for (int i=0; i < input.length(); i++){
char current = input.charAt(i);
switch (current){
case'+':
case'-':
processOperator(current, 1);
break;
case'*':
case'/':
processOperator(current, 2);
break;
case'(':
stack.push(current);
break;
case')':
processParen(current);
break;
default:
output.push(current);
break;
}
}
while (!stack.isEmpty())
output.push(stack.pop());
}
// Method to handle precedence and operate on Operators
publicvoid processOperator(char operate,int precA){
int precB;
while (!stack.isEmpty()){
if (stack.view().equals('('))// If operator is a (
break;// leave well enough alone.
else{
if (stack.view().equals('+') || stack.view().equals('-'))
precB = 1;
else
precB = 2;
}
if (precA < precB){// If the precedence of our
break;// top operator is less than the new
}// leave well enough alone.
else{
output.push(stack.pop());
}
}
stack.push(operate);
}
// Extra method to handle Parenthesis
publicvoid processParen(char incoming){
while (!stack.isEmpty()){
if (stack.view().equals('(')){
stack.pop();
break;
}
else{
output.push(stack.pop());
}
}
}
public String returnOutput(){
return output.returnOutput();
}
}
lifoStack.java
import java.util.LinkedList;
publicclass lifoStack{
static LinkedList lifoStack =new LinkedList();
public Object pop(){
Object temp = lifoStack.getLast();
lifoStack.removeLast();
return temp;
}
publicvoid push(Object o){
lifoStack.add(o);
}
public Object view(){
return lifoStack.getLast();
}
publicboolean isEmpty(){
return lifoStack.isEmpty();
}
public String returnOutput(){
return lifoStack.toString();
}
}
