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Thread: Need some help getting this code to run...

  1. #1
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    Default Need some help getting this code to run...

    I compiled my code but it will not output. I get an error that points to

    public String toString()
        {
            String s= "ALL PACKETS \n";
            for(Packet mypacket : shipment)
                s += mypacket;
            return s;

    it says java.lang.NullException Null...

    public class Packet
    {
        // instance variables - replace the example below with your own
        private int idNumber;
        private String state;
        public double weight;
     
        public Packet(int id, String s, double d)
        {
            this.idNumber = id;
            this.state = s;
            this.weight = d;
        }
     
        public boolean isHeavy()
        {
            if (weight > 12)
                return true;
            else
                return false;
        }
     
        public boolean isLight()
        {
            if (weight < 7)
                return true;
            else
                return false;
        }
     
        public String toString()
        {
            return "ID: " + idNumber + "\t" + "Weight: " + weight + "\t" + "State: " + state;
        }
    }

    import java.util.ArrayList;
    import java.util.Scanner;
    import java.io.*;
     
    public class Packages
    {
        double totalWeight;
        public ArrayList<Packet> shipment;
     
        public void Packages () throws IOException
        {
            totalWeight = 0.0;
            shipment = new ArrayList<Packet>();
            Scanner fileScan;
     
            fileScan = new Scanner (new File("allPackets.txt"));
            // Read and process each line in the file
            while (fileScan.hasNext())
            {
                int id = fileScan.nextInt();
                String s = fileScan.next();
                double d = fileScan.nextDouble();
     
                Packet p = new Packet (id, s, d);
                shipment.add(p);
            }
            System.out.println(shipment);
        }
     
        public void displayLightPackages()
        {
            System.out.println("ALL LIGHT PACKAGES \n");
            for(Packet mypacket : shipment)
                if (mypacket.isLight())
                    System.out.println(mypacket);
        }
     
        public void displayHeavyPackages()
        {
            System.out.println("ALL HEAVY PACKETS \n");
            for(Packet mypacket : shipment)
                if (mypacket.isHeavy())
                    System.out.println(mypacket);
        }
     
        public void displayOtherPackages()
        {
            for(Packet mypacket : shipment)
                if(mypacket.isHeavy() || mypacket.isLight())
                    System.out.println("ALL REGULAR PACKETS \n");
                else
                    System.out.println(mypacket);  
        }
     
        public void displayTotalWeight()
        {
            for(Packet mypacket : shipment)
                totalWeight += mypacket.weight;
                System.out.println ("Total weight of all packets is " + totalWeight);
        }
     
        public void displayAverageWeight()
        {
            for(Packet mypacket : shipment)
            {
                totalWeight = shipment.size();
            }
            System.out.println("Average weight of all packets is " + totalWeight);
        }
     
        public String toString()
        {
            String s= "ALL PACKETS \n";
            for(Packet mypacket : shipment)
                s += mypacket;
            return s;
        }
     
    }

    public class TestPackages
    {
        public static void main(String[] args)
        {
            Packages mypacket = new Packages();
            System.out.println(mypacket);
        }
    }

    Any help on what I am doing wrong would be great! Thanks.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    Please copy and paste the full text of the error message.
    If you know what line the variable with the null value is on, look at that line, find the variable with the null value and then backtrack in the code to see why that variable does not have a valid value.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Need some help getting this code to run...

    for(Packet mypacket : shipment)

    It says, java.lang.NullPointerException
    at Packages.toString(Packages.java:78)
    at java.lang.String.valueOf(String.java:2854)
    at java.io.PrintStream.println(PrintStream.java:821)
    at TestPackages.main(TestPackages.java:10)

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    Try debugging the code by adding some println() statements that print out the value of the shipment variable every time its value is changed and set to see why it has a null value.
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: Need some help getting this code to run...

    what will I be printing in...string s, int id, double d? in the array loop?

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    Just print the value of the shipment variable every time it is changed:
    System.out.println("s1="+shipment);
    change the digit to be unique in each call to println() so you can tell which println() printed the output.
    If you don't understand my answer, don't ignore it, ask a question.

  7. #7
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    Default Re: Need some help getting this code to run...

    I got the code to compile and run but it only outputs "ALL PACKETS" text...I don't know what's going on.

    public class Packet extends Packages
    {
        // instance variables - replace the example below with your own
        private int idNumber;
        private String state;
        public double weight;
     
        public Packet(int id, String s, double d)
        {
            this.idNumber = id;
            this.state = s;
            this.weight = d;
        }
     
        public boolean isHeavy()
        {
            if (weight > 12)
                return true;
            else
                return false;
        }
     
        public boolean isLight()
        {
            if (weight < 7)
                return true;
            else
                return false;
        }
     
        public String toString()
        {
            String s = "ID: " + idNumber + "\t" + "Weight: " + weight + "\t" + "State: " + state;
            return s;
        }
    }

    public class Packages
    {
        double totalWeight;
        public ArrayList<Packet> shipment = new ArrayList<Packet>();;
     
        public void Packages () throws IOException
        {
            totalWeight = 0.0;
            ArrayList<Packet> current = new ArrayList<Packet>();
            Scanner fileScan;
     
            fileScan = new Scanner (new File("allPackets.txt"));
            // Read and process each line in the file
            while (fileScan.hasNext())
            {
                int id = fileScan.nextInt();
                String s = fileScan.next();
                double d = fileScan.nextDouble();
     
