Sort an ArrayList

On the below code, how would i sort the ArrayList 'inv' by the 3rd element from greatest to smallest number.
I.E.: 30, 22, 85, 75, 40, 45
StoreInfo si = new StoreInfo();
        ArrayList sup = new ArrayList();
        ArrayList inv = new ArrayList();
        Supplier s = new Supplier("Hills", "777-123-2222");
        Supplier s1 = new Supplier("Bantam", "430-126-0978");
        Supplier s2 = new Supplier("Time, Inc.", "967-534-7676");
        Supplier s3 = new Supplier("Starbucks", "897-888-4534");
        Supplier s4 = new Supplier("Prentice Hall", "678-555-9876");
        sup.add(s);
        sup.add(s1);
        sup.add(s2);
        sup.add(s3);
        sup.add(s4);
        inv.add(new Book(s1, "Java for Fun", 30, 45.50, "Wolff"));
        inv.add(new Book(s4,"Databases for Dummies", 22, 65.50, "Riggins"));
        inv.add(new Magazine(s2, "Time", 85, 6.00, new Day(2004,3,5)));
        inv.add(new Magazine(s2,"Sports Illustrated", 75, 5.50, new Day(2004,2,20)));
        inv.add(new Coffee(s,"Breakfast Jolt",40,9.95,"2 pounds"));
        inv.add(new Coffee(s3,"Afternoon Snooze",45,12.95, "1 pound"));

ok, i read that entire tutorial twice even still a few more questions.
here is my compareTo method implementing the comparable:
public int compareTo(Object obj) throws ClassCastException {
          if (!(obj instanceof Inventory))
            throw new ClassCastException("An Inventory object expected.");
          int anotherInv = ((Inventory) obj).getNumInStock();
          return this.numInStock - anotherInv;
     }and this is where i use it
public void sortPrint(ArrayList inventory)
         Collections.sort(inventory);
         int size= inventory.size();
         for (int i=0; i<size; i++)
              Inventory inv = (Inventory) inventory.get(i);
              inv.toString();
    }from what i got from that turorial, you use the Collections call to use the compareTo method?? is this true, if so why?
here is the error i get:
java.lang.ClassCastException
     at java.util.Arrays.mergeSort(Arrays.java:1122)
     at java.util.Arrays.sort(Arrays.java:1073)
     at java.util.Collections.sort(Collections.java:109)
     at cornerBookStore.StoreInfo.sortPrint(StoreInfo.java:336)
     at cornerBookStore.CornerBookStore.main(CornerBookStore.java:91)
Exception in thread "main"
thanks for your time!

Similar Messages

I wanna sort my arraylist

If I wanna sort my arraylist, can I make it???How?
Thanx!

Do you know how to use interfaces? Do you understand OOP and Java? Once nice thing about using generic types like an interface, is that you can easily switch out different algorithms. For example, if you are doing something like this:
ArrayList myList = new ArrayList();
and throughout your code you have methods like:
public void doSomething(ArrayList list)
you limit youself to using nothing but ArrayLists. What happens if your code suddenly is used by more than one concurrent person? You will no doubt run into mult-threading issues using an ArrayList. However, Vector, which is identical to an ArrayList but is thread-safe, can be swapped for an ArrayList with no changes to your code. You MUST use the methods defined in the interface however, when working on the ArrayList or Vector. As an example, there are times when you may want to "speed up" your code. When using a Vector, you lose a lot of speed because a Vector has a lot of syncronized overhead in it. This is to ensure that multiple threads do not lock or cause any problems while accessing the same Vector in memory. At times, you may find out that a certain Vector is NEVER accessed by more than one thread. Therefore, its a perfect candidate for turning into an ArrayList, as the ArrayList has no synchronized overhead and is thus much faster. But, if you are not using the List interface methods, you may find you need to change a lot more code for this to work.
List is an interface (java.util.List) that defines methods like add, remove, get, and so on. As the other replier said, you can use the Collection framework to sort a list, and do other things with it. If you use a Vector ad usee addElement(), removeElement, etc, they wont work with ArrayList or LinkedList. The List interface is implemented by ArrayList, Vector and LinkedList. Each has their benefits. For example, if you write a program using a Vector, and it does a LOT of moving items back and forth (such as sorting), a LinkedList will be faster because the way they work, they just move a pointer to move the entire contents of the list. A Vector and ArrayList have to copy the data. However, searching is done much faster on Vectors and ArrayLists. So if you need to do a lot of searching, then one of those two would be better. If you need to do all three (or more), then you need to experiment. The nice thing is, as long as you use the List interface everywhere (except when instantiating the objects), you can simply do a find/replace in your code to replace ArrayList with Vector or LinkedList and you are all set.
Hope that helps.

