Hello all! I have made a program which reads out text files and key input and displays it in app window, the key input can be read by a (not fully implemented) parser which in turn executes commands. Think of it of a really low end version of a terminal or a dos like program. I am making this system object oriented for ease of use. But I am running in some problems, the text read out works fine but keypress handler crashes after a single keypress (EDIT: stack trace in next post). My code is getting a bit muddled and have little experience with OOP, I am having some trouble seeing were I went wrong. Main controller and classes pasted below. Sorry for the long post, but would appreciate any input.

int leftmargin = 10;
float fSize = 16;
int maxLength = 65;
TextEngine sendLine = new TextEngine();
TextEngine displayText = new TextEngine();
KeyInput newKey = new KeyInput();
Parser newParse = new Parser();
//int row = 0;

void setup()
{
 size(600,400);
 background(0);
 fill(0,255,0);
 textFont(loadFont("DOS16.vlw"),fSize);
 //displayText.updateText(loadStrings("test.txt"));
}

void draw(){}

void keyPressed()
{
 char k;
 k = (char)key;
 newKey.update(k);
 println("key pressed");
}

/*
             code below meant for second tab
*/

class Parser
{
 String tempLine;
 Boolean cmdSent;
 Boolean lastLetter;
 String inputString;
 Parser()
 {
   tempLine = "";
   cmdSent = false;
   lastLetter = false;
 }
 void sendParse(String input)
 {
   inputString = input;
   inputString = inputString.toLowerCase();
   for (int i = 0; i < inputString.length(); i++)
   {
    //println("char " + i);
    char outLetter = inputString.charAt(i);
    if (i == inputString.length()-1)
    {
      lastLetter = true;
      //println("last letter " + outLetter);
      charChecker(outLetter);
    } else {
      charChecker(outLetter);
    }
  }
}

void charChecker(char inLetter)
{
 //println("char pass");
 if (lastLetter == true)
 {
   //println("trigger last");
   if (inLetter != 32)
   {
     tempLine = tempLine + inLetter;
   }
   cmdNounSplitter();
   lastLetter = false;
   } else {
     switch(inLetter)
     {
     case 32: // space
     cmdNounSplitter();
     break;
     case 8: //backspace
     case 10: //enter pc
     case 13: // enter mac
     case 65535:
     case 127:
     case 27:
     break;
     default:
     tempLine = tempLine + inLetter;
     //print(tempLine);
     break;
     }
   }
 }

 void cmdChecker(String inputLine)
 {
   if(inputLine.equals("cd"))
   {
     println("cd command");
   } else if(inputLine.equals("run"))
   {
     println("run command");
   } else {
     println("error");
   }  
   tempLine = "";
 }

 void nounChecker(String inputLine)
 {
   if(inputLine.equals("bla"))
   {
     println("bla noun!");
   } else if (inputLine.equals("text.txt"))
   {
     println("text noun!");
   } else {
     println("error");
   }
   tempLine = "";
   cmdSent = false;
 }

 void cmdNounSplitter()
 {
   if (tempLine.equals("") == false)
   {
     //println("not empty");
     if (cmdSent == false)
     {
       //println("cmdSent !");
       //println(tempLine);
       cmdSent = true;
       cmdChecker(tempLine);
     } else {
       //println("noun sent!");
       //println(tempLine);
       nounChecker(tempLine);
     }
   }
 }
}
class TextEngine
{
 Boolean moreText;
 int nrStartChar;
 int numRow;
 int numLines;
 int row;
 String[] lineBuff;
 TextEngine()
 {
   moreText = false;
   nrStartChar = 0;
   numRow = 25;
   row = 0;
   lineBuff = new String[numRow];
 }
   void updateText(String lines[])
 {
   background(0);
   //println("there are " + lines.length + " lines");
   println(lines.length);
   numLines = lines.length;
   for (int i=0; i < lines.length; i++)
   {
   //println(lines[i]);
     if (i < numRow-1)
     {
       lineBuff[i] = lines[i];
     } else {
       moreText = true;
       println("there is more text");
     }
   }
   //lineBuff[row] = "";
   float uPos;
   char k;
   //println(row);
   //uPos = textWidth(lineBuff[row]) + leftmargin;
   int maxLines;
   if(numLines > numRow)
   {
     maxLines = numRow -1;
     } else {
       maxLines = numLines;
     }
   for(int i = 0; i < maxLines; i++)
   {
     println("going " + i + " times");
     text(lineBuff[i],leftmargin,(i*fSize)+fSize);
     row = row + 1;
   }
 }
}

class KeyInput
{
 String[] activeInput;
 KeyInput()
 {
   activeInput = new String[1];
   activeInput[0] = "";
 }
 void update(char m)
 {
 switch(m)
   {
     case 8:
     if (activeInput[0].length() > 0)
    {
      println("backspace");
      activeInput[0] = activeInput[0].substring(0, activeInput[0].length()-1);
      println(activeInput[0]);
      sendLine.updateText(activeInput);
    }
     break;
     case 10:
     case 13:
     String activeString = activeInput[0];
     newParse.sendParse(activeString);//send to parser!
     break;
     case 65535:
     case 127:
     case 27:
     break;
     default:
     if(activeInput[0].length() < maxLength)
     {
       activeInput[0] = activeInput[0] + m;
       sendLine.updateText(activeInput);
     }
     break;
     }
   }
 }

"would appreciatie any input" Lol!

JR is right in the sense you should check if you have some characters in your input before deleting them, but as long as you don't hit backspace, there shouldn't be a problem.
I too fails to see why you create an array of strings and use only one item of the array. If you plan to have an history of commands, you should put activeInput as member of the class, not as local variable of the method. Otherwise you won't store any character there.

"null point exception somewhere in the keypress and/or textengine classes"
Again JR is right: these exceptions show a detailed stack trace so we can know the exact location of the issue. Your vague report doesn't help...

I guess the problem occurs here:
uPos = textWidth(lineBuff[row]) + leftmargin;

... and you probably haven't initialized that position in the array.