                Packet p = new Packet (id, s, d);
                current.add(p);
            }
            System.out.println(current);
        }
     
        public void displayLightPackages()
        {
            System.out.println("ALL LIGHT PACKAGES \n");
            for(Packet mypacket : shipment)
                if (mypacket.isLight())
                    System.out.println(mypacket);
        }
     
        public void displayHeavyPackages()
        {
            System.out.println("ALL HEAVY PACKETS \n");
            for(Packet mypacket : shipment)
                if (mypacket.isHeavy())
                    System.out.println(mypacket);
        }
     
        public void displayOtherPackages()
        {
            for(Packet mypacket : shipment)
                if(mypacket.isHeavy() || mypacket.isLight())
                    System.out.println("ALL REGULAR PACKETS \n");
                else
                    System.out.println(mypacket);  
        }
     
        public void displayTotalWeight()
        {
            for(Packet mypacket : shipment)
                totalWeight += mypacket.weight;
                System.out.println ("Total weight of all packets is " + totalWeight);
        }
     
        public void displayAverageWeight()
        {
            for(Packet mypacket : shipment)
            {
                totalWeight = shipment.size();
            }
            System.out.println("Average weight of all packets is " + totalWeight);
        }
     
        public String toString()
        {
           String s = "ALL PACKETS \n";
           for(Packet mypacket : shipment)
                s += mypacket;
           return s;
        }
    }

    public class TestPackages
    {
        public static void main(String[] args)
        {
            Packages shipment = new Packages();
            System.out.println(shipment);
        }
    }

  8. #8
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    I don't know what's going on.
    Try debugging the code by adding println() statements that print out the value of shipment at all locations in the code where the value of the variable: shipment is changed. You need to see if and when the value of and contents of shipment changes.
    See post#6
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
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    Default Re: Need some help getting this code to run...

    I added this

    public String toString()
        {
           if (shipment == null)
               return "shipment is null";
           else{
           String s = "ALL PACKETS \n";
           for(Packet mypacket : shipment)
                s += mypacket;
           return s;}
        }

    It did return shipment is null.

    but whyyyy I read the file in right...I believe. I don't know if my array is set-up right.

  10. #10
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    What was printed out when you added all the println() calls I suggested to the code?
    You need to add lots of println() statements to see what the code is doing.
    The code you posted looks like it added just one println and that is NOT what I was suggesting as a place to add any.
    If you don't understand my answer, don't ignore it, ask a question.

  11. #11
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    Default Re: Need some help getting this code to run...

    Quote Originally Posted by Norm View Post
    What was printed out when you added all the println() calls I suggested to the code?
    You need to add lots of println() statements to see what the code is doing.
    The code you posted looks like it added just one println and that is NOT what I was suggesting as a place to add any.
    OK I figured it out...my array now prints the input file.

    BUT my methods (displayHeavyWeight, displayLightWeight...etc) are not being displayed or called I believe...how do I go about implementing that? When I run the output my strings do not show up...

     
    import java.util.ArrayList;
    import java.util.Scanner;
    import java.io.*;
     
    public class Packages
    {
        double totalWeight;
        public ArrayList<Packet> shipment;
     
        public Packages () throws IOException
        {
            totalWeight = 0.0;
            shipment = new ArrayList<Packet>();
            String currentPacket;
            Scanner fileScan, packetScan;
     
            fileScan = new Scanner (new File("allPackets.txt"));
            // Read and process each line in the file
            while (fileScan.hasNext())
            {
                currentPacket = fileScan.nextLine();
                packetScan = new Scanner (currentPacket);
     
                int id = packetScan.nextInt();
                double d = packetScan.nextDouble();
                String s = packetScan.next();
     
                Packet temp = new Packet (id,d,s);
                shipment.add(temp);
            }
            System.out.println(shipment);
        }
     
        public void displayLightPackages()
        {
            System.out.println("ALL LIGHT PACKAGES \n");
            for(Packet thepacket : shipment)
                if (thepacket.isLight())
                    System.out.println(thepacket);
        }
     
        public void displayHeavyPackages()
        {
            System.out.println("ALL HEAVY PACKETS \n");
            for(Packet thepacket : shipment)
                if (thepacket.isHeavy())
                    System.out.println(thepacket);
        }
     
        public void displayOtherPackages()
        {
            for(Packet thepacket : shipment)
                if(thepacket.isHeavy() || thepacket.isLight())
                    System.out.println("ALL REGULAR PACKETS \n");
                else
                    System.out.println(thepacket);  
        }
     
        public void displayTotalWeight()
        {
            for(Packet thepacket : shipment)
                totalWeight += thepacket.weight;
                System.out.println ("Total weight of all packets is " + totalWeight);
        }
     
        public void displayAverageWeight()
        {
            for(Packet thepacket : shipment)
            {
                totalWeight = shipment.size();
            }
            System.out.println("Average weight of all packets is " + totalWeight);
        }
     
        public String toString()
        {
               String s = "ALL PACKETS \n";
               for(Packet thepacket : shipment)
               s += thepacket + "\n";
               return s;
        }  
    }

  12. #12
    Super Moderator Norm's Avatar
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    Default Re: Need some help getting this code to run...

    my strings do not show up...
    Which methods are you calling? Do each of them have a println() at the start of the method?

    To call a method, you must have a reference to an instance of the class with the method and use that:
    TheClass aRefToClass = new TheClass();
     ....
    aRetfToClass.theMethodToCall(): // call a method in TheClass
    If you don't understand my answer, don't ignore it, ask a question.

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