I have an ArrayList of Items and I want to be able remove one Item from the list by entering only one Item attribute, for example its number (int ItemNumber). I also wanna do the same when I check Item quantities.
These are my equals() & contains() methods, do I need to make any changes here?
public boolean contains(T anEntry) {
boolean found = false;
for (int index = 0; !found && (index < numberOfEntries); index++) {
if (anEntry.equals(list[index]))
found = true;
}//end for
return found;
} // end contains
public boolean equals(Object object){
Item item = (Item) object;
if (itemNo == item.itemNo)
return true;
return false;
}
If you change the class Item equals() and compareTo() methods, so that they check only one object field, such as a quantity, it could result in strange behavior in other parts of your application. For example, two items with different itemNo, itemName, and itemPrice, but with the same quantities could be considered equal. Besides, you wouldn't be able to change the comparison attribute without changing the equals() code every time.
Also, creating a custom contains() method makes no sense, since it belongs to the ArrayList class, and not to Item.
If you can use Java 8, a clean way to do it is to use the new Collection's removeIf method:
Suppose you have an Item class with the num and name properties:
class Item {
final int num;
final String name;
Item(int num, String name) {
this.num = num;
this.name = name;
}
}
Given a List<Item> called items and an int variable called number, representing the number of the item you want to remove, you could simply do:
items.removeIf(item -> item.num == number);
If you are unable to use Java 8, you can achieve this by using custom comparators, binary search, and dummy objects.
You can create a custom comparator for each attribute you need to look for. The comparator for num would look like this:
class ItemNumComparator implements Comparator<Item> {
#Override
public int compare(Item a, Item b) {
return (a.num < b.num) ? -1 : ((a.num == b.num) ? 0 : 1);
}
}
Then you can use the comparator to sort and search for the desired elements in your list:
public static void main(String[] args) {
List<Item> items = new ArrayList<>();
items.add(new Item(2, "ball"));
items.add(new Item(5, "cow"));
items.add(new Item(3, "gum"));
Comparator<Item> itemNumComparator = new ItemNumComparator();
Collections.sort(items, itemNumComparator);
// Pass a dummy object containing only the relevant attribute to be searched
int index = Collections.binarySearch(items, new Item(5, ""), itemNumComparator);
Item removedItem = null;
// binarySearch will return -1 if it does not find the element.
if (index > -1) {
// This will remove the element, Item(5, "cow") in this case, from the list
removedItem = items.remove(index);
}
System.out.println(removedItem);
}
To search for another field like name, for example, you would need to create a name comparator and use it to sort and run the binary search on your list.
Note this solution has some drawbacks though. Unless you are completely sure that the list didn't change since the last sort, you must re-sort it before running the binarySearch() method. Otherwise, it may not be able to find the correct element. Sorting complexity is O(nlogn), so running it multiple times can get quite expensive depending on the size of your list.
Do you want to remove an object at a specific index? I'm not entirely sure what you mean by 'number field'. If so, jump to method: remove(int):
http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#remove%28int%29
EDIT: If you want to find/adjust a field of an object in the Array list, you can do this (piece of my own code):
public boolean studentHasISBN(ArrayList<Student> st, String s){
for(Student j : st) {
if(s.equals(j.getRentedBookISBN()))
return true;
}
return false;
}
All you have to do is iterate through the list, and search through the field that you want to find. Then use the remove(int) method.
simply use the remove function of ArrayLists in Java:
theNameOfYourArrayList.remove(ItemNumber);
to remove the element which has the index (int ItemNumber)
to check if the element with item number (int ItemNumber) exists in your ArrayList (hypothetically called theNameOfYourArrayList):
theNameOfYourArrayList.get(ItemNumber);
I'm going to assume that by 'number field' you mean that you invoked ArrayList with the Integer data type. I have a few different solutions to your problem:
ArrayLists, assuming that the ArrayList is ArrayList<Integer> numList = new ArrayList<Integer>(); you can simply write a method that will search 'numList' and delete the index that the number is. The problem is, contains and find in ArrayLists can be slow.
public void deleteNumField(int field) {
// this will stop any error if field isn't actually in numList
// and it will remove the first index of field in the ArrayList
if(numList.contains(field)) numList.remove(numList.find(field));
}
HashSets, HashSets are a handy data type that is like an ArrayList, except, its data is its 'index' (sortof). I won't go in depth about how they work, but I will say that searching in them is considered O(1). This will make your deletion really easy, and fast. Note: the HashSet assumes there are no duplicate numbers, if there are use a HashMap.
