Question

having problem with null pointer exception and i read few article bout that error and still coundnt figure out what the problem.

The error happen at CompatibleActivity[topIndex]=new Activity(aNum,bNum,c);

topIndex=0 by the way.

Can anyone highlight the problem im having?

here my class

public class Schedulingtest {


public static void main (String[] args) throws IOException
{   

   Scanner fileScan;

   fileScan=new Scanner(new File("data.txt"));

    Schedule compatibility = new Schedule(); 


    while(fileScan.hasNext())
    {String url=fileScan.nextLine();
    compatibility.addActivity(url);


    }

}


public class Schedule{


import java.util.Scanner;
import java.util.Arrays;

public class Schedule {
Activity[] CompatibleActivity;
int totalTime=0,topIndex=0;
Scanner urlScan;

public Schedule(){
Activity[] CompatibleActivity=new Activity[30];

}


public int getTotalTime()
{return totalTime;
}     

public void addActivity(String entry){

    urlScan=new Scanner(entry);
    urlScan.useDelimiter(" ");

     String c=null;
    int aNum = 0,bNum=0;

    while(urlScan.hasNext())
    {String a=urlScan.next();
     String b=urlScan.next();
     c=urlScan.next();

     aNum=Integer.parseInt(a);
     bNum=Integer.parseInt(b); 

             }    
    CompatibleActivity[topIndex]=new Activity(aNum,bNum,c);
    topIndex++;

    System.out.println("Activity added:  start "+aNum+ " stop "+bNum+" "+c ); 
}    

}

Activity Class

public class Activity {

private int start,stop,duration;
private String name;

public Activity(int Start,int Stop,String Name)
{
start=Start;
stop=Stop;
name=Name;
duration=Stop-Start;   
}    

public String getName()
{return name;
}        

public int getStart()
{return start;
}

public int getStop()
{return stop;
}        

public int getDuration()
{return duration;
}    

public boolean compatible(int Start1,int Stop1,int toMatchsideStart,int toMatchsideStop)
{
   int Start=Start1;
   int Stop=Stop1;
   int toMatchStart=toMatchsideStart;
   int toMatchStop=toMatchsideStop;

if(toMatchStop<=Start)
{return true;
}
if(toMatchsideStart>=Stop)
{return true;
}
else
{return false;}    
}        


public String toString()
{return( name+"<"+start+","+stop+">"); }        
}

Answer 1

Most likely you have CompatibleActivity declared in your class as

private Activity[] CompatibleActivity;

which declares a reference and initializes it to null by default. You need to assign it a real array with enough elements (e.g. in your constructor):

CompatibleActivity = new Activity[myBigNumber];