我可能有一个空指针异常，我只是不知道在哪里

Question

I am making a simple- code generator (non-random) and I've run into a problem. The user inputs data into 7 textfields; then contents of which are then placed into an array of type String. My code then takes that array, analyses the different elements and spits out a code that is usable (by me).

However, in my attempt to standardize the String array (aka make every element a lowercase String), it causes the entire program to stop working and I get what appears to be a null pointer exception.

My specific error message is as such:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
 at inventorycodeconverter.codeDisplay.calculateData(codeDisplay.java:48)
    at inventorycodeconverter.InventoryCodeConverterView.button1ActionPerformed(InventoryCodeConverterView.java:315)
    at inventorycodeconverter.InventoryCodeConverterView.access$900(InventoryCodeConverterView.java:23)
    at inventorycodeconverter.InventoryCodeConverterView$5.actionPerformed(InventoryCodeConverterView.java:211)
    at java.awt.Button.processActionEvent(Button.java:392)
    at java.awt.Button.processEvent(Button.java:360)
    at java.awt.Component.dispatchEventImpl(Component.java:4630)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

The section of code that appears to be giving the trouble is goes as such:

public void calculateData(){

     int i;
     for (i = 0; i < calculationBuffer.length; i++){ // calculationBuffer is defined and initialized elsewhere.

         calculationBuffer[i] = calculationBuffer[i].toLowerCase();// this is line 48 where the first error appears to be.
     }


     if (calculationBuffer[0].equals("cap")){
           displayBuffer[0] = "CAP"; // displayBuffer is defined and initialized elsewhere.
     }
     else {
         displayBuffer[0] = "#Error#";
     }


     if (calculationBuffer[1].equals("20/400")) {
           theTA.append("B");
     }
     else if (calculationBuffer[1].equals("20/410")){
           displayBuffer[1] = "C";
     }
     else if (calculationBuffer[1].equals("24/410")){
         displayBuffer[1] = "F";
     }
     else if (calculationBuffer[1].equals("24/415")) {
         displayBuffer[1] = "Y";
     }
     else if (calculationBuffer[1].equals("28/400")){
         displayBuffer[1] = "H";
     }
     else if (calculationBuffer[1].equals("28/415")) {
         displayBuffer[1] = "X";
     }
     else if (calculationBuffer[1].equals("28/410")) {
         displayBuffer[1] = "I";
     }
     else if (calculationBuffer[1].equals("33/400")) {
         displayBuffer[1] = "J";
     }
     else if (calculationBuffer[1].equals("38/400")) {
         displayBuffer[1] = "K";
     }
     else if (calculationBuffer[1].equals("43/400")) {
         displayBuffer[1] = "M";
     }
     else if (calculationBuffer[1].equals("45/400")) {
         displayBuffer[1] = "N";
     }
     else if (calculationBuffer[1].equals("58/400")) {
         displayBuffer[1] = "Q";
     }
     else if (calculationBuffer[1].equals("70/400")) {
         displayBuffer[1] = "S";
     }
     else if (calculationBuffer[1].equals("89/400")) {
         displayBuffer[1] = "T";
     }

 }


 public void displayData() {
      String displayString;
      displayString = Arrays.toString(displayBuffer);
      StringTokenizer st = new StringTokenizer(displayString, ",");

      while(st.hasMoreTokens()){
          String finalText = st.nextToken();

          theTA.append(finalText);
      }
 }

Answer 1

Somewhere you have declared you array as following (probably):

String[] calculationBuffer = new String[size];

It'll intialize all the indexes with null s. and hence you are getting NPE.

You should fill all the indexes in a loop as following, before using the array

for(int i=0; i<size; i++)
{
    calculationBuffer[i] = "A String";
}

Answer 2

If you are declaring your string array like this String[] calculationBuffer = new String[size];

then all string element are initialized by Null so if you are not adding value in this string array at somewhere else then you can update your code like this

for (i = 0; i < calculationBuffer.length; i++){ 
      **calculationBuffer[i]="";**
     calculationBuffer[i] = calculationBuffer[i].toLowerCase();// this is line 48 where the first error appears to be.
 }

Thanks

Answer 3

where do you fill your calculationBuffer ?

the error you get indicated that calculationBuffer[i] is null meaning you didnt correctly fill your initial array.