HashSet<Integer> numList = new HashSet<Integer>();
public void deleteNumField(int field) {
// this will stop errors from attempting to remove a
// non-existant element, and remove it if it exists.
if(numList.contains(field)) numList.remove(field);
}
For more information on HashMaps, HashSets and ArrayLists, see:
http://docs.oracle.com/javase/8/docs/api/
Related
I have a list of Strings named cardNameAndDescription in a class called GlobalDataHolderand i am sorting these Strings alphabetically with the Collections.sort(); method. However, each String in this list is associated with an image in another list called mGLBIcons which is located inside an Adapter class' called UserBoxGlbImageAdapter. When the List<String> gets sorted, I need to sort the List<Integer> (images) so that the image to String association doesn't get messed up.However, when I try and swap the images based on the custom Comparator's results, the images do not at all match with the Strings.Can someone help me understand what I am doing wrong here?I think I might have misunderstood how the Comparator class sorts collections. Here's my code:
GlobalDataHolder.cardNameAndDescription.sort(new Comparator<String>() {
#Override
public int compare(String s1, String s2) {
int s1Pos = GlobalDataHolder.cardNameAndDescription.indexOf(s1);
int s2Pos = GlobalDataHolder.cardNameAndDescription.indexOf(s2);
if(s1.equalsIgnoreCase(s2)) {
return 0;
} else if(s1.compareToIgnoreCase(s2) > 0) { //greater than s2
Log.v("card_names","1");
Collections.swap(UserBoxGlbImageAdapter.mGLBIcons,s1Pos,s2Pos);
// UserBoxGlbImageAdapter.refreshFragmentView(UserBoxGLBFragment.getUserBoxAdapter());
return 1;
} else if(s1.compareToIgnoreCase(s2) < 0) { // less than s2
Log.v("card_names","-1");
Collections.swap(UserBoxGlbImageAdapter.mGLBIcons,s2Pos,s1Pos);
// UserBoxGlbImageAdapter.refreshFragmentView(UserBoxGLBFragment.getUserBoxAdapter());
return -1;
} else {
return 100; // error code
}
}
});
Example: The List of Strings contains 3 elements in this order : "Terrifying Power","SSJ4","Androids". After the sorting process, their order is : "Androids","SSJ4","Terrifying Power".
Here is the starting order of the images:
and here is the right order:
And after the sorting process happens, the images just swap places and remain in the wrong order until i've run the sorting process like 20 times(i know i'm using the swap() method but i only want this to sort the images as needed so that they match up with the strings).
Working with a list of Object where one of the item has an empty String. Trying to write method which would return a sorted list. By sorting means the first item value of the list should always be an empty String.
Since I don't want to manipulate the unsorted list, I am creating a new list to sort.
So far my code is:
private List<LoggerConfig> sort(List<LoggerConfig> unSortedList) {
List<LoggerConfig> sortedList = new ArrayList<LoggerConfig>(unSortedList);
//What to do here
return sortedList;
}
Looked at lot of SO posts but very confused.
You can trust the String.compareTo to match the order you seek. Here is a Comparator :
new Comparator<LoggerConfig>() {
#Override
public int compare(LoggerConfig o1, LoggerConfig o2) {
return (o1.getName().compareTo(o2.getName()));
}
};
or directly implementing Comparable in the specific class (here Dummy)
class Dummy implements Comparable<Dummy>{
String name;
public int compareTo(Dummy o) {
return this.name.compareTo(o.name);
}
}
The why :
The String.compareTo check the first characters of both to find a difference (until the smallest length of both), if they match, the lengths are use to make the difference, the longest will be after the shortest (shortest.compareTo(longuest) will return an negative value (the length difference)).
