How do read from a .txt file and put data into an array list of an object?

Question

What I have written so far works from my current knowledge of basic arrays, but I just don't understand how to use an arraylist, or how to read from a file. What I have written so far works. Any links or advice to help fix my code to read from a file and use an arraylist would be greatly appreciated. Thank you.

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public class TestPackages
{
   public static void main (String[] args) 
    {
       Packages testPackages = new Packages();

       testPackages.addPacket(1001, 7.37, "CA");
       testPackages.addPacket(1002, 5.17, "CT");
       testPackages.addPacket(1003, 11.35, "NY");
       testPackages.addPacket(1004, 20.17, "MA" );
       testPackages.addPacket(1005, 9.99, "FL");
       testPackages.addPacket(1006, 14.91, "VT");
       testPackages.addPacket(1007, 4.97, "TX");

       System.out.println(testPackages); 
    }
}

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import java.util.Scanner;
import java.io.*;
import java.util.ArrayList;

public class Packages
{
   private Packet[] shipment;
   private int count;
   private double totalWeight;

   public Packages()
   {
      shipment = new Packet[100];
      count = 0;
      totalWeight = 0.0;
   }

   public void addPacket (int idNumber, double weight, String state)
   {
      if(count == shipment.length)
         increaseSize();

      shipment[count] = new Packet (idNumber, weight, state);
      totalWeight += weight;
      count++;
   }

   public String toString()
   {
      String report;

      report = "All Packets\n";
      for(int num = 0; num < count; num++)
         report += shipment[num].toString() + "\n";

      report += "Total Weight:\t" +totalWeight+" pounds";
      return report;
   }

   private void increaseSize()
   {
      Packet[] temp = new Packet[shipment.length * 2];

      for (int num = 0; num < shipment.length; num++)
         temp[num] = shipment[num];

      shipment = temp;
   } 
}

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public class Packet
{
   private int idNumber;
   private double weight;
   private String state;

   public Packet(int idNumber, double weight, String state)
   {
      this.idNumber = idNumber;
      this.weight = weight;
      this.state = state;
   }

   public boolean isHeavy(double weight)
   {
      return (weight > 10);
   }

   public boolean isLight(double weight)
   {
      return (weight < 7);
   }

   public String toString()
   {
      String description = "ID number: " + idNumber + "\tWegiht: " + weight + "\tState: " 
      + state;

      return description;
   }
}

Answer 1

Try this link to learn how to read from file with JAVA

How to read a large text file line by line using Java?

You can try the following code to learn how to use Arraylist (There are many more tutorials in the web)

Packet p1=new Packet (1001, 7.37, "CA");
Packet p2=new Packet (1002, 17.5, "SF");
Packet p3=new Packet (1003, 13.8, "DF");
Packet p4=new Packet (1004, 14.4, "XC");

ArrayList<Packet> arr= new ArrayList<Packet>();

arr.add(p1);
arr.add(p2);
arr.add(p3);
arr.add(1,p4);

System.out.println("return the 2st element- " + arr.get(1) );
System.out.println("return the 4rd element- " + arr.get(3) );

System.out.println("Size of ArrayList after insertion- " + arr.size());
System.out.println("Elements of ArrayList after insertion- " + arr);

arr.remove(p2); 
arr.remove(2);

System.out.println("Size of ArrayList after deletion- " + arr.size());
System.out.println("Elements of ArrayList after deletion- " + arr);

Answer 2

There are many ways to read a file line by line in java. But I prefer using commons-io utils. It's a very useful tool that provide reading from a file. Here is a one line example.

List<String> lines = FileUtils.readLines(new File(fileName));