Thread: 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.

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)

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

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);
    }
}

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.

Originally Posted by Norm

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;
    }  
}