In this case, "".compareTo("abc"), there is no character in the empty String, so the first check is skipped and the length is use to compare the Strings, so an empty String will always be seen as first compare to any "non-empty" String
An example with the previous Dummy class (just need to add the Constructor Dummy(String):
public class Main {
public static void main(String[] args) {
List<Dummy> dummies = new LinkedList<Dummy>();
dummies.add(new Dummy("abc.com.core"));
dummies.add(new Dummy(""));
dummies.add(new Dummy("abc.com.core.def"));
System.out.println("BEFORE : " + dummies);
Collections.sort(dummies);
System.out.println("AFTER : " + dummies);
}
}
Output :
BEFORE : [abc.com.core, , abc.com.core.def]
AFTER : [, abc.com.core, abc.com.core.def]
You can place this condition in your comparator so that elements with an empty value are considered "less" than other elements, so that it shows up at the beginning of the sorted list. Try something like this:
Collections.sort(sortedList, new Comparator<LoggerConfig>() {
#Override
public int compare(LoggerConfig o1, LoggerConfig o2) {
if(o1.getName().isEmpty(){
return -1;
}
if(o2.getName().isEmpty(){
return 1;
}
return (o1.getName().compareTo(o2.getName()));
}
});
I didn't test this, but this should make the idea clear. If the empty element shows up at the end of the list, swap the -1 and the 1.
If your List is huge and sorting takes a lot of time, it might be a better idea to remove the empty element before sorting, then sort, then place the element at the beginning.
The Comparator solution seems feasible to me; what you're missing is implementing the compare method so that it does what you want.
Collections.sort(sortedList, new Comparator<LoggerConfig>() {
#Override
public int compare(LoggerConfig o1, LoggerConfig o2) {
if(o1.getName().equals("")){
return -1;
} else if(o2.getName().equals("")) {
return 1;
} else {
return (o1.getName().compareTo(o2.getName()));
}
}
});
As per Java docs, the Comparator has a compare method that returns an int which is
less than 0 if the first argument is less than the second
0 if the arguments are equal
greater than 0 if the first argument is greater than the second
So the Comparator you need should return the comparison of the two strings if they're both different from "", and -1 (or 1) if the first (or second) String is empty.
I need a Collection that sorts the element, but does not removes the duplicates.
I have gone for a TreeSet, since TreeSet actually adds the values to a backed TreeMap:
public boolean add(E e) {
return m.put(e, PRESENT)==null;
}
And the TreeMap removes the duplicates using the Comparators compare logic
I have written a Comparator that returns 1 instead of 0 in case of equal elements. Hence in the case of equal elements the TreeSet with this Comparator will not overwrite the duplicate and will just sort it.
I have tested it for simple String objects, but I need a Set of Custom objects.
public static void main(String[] args)
{
List<String> strList = Arrays.asList( new String[]{"d","b","c","z","s","b","d","a"} );
Set<String> strSet = new TreeSet<String>(new StringComparator());
strSet.addAll(strList);
System.out.println(strSet);
}
class StringComparator implements Comparator<String>
{
#Override
public int compare(String s1, String s2)
{
if(s1.compareTo(s2) == 0){
return 1;
}
else{
return s1.compareTo(s2);
}
}
}
Is this approach fine or is there a better way to achieve this?
EDIT
Actually I am having a ArrayList of the following class:
class Fund
{
String fundCode;
BigDecimal fundValue;
.....
public boolean equals(Object obj) {
// uses fundCode for equality
}
}
I need all the fundCode with highest fundValue
You can use a PriorityQueue.