Questions on filtering and sorting an arraylist

Hello All,
I'm trying to figure out a better way to do this code, but what I'm doing is taking an arrayList and filtering it using iter.remove(), based on a criteria I have, then take the results and sort them. The code below works, but I was seeing class cast exceptions when I tried to do a ss.addAll(list);
, but it worked when I just added the set object. Why is this and is there a better way to do all this?
Set set = new HashSet();
List retList = new ArrayList();
SortedSet ss = new TreeSet();
for(Iterator iter = list.iterator(); iter.hasNext();){
.//More code...
iter.remove();
//Remove duplicates.
set.addAll(list);
ss.add(set);
retList.addAll(set);
return retList;

Your current code is adding the set (the hash set) as one element (the only element?) of the sorted set. Is this what you want to do? Would you rather add each element of the set to the sorted set?
I don't know about the class cast exceptions. If you want somebody's comment on this part, you may consider posting code and stack trace.
I think I'd rely on Collections.sort() for sorting rather than TreeSet. If you prefer TreeSet, why not skip the hash set in between?
Hope this helps.
Please remember the code button/tags.

Need help Sorting an Arraylist of type Object by Name

Hey guys this is my first time posting on this forum so hopefully i do it the right way. My problem is that i am having difficulties sorting an Array list of my type object that i created. My class has fields for Name, ID, Hrs and Hrs worked. I need to sort this Array list in descending order according to name. Name is the last name only. I have used a bubble sort like this:
public static void BubbleSort(TStudent[] x){
TStudent temp = new TStudent();
        boolean doMore = true;
        while (doMore) {
            doMore = false;
            for (int i=0; i < x.length-1; i++) {
               if (x.stuGetGPA() < x[i+1].stuGetGPA()) {
// exchange elements
temp = x[i]; x[i] = x[i+1];
x[i+1] = temp;
doMore = true;
before many time to sort an array of my class TStudent according to GPA. This time though i tried using an Array list instead of just a simple array and i can't figure out how i would modify that to perform the same task. Then i googled it and read about the Collections.sort function. The only problem there is that i know i have to make my own comparator but i can't figure out how to do that either. All of the examples of writing a comparator i could find were just using either String's or just simple Arrays of strings. I couldn't find a good example of using an Array list of an Object.
Anyways sorry for the long explanation and any help anyone could give me would be greatly appreciated. Thanks guys
Edited by: Brian13 on Oct 19, 2007 10:38 PM

ok still having problems I have this line of code to try and all Arrays.sort
Arrays.sort(Employee, TEmployee.FirstNameComparator);Then in my class TEmployee i have
public static Comparator FirstNameComparator = new Comparator() {
    public int compare(Object employee, Object anotherEmployee) {
      String lastName1 = ((TEmployee) employee).getEmpName().toUpperCase();
      String lastName2 = ((TEmployee) anotherEmployee).getEmpName().toUpperCase();
       return lastName1.compareTo(lastName2);
};Here is the rundown of what i have for this. I have 2 classes. One is called TEmployee and that class holds fields for Name, ID, Hrs Worked, Hourly Rate. Then i have another class called myCompany that holds an ArrayList of type TEmployee. Then i have my main program in a Jframe.
So maybe i am putting them in the wrong spots. The code in TEmployee thats make the comparator is fine. However when i try to call Arrays.sort from class myCompany i get this
cannot find symbol
symbol: method sort (java.util.Array list<TEmployee>.java.util.Comparator
location: class java.util.Arrays
I have to put the comparator in TEmployee right because thats where my fields for Name are? Do i call the arrays.sort from my main program or do i call that from the class myCompany where my ArrayList of TEmployees are stored?
Again guys thanks for any help you could give me and if you need any code to see what else is going on just let me know.

