java - NullPointerException in an Array Stack -

i'm trying implement stack using array program reverses sound clips, taking pieces of .dat file stack , popping them off 1 one. i'm getting nullpointerexception in pop() method whenever run client program, , can't figure out why.

here stack implementation:

import java.util.emptystackexception; public class arraystack  implements dstack { private int size = 0; private static int default_capacity = 10; private double arrayst[];   public arraystack(){     arrayst = new double[default_capacity]; }   public boolean isempty(){     boolean empty = true;     if(arrayst.length != 0) {         empty = false;     }      return empty;  } public void push(double d){     if(size == arrayst.length){         increasecapacity();     }     arrayst[size++] = d;  } public double pop(){     boolean empty = isempty();     if(empty == true){         throw new emptystackexception();     }     double d = arrayst[--size];     arrayst[size] = null;     return d;  }  public double peek(){     boolean empty = isempty();     if(empty == true){         throw new emptystackexception();     }     double stacktop = pop();     return stacktop;  } private double[] increasecapacity(){     int arraylength = arrayst.length;            int newcapacity = arrayst.length*2;     double[] increasearray = new double[newcapacity];     for(int = 0; < arraylength; i++){         arrayst[i] = increasearray[i];      }     return increasearray; }   } 

here client program:

    import java.io.bufferedreader;     import java.io.bufferedwriter;     import java.io.filereader;     import java.io.filewriter;     import java.io.ioexception;     import java.io.printwriter;     import java.util.stringtokenizer;  /**  * read .dat file , reverse it.  *   * @version cse373, 14sp  */ public class reverseredux {      @suppresswarnings("unused")     public static void main(string[] args) {         if (args.length != 3) {             system.err.println(" incorrect number of arguments");             system.err.println(" usage: ");             system.err                     .println("\tjava reverse <stack type> <input file> <output file>");              system.exit(1);         }          boolean uselist = true;         if (args[0].compareto("list") == 0)             uselist = true;         else if (args[0].compareto("array") == 0)             uselist = false;         else {             system.err.println("\tsaw " + args[0]                     + " instead of list or array first argument");             system.exit(1);         }          try {             //             // set input file read, , output file write             //             bufferedreader filein = new bufferedreader(new filereader(args[1]));             printwriter fileout = new printwriter(new bufferedwriter(                     new filewriter(args[2])));              //             // read first line of .dat file sample rate.             // want store sample rate value in variable,             // can ignore "; sample rate" part of line.             // step through first line 1 token (word) @ time             // using stringtokenizer. fourth token 1             // want (the sample rate).             //             stringtokenizer str;             string oneline;             int samplerate;             string strjunk;              oneline = filein.readline();              str = new stringtokenizer(oneline);             strjunk = str.nexttoken(); // read in semicolon             strjunk = str.nexttoken(); // read in "sample"             strjunk = str.nexttoken(); // read in "rate"              // read in sample rate         samplerate = integer.parseint(str.nexttoken());          //         // read in remainder of file on line @ time.         // values in first column thrown away.         // place values second column on stack.         // stop reading if reach end of file.         //          dstack s;         if (uselist)             s = new liststack();         else             s = new arraystack();         string timestep;         double data;          int count = 0;         while ((oneline = filein.readline()) != null) {             if (oneline.charat(0) == ';') {                 continue;             }             str = new stringtokenizer(oneline);              // read in time step value first column             timestep = str.nexttoken();             // read in data value second column             data = double.parsedouble(str.nexttoken());             s.push(data);             count++;         }          system.out.println(count + " samples in file");          //         // print data values output .dat file.         // first, output header line:         // "; sample rate <sample rate>"         //         fileout.println("; sample rate " + samplerate);          // since first column consists of numbers start         // @ 0 , increase 1/samplerate every time slice, we'll         // use numsteps recalculate these numbers.         int numsteps = 0;          // finally, print values in reverse order (by popping         // them off stack). first column consists of numbers         // start @ 0 , increase 1/samplerate per row,         // we'll use numsteps/samplerate recalculate appropriate         // values. print tab uniform spacing.          while (!s.isempty()) {             fileout.println((double) numsteps / samplerate + "\t" + s.pop());             numsteps++;         }          //         // close files         //         filein.close();         fileout.close();      } catch (ioexception ioe) {         system.err                 .println("error opening/reading/writing input or output file.");         system.exit(1);     } catch (numberformatexception nfe) {         system.err.println(nfe.tostring());         system.err.println("error in file format");         system.exit(1);     } } 

}

thanks help!

i guess line reason:

double d = arrayst[--size]; 

you have null in array position , converting primitive causes npe