Help with a school project please

Question

Finally got the code to compile without errors, but now it is giving me a null pointer exception and I can't see it (blind, maybe)
Can someone help me see what I have done wrong?
Exception:
Exception in thread "main" java.lang.NullPointerException
at Calc2.getInput(Calc2.java:38)
at Calc2.main(Calc2.java:18)
import java.util.*;
public class Calc2 {
     static MyStack stack;
     public Calc2 () {
          stack = new MyStack();
     public static void main(String[]args) {
          for (int i=0; i<args.length; i++) {
               if (testNext(i, args)){
                    getInput(i, args);
               } else {
                    try {
                         startCalculation(i, args);
                    } catch (StackException exc) {
                         System.out.println("Stack Exception");
                    }//end try catch
                    try {
                         System.out.println(stack.peek());
                    } catch (StackException exc) {
                         System.out.println("Stack Exception");
                    }//end try catch
     } //end main
     public static void getInput(int i, String args) {
          try {
               int num = Integer.parseInt(args);
               stack.push(num);
          } catch (StackException exc) {
               System.out.println("Stack Exception");
          }//end try catch
     }//end getInput
     public static boolean testNext(int i, String[]args) {
          if (args[i].startsWith("1")
               ||args[i].startsWith("2")
               ||args[i].startsWith("3")
               ||args[i].startsWith("4")
               ||args[i].startsWith("5")
               ||args[i].startsWith("6")
               ||args[i].startsWith("7")
               ||args[i].startsWith("8")
               ||args[i].startsWith("9")) {
               return true;
          } else {
               return false;
     }//end testNext
     public static void startCalculation(int i, String[]args) throws StackException {
          try {
               String getB = (String)stack.pop();
               String getA = (String)stack.pop();
               int b = Integer.parseInt(getB);
               int a = Integer.parseInt(getA);
               int result;
               if (args[i]=="+") {
                    result=a+b;
                    if (stack.isEmpty()==false)
                         stack.push(result);
               } else if (args[i]=="-") {
                    result=a-b;
                    if (stack.isEmpty()==false)
                         stack.push(result);
               } else if (args[i]=="x") {
                    result=a*b;
                    if (stack.isEmpty()==false)
                         stack.push(result);
               } else if (args[i]=="/") {
                    result=a/b;
                    if (stack.isEmpty()==false)
                         stack.push(result);
               } else {
                    throw new StackException("Invalid Exception");
               }//end ifs
               if (i > args.length && stack.isEmpty()==false) {
                    startCalculation(i++, args);
               } else {
                    System.out.println(result);
               }//end if
          } catch (StackException exc) {
               System.out.println("Stack Exception");
          }//end try catch
     }//end startCalculation
}//end Calc2
Edited by: DilutedEgo on Oct 22, 2007 8:12 PM                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

Answer 1

Your stack is null. It only gets initialised inside the Calc2 constructor but since you never create a Calc2 object the stack will remain null.
Also never use == to compare Strings, use the equals method instead.