Can I sort an ArrayList basd on a data member?

Hello everyone.
Currently my program stores a lot of objects in an ArrayList.
Each of these objects has the following data members:
public class ClassEntity
     //RODM ID VERSION
     private String idClassName;
     private String idFieldName;
     //ENGLISH VERSION
     private String englishClassName;
     private String englishFieldName;
     private String fieldType;
}I have the following data structure:
List<ClassEntity> classEntityList = new ArrayList<ClassEntity>();Now is there a way to order the Objects inside the ArrayList so they are in alphabetical order based on the englishFieldName data member?
I currently iterate through the List and write it to a file and the List is out of order so the writing to file is out of order.
I saw collections has a .sort() method, but I'm not sure how I would tell the .sort() to sort based on englishFieldName. In C++ I would overload the < operator I'm still new to data structures in Java though.
Thanks!

Implement the Comparable Interface in your class, as my example in this posting shows:
http://forum.java.sun.com/thread.jspa?forumID=31&threadID=598401

Need help sorting an ArrayList of strings by an in common substring

I'm trying to sort a .csv file, which I read into an ArrayList as strings. I would like to sort by the IP address within the string. Can someone please help me figure out the best way to sort these strings by a common substring each has, which is an IP address.
I'm pretty sure that I need to use Collections some how, but I don't know where to begin with them besides trying to look online for help. I had no luck finding any good examples, so I came here as a last resort.
Thanks for any help provided.

You need to write your own Comparator class. In the compare() method you substring out the two IP addresses, compare them and return appropriate value. Then you call Collections.sort() and pass your list and comparator.

Sorting a arraylist user defined object

I'm curious as to how I would go about sorting an object that I created. I would like to sort the object by the name variable in the Employee class. I tried just doing Collections.sort(staff);, but that throws the 'ClassCastException' exception. Here is the code that I was working with.
import java.util.*;
import java.util.Collections.*;
public class ArrayListTest {
     public static void main (String args[]) {
          ArrayList staff = new ArrayList();
          staff.add(new Employee("Harry Cracker", 50000, 1989, 10, 1));
          staff.add(new Employee("Tony Cracker", 40000, 1990, 3, 15));
          staff.add(new Employee("carl Cracker", 75000, 1987, 12, 15));
          for (int i=0; i<staff.size(); i++) {
               Employee e = (Employee)staff.get(i);
               e.raiseSalary(5);
          try {
               Collections.sort(staff);
          catch (ClassCastException e) {
               //e.printStackTrace();
               System.out.println("got here1");
          catch (UnsupportedOperationException e) {
               //e.printStackTrace();
               System.out.println("got here1");
          for (int i=0; i<staff.size(); i++) {
               Employee e = (Employee)staff.get(i);
               System.out.println("name=" + e.getName() + "salary=" + e.getSalary() + ",hireDay=" + e.getHireDay());
class Employee {
     public Employee(String n, double s, int year, int month, int day) {
          name = n;
          salary = s;
          GregorianCalendar calendar = new GregorianCalendar(year, month-1, day);
          hireDay = calendar.getTime();
     public String getName() {
          return name;
     public double getSalary() {
          return salary;
     public Date getHireDay() {
          return hireDay;
     public void raiseSalary(double byPercent) {
          double raise = salary * byPercent / 100;
          salary += raise;
     private String name;
     private double salary;
     private Date hireDay;
}WHile I'm on this example if someone could explain what the Employee class is, shouldn't that be defined in a seperate file? It's not a inner class because it's not defined inside another class, but what would it be called and how come it works that way. If I go and put a private/public statement in front of it I get an error message. Thanks for the help.

