Cant seem to figure out sorting an array of objects in java

Question

this is the class for the objects that fill my array

class InventoryItem implements Comparable<InventoryItem>{
String name;
double price = 0.0;
int quantity = 0;
int itemCount = 0;
public InventoryItem(String newName, double newPrice, int newQuantity){
    name = newName;
    price = newPrice;
    quantity = newQuantity;
    itemCount++;
}

public String toString(){
    String output = name + ", at " + Program4.moneyString(price) + " each.";
    output += " We have " + getQuantity() + " of them.\n";
    return output;
}

public String getName(){
    return name;
}

public int getQuantity(){
    return quantity;
}

public double getPrice(){
    return price;
}

public int compareTo(InventoryItem other){
    return getName().compareTo(other.getName());
}
}

and my call to sort the array

public static void sortByName(InventoryItem[] array){
    Arrays.sort(array);
}

this gives me the errors

Exception in thread "main" java.lang.NullPointerException
at java.util.ComparableTimSort.binarySort(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at CSC120Program4.Program4.sortByName(Program4.java:97)
at CSC120Program4.Program4.processUserChoice(Program4.java:121)
at CSC120Program4.Program4.main(Program4.java:134)

I'm not quite experienced enough to know how to fix this or really what I'm doing wrong.

edit: array being built:

static InventoryItem[] inventoryItems = new InventoryItem[100];
public static void readAndStoreInventory(InventoryItem[] items, Scanner input){

    String[] params = new String[3];
    while(input.hasNextLine()){
        String name = "";
        double value = 0.0;
        int quantity = 0;
        params = (input.nextLine().split("#"));
        name = params[0];
        value = Double.parseDouble(params[1]);
        quantity = Integer.parseInt(params[2]);
        items[lastInUse + 1] = new InventoryItem(name, value, quantity);
        lastInUse++;
    }
    input.close();

the file being read is as follows:

wooden baseball bat#3.67#10
rubber mallet#5.50#20
duffel bag#10.20#11
gold bar#9999.99#1
hitman rental#20000.00#99
minivan#15000.21#3
half full cup#2.00#10
half empty cup#10.00#10
lawn chair#12.34#100
used golf ball#0.99#2
cosby sweater#250.00#1
CSC120 cheatsheet#0.01#1

Where is your array in the code? I don't see any arrays initialized in your code.. — ryekayo
– ryekayo, Commented Nov 12, 2015 at 20:08

Michael Möbius · Accepted Answer · 2015-11-12 20:44:29Z

2

You are creating an array with 100 elements but you only creating a few items. So when you sort it, it will also sort these null objects. You should use a List for this purpose.

You could also try to add a null-ptr check:

public int compareTo(InventoryItem other){
if (other != null){
    return getName().compareTo(other.getName());
}
return 0
}

A dirty fix would be to get the size of the elements and sort only the added elements: (Better way would be to use a list from the start)

public static void sortByName(InventoryItem[] array){
    int counter = 0;
    for (int i = 0; i < array.length; i ++)
            if (array[i] != null){
            counter ++;
            break;
        }
    Arrays.sort(array, 0, counter-1);
}

edited Nov 12, 2015 at 20:44

answered Nov 12, 2015 at 20:17

Michael Möbius

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2 Comments

ryekayo Over a year ago

@JoshPeel, this is not supposed to fix the underlying issue. This is simply checking if the values within the array are not null..

brso05 Over a year ago

@ryekayo actually it should fix the issue...the null elements should be at the end of the array when done.

Community · Accepted Answer · 2017-05-23 11:51:56Z

0

Arrays.sort(Object[]) expects that all elements in the array are != null since it demands that

all elements in the array must be mutually comparable (that is, e1.compareTo(e2) ...

What you see is the result of null.compareTo(something).

Your InventoryItem[] array has presumable 100 slots, but not all of them are filled.

To fix that, you could

copy the non-null data into a correctly sized array
sort just the filled subset via Arrays.sort(Object[] array, int fromIndex, int toIndex) (with 0, lastIndexUsed as params).
Provide a Comparator that can handle null. With one the algorithm is no longer null.compareTo(something) but comparator.compare(null, something) which does not result in a NPE. See How to sort an array of objects containing null elements?
use a List

edited May 23, 2017 at 11:51

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answered Nov 12, 2015 at 20:20

zapl

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1 Comment

Josh Peel Over a year ago

THANK YOUUUUU. We have to use arrays due to it being to my professors guidelines for the assignment so using a list wasnt an option unfortunately. its dumb i know. but using the index values made it work flawlessly

djechlin · Accepted Answer · 2015-11-12 20:13:55Z

0

Exception in thread "main" java.lang.NullPointerException

Probably means a null is having properties accessed on it. Probably in here:

public int compareTo(InventoryItem other){
  return getName().compareTo(other.getName());
}

So one of the name is null. So probably here:

params = (input.nextLine().split("#"));
name = params[0];

answered Nov 12, 2015 at 20:13

djechlin

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Comments

ergonaut · Accepted Answer · 2015-11-12 20:16:21Z

0

The InventoryItem array is initialized as 100, and if you don't read in 100 rows, then you'll have null pointers for the non-existent indexes. You'll need to use a List, rather than a java array.

List<InventoryItem> inventoryItems = new List<InventoryItem>();

then when you add them do something like this,

inventoryItems.add(new InventoryItem(name, value, quantity));

edited Nov 12, 2015 at 20:16

answered Nov 12, 2015 at 20:14

ergonaut

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1 Comment

Josh Peel Over a year ago

its initialized at -1

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