It doesn't know how to sort Employee, you have to implement Comparable or create a Comparator. You could change your Collections.sort() method to something like this: (remove the try and catch also)Collections.sort(staff, new Comparator()
    public int compare(Object o, Object o2)
        String s = ((Employee)o).getName();
        String s2 = ((Employee)o2).getName();
        return String.CASE_INSENSITIVE_ORDER.compare(s, s2);
});That's at least one way.

Any good way to sort an ArrayList??

I have a ArrayList now which must be sorted by the order such as:
All
string
message
number
float
My arrayList may only contains some of them, lets say{"float","string","number"}. how can I make this sort?
thanks for any advice.

Use Collections.sort, and write your ownComparator.
If you store String objects you don't need a
Comparator because String implements the Comparable
Interface so their ordering is already defined. You
just use Collections.sort on the ArrayList.Except that the ordering required by the original
poster is goofy (non-alpha ordering), so the default
String comparitor will not suffice...
So, the answer is: "Use Collections.sort, and write
your own Comparator".
(Darned those pesky requirements :-) )
- KAnd it's pesky when the OP never comes back and makes clarifications. I interpreted the list given as the input list, not the resulting sorted list. If it was already sorted he wouldn't need a program at all would he! -:)

Sort Parallel ArrayList

Ok I have 4 arraylists that are in parallel. There elements line with the elements in the other array list. I know how to sort one list using the collection method. But is there a way to sort multiple list in reference to only one original list.
I'm sure there' s better ways to do this.... But is this possible?

I have defined my own interface "Sortable" (Google shows I'm not the only one with this idea):
public interface Sortable
public int size();
public int compare(int firstIndex, int secondIndex);
public void swap(int firstIndex, int secondIndex);
}I then have a sort method that takes a Sortable, which I usually define as an anonymous inner class.
This is best for cases when I have a list of some class, and then I get a list of something else which is used solely to determine how to sort the first list. Rather than creating a potentially large list of new objects solely for the purpose of sorting, I just use Sortable to sort it in place. The down side is that you have to implement the sorting methods yourself (I'm gonna have to submit this as an RFE to the JDK). All the existing sort methods using Comparable and Comparator require discrete objects; the Sortable interface only requires an object that is addressable by index.
Edited by: paul.miner on May 13, 2008 1:28 PM

Sorting an ArrayList based on the value of a field in one of the objects.

Hi,
I have the following problem:
A server app returns an array list of simple objects with a bunch of fields.
One of these fields is an int.
How do I go about sorting the list based on the size of the int?
Thanks in advance!

Thanks a lot.
The Comparator solution is actually ridiculously simple.....
public class MyComparator implements Comparator {
public int compare(Object o1, Object o2) {
     if (Integer.parseInt(((ZMData)o1).getAantalposts())>Integer.parseInt(((ZMData)o2).getAantalposts())) {
          return 1;
     if (Integer.parseInt(((ZMData)o1).getAantalposts())<Integer.parseInt(((ZMData)o2).getAantalposts())) {
          return -1;
     return 0;
}Thanks again.

Problems with sort, search and write objects o an ArrayList

Hi
Lets say that i have two subclasses (the program is not finished so in the end it can be up to 34 classes) of an abstract superclass. I also got one class which basicly is a register in which i've created an ArrayList of the type <abstractClass>. This means that i store the two subclasses in the arrayList. no problems so far i think (at least eclipse doesn't mind).
1. now, i want to be able to sort the arrayList aswell as search thorugh it. I've tried Collections.sort(arrayList) but it doesn't work. So i have no idea how to solve that.
2.The search-method i made doesn't work as good as i hoped for either. I ask the user to first decide what to search for (choose subclass) and then input the same properties as the subclass we search for. I create a new object of the subclass with these inputs and try arrayList.contains(subClassObject)
it runs but it returns +"false"+ even if i create an object with the exact same properties.
3. If i want to write this arrayList to a txtFile so i can import it another time. Which is the best method? first i just thought i'd convert the arrayList to string and then print every single object to a textfile as strings. that worked but i have no good idea how to import that into the same arrayList later. Then i found ObjectOutputStream and import using inputStream.nextObject(). But that doesn't work :(
Any ideas?
Thank you!
Anton

lavalampan wrote:
Hi
Lets say that i have two subclasses (the program is not finished so in the end it can be up to 34 classes) of an abstract superclass. I also got one class which basicly is a register in which i've created an ArrayList of the type <abstractClass>. This means that i store the two subclasses in the arrayList. no problems so far i think (at least eclipse doesn't mind).
1. now, i want to be able to sort the arrayList aswell as search thorugh it. I've tried Collections.sort(arrayList) but it doesn't work. So i have no idea how to solve that. Create a custom comparator.
>
2.The search-method i made doesn't work as good as i hoped for either. I ask the user to first decide what to search for (choose subclass) and then input the same properties as the subclass we search for. I create a new object of the subclass with these inputs and try arrayList.contains(subClassObject)
it runs but it returns +"false"+ even if i create an object with the exact same properties.Implement hashCode and equals.
>
3. If i want to write this arrayList to a txtFile so i can import it another time. Which is the best method? first i just thought i'd convert the arrayList to string and then print every single object to a textfile as strings. that worked but i have no good idea how to import that into the same arrayList later. Then i found ObjectOutputStream and import using inputStream.nextObject(). But that doesn't work :(Depends on what your requirement is, but yes, Serialization might work for you. Your classes should in that case implement Serializable.
Kaj

Sorting a String Array in alpha order (Using ArrayList)

Hello all!
I am pretty new to Java and am trying to do a, seemingly, simple task; but cannot get the syntax right. I am trying to sort an ArrayList in aplha order based off of the variable String Product. Any help would be awesome!
First Class:
//This program stores all of the variables for use in Inventory.java
public class Vars {
     // defining all variables
     private String product;
     private String prodCode;
     private double price;
     private double invCount;
     // Begin listing getters for class
     public Vars(String product) {
          this.product = product;
     public String getProdCode() {
          return prodCode;
     public String getProduct() {
          return product;
     public double getPrice() {
          return price;
     public double getInvCount() {
          return invCount;
     // Declaring the total variable
     public double total() {
          return invCount * price;
     // Begin listing setters for variables
     public void setProduct(String product) {
          this.product = product;
     public void setProdCode(String prodCode) {
          this.prodCode = prodCode;
     public void setInvCount(double invCount) {
          this.invCount = invCount;
     public void setPrice(double price) {
          this.price = price;
Second Class:
//This program will use the variables stored in Vars.java
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class Inventory {
     public static void main(String args[]) {
          Scanner input = new Scanner(System.in);
          //defining and declaring local variables
          String exit;
          int counter;
          double gtotal;
          exit = "exit";
          counter = 0;
          gtotal = 0;
          //beginning message and beginning of the loop
          System.out.print("Welcome to the Inventory Contol System\nPlease enter the Product name: ");
          System.out.println();
          System.out.println("When finished entering in the data, please enter EXIT in place of Product name.");
          String name = input.next();
          List<Vars> var = new ArrayList<Vars>(); //creating arraylist object
          //loop while exit is not entered, exit loop once exit is typed into product variable
          while (name.compareToIgnoreCase(exit) != 0) {
               Vars vars = new Vars(name); //calling Vars methods
               counter = counter ++;
               System.out.println("Please enter the Product Code: ");
               vars.setProdCode(input.next());
               System.out.println("Inventory Count: ");
               vars.setInvCount(input.nextDouble());
               //Making sure that the value entered is a positive one
               if(vars.getInvCount() < 0){
                    System.out.println("Please enter a positive number: ");
                    vars.setInvCount(input.nextDouble());
               System.out.println("Price per product:");
               vars.setPrice(input.nextDouble());
               //Making sure that the value entered is a positive one
               if(vars.getPrice() < 0) {
                    System.out.println("Please enter a positive number: ");
                    vars.setPrice(input.nextDouble());
               System.out.println("Please enter the next Product name, or enter EXIT: ");
               name = input.next();
               gtotal += vars.total(); //calculation for Grand Total of all products entered
               var.add(vars);
               static void slectionSort(int[] product) {
               for (int lastPlace = product.length-1; lastPlace > 0; lastPlace--) {
               int maxLoc = 0;
               for (int j = 1; j <= lastPlace; j++) {
               if (product[j] > product[maxLoc]) {
               maxLoc = j;
               int temp = product[maxLoc];
               product[maxLoc] = product[lastPlace];
               product[lastPlace] = temp;
          //Sorting loop function will go here, before the information is displayed
          //Exit message and array list headers
          System.out.print("Thank you for using the Inventory program.\nHave a wonderful day.\n");
          System.out.printf("Code\tProduct\tCount\tPrice\tTotal\n");
          System.out.println();
          //For loop to display all entries via an ArrayList
          for (Vars vars : var) {
               System.out.printf("%5s\t%5s\t%.01f\t$%.02f\t$%.02f\n\n", vars.getProdCode(),
                         vars.getProduct(), vars.getInvCount(), vars.getPrice(),vars.total());
          //Displays the amount of entries made and Grand Total of all products entered
          System.out.println();
          System.out.printf("The number of products entered is %d\n", var.size());
          System.out.printf("The Grand Total cost for the products entered is $ %s\n", gtotal);
          System.out.println();
}

go through below links, you can easily understand how to sort lists.
http://www.onjava.com/lpt/a/3286
http://java.sun.com/docs/books/tutorial/collections/interfaces/order.html
http://www.javaworld.com/javaworld/jw-12-2002/jw-1227-sort.html

How to Sort ArrayList

Hi
I have class like below , i store them in a ArrayList, is it possible to sort this ArrayList by name or by date.
The class is as below
public class MyInfo
private String name;
private Date date;
//have getters and setters for name and date
ArrayList data = new ArrayList();
data.add(instance of MyInfo)
Ashish

Comparable doesn't really apply here IMO, since he
uses different ordering criteria - name and date.Did you read the link? The tutorial contains useful information on the Comparator as well as the Comparable interface, which is why I posted it. I believe you're objecting only to the title of the article.
:o)

Sort ArrayList by length

I need to remove the string(s) with the least number of letters from an ArrayList.
How can I sort the ArrayList by the length of the strings contained in it?

Why would you need to?
1.5 (you should be using an ArrayList<String>)
list.get(i).length();Earlier (without generics)
((String)list.get(i)).length();This is of course irrelevent to impelementing a
simple Comparator and passing it to Collections.Or
list.get(i).toString().length();
Or
new StringBuilder(list.get(i).toString().toCharArray()).toString();

Sort ArrayList of HashMaps

Hello all
I have Arraylist that contains HashMaps
how can I sort the ArrayList by some value of HashMaps?
e.g. I have following HashMaps that are put inside a list and need to sort them by there value
ArrayList<HashMap> al = new ArrayList<HashMap>();
HashMap map1 = new HashMap();
map1.add( new Integer( 2 ), "two" );
map1.add( new Integer( 4 ), "four" );
HashMap map2 = new HashMap();
map2.add( new Integer( 22 ), "two2" );
map2.add( new Integer( 42 ), "four2" );
HashMap map3 = new HashMap();
map3.add( new Integer( 23 ), "two" );
map3.add( new Integer( 43 ), "four" );
al.add(map1);
al.add(map2);
al.add(map3);Thanks

Create a Comparator<HashMap> and pass that to Collections.sort. Write the comparator to use whatever rules you need to determine whether a map is "less than" another.
http://java.sun.com/javase/6/docs/api/java/util/Comparator.html
http://java.sun.com/javase/6/docs/api/java/util/Collections.html#sort(java.util.List, java.util.Comparator)
http://java.sun.com/docs/books/tutorial/collections/interfaces/order.html
http://www.javaworld.com/javaworld/jw-12-2002/jw-1227-sort.html

Sort an ArrayList

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