PriorityQueue<Integer> pQueue = new PriorityQueue<Integer>();
PriorityQueue(): Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
This is a link to doc: https://docs.oracle.com/javase/8/docs/api/java/util/PriorityQueue.html
I need all the fundCode with highest fundValue
If that's the only reason why you want to sort I would recommend not to sort at all. Sorting comes mostly with a complexity of O(n log(n)). Finding the maximum has only a complexity of O(n) and is implemented in a simple iteration over your list:
List<Fund> maxFunds = new ArrayList<Fund>();
int max = 0;
for (Fund fund : funds) {
if (fund.getFundValue() > max) {
maxFunds.clear();
max = fund.getFundValue();
}
if (fund.getFundValue() == max) {
maxFunds.add(fund);
}
}
You can avoid that code by using a third level library like Guava. See: How to get max() element from List in Guava
you can sort a List using Collections.sort.
given your Fund:
List<Fund> sortMe = new ArrayList(...);
Collections.sort(sortMe, new Comparator<Fund>() {
#Override
public int compare(Fund left, Fund right) {
return left.fundValue.compareTo(right.fundValue);
}
});
// sortMe is now sorted
In case of TreeSet either Comparator or Comparable is used to compare and store objects . Equals are not called and that is why it does not recognize the duplicate one
Instead of the TreeSet we can use List and implement the Comparable interface.
public class Fund implements Comparable<Fund> {
String fundCode;
int fundValue;
public Fund(String fundCode, int fundValue) {
super();
this.fundCode = fundCode;
this.fundValue = fundValue;
}
public String getFundCode() {
return fundCode;
}
public void setFundCode(String fundCode) {
this.fundCode = fundCode;
}
public int getFundValue() {
return fundValue;
}
public void setFundValue(int fundValue) {
this.fundValue = fundValue;
}
public int compareTo(Fund compareFund) {
int compare = ((Fund) compareFund).getFundValue();
return compare - this.fundValue;
}
public static void main(String args[]){
List<Fund> funds = new ArrayList<Fund>();
Fund fund1 = new Fund("a",100);
Fund fund2 = new Fund("b",20);
Fund fund3 = new Fund("c",70);
Fund fund4 = new Fund("a",100);
funds.add(fund1);
funds.add(fund2);
funds.add(fund3);
funds.add(fund4);
Collections.sort(funds);
for(Fund fund : funds){
System.out.println("Fund code: " + fund.getFundCode() + " Fund value : " + fund.getFundValue());
}
}
}
Add the elements to the arraylist and then sort the elements using utility Collections.sort,. then implement comparable and write your own compareTo method according to your key.
wont remove duplicates as well, can be sorted also:
List<Integer> list = new ArrayList<>();
Collections.sort(list,new Comparator<Integer>()
{
#Override
public int compare(Objectleft, Object right) {
**your logic**
return '';
}
}
)
;
I found a way to get TreeSet to store duplicate keys.
The problem originated when I wrote some code in python using SortedContainers. I have a spatial index of objects where I want to find all objects between a start/end longitude.
The longitudes could be duplicates but I still need the ability to efficiently add/remove specific objects from the index. Unfortunately I could not find the Java equivalent of the Python SortedKeyList that separates the sort key from the type being stored.
To illustrate this consider that we have a large list of retail purchases and we want to get all purchases where the cost is in a specific range.
// We are using TreeSet as a SortedList
TreeSet _index = new TreeSet<PriceBase>()
// populate the index with the purchases.
// Note that 2 of these have the same cost
_index.add(new Purchase("candy", 1.03));
Purchase _bananas = new Purchase("bananas", 1.45);
_index.add(new Purchase(_bananas);
_index.add(new Purchase("celery", 1.45));
_index.add(new Purchase("chicken", 4.99));
// Range scan. This iterator should return "candy", "bananas", "celery"
NavigableSet<PriceBase> _iterator = _index.subset(
new PriceKey(0.99), new PriceKey(3.99));
// we can also remove specific items from the list and
// it finds the specific object even through the sort
// key is the same
_index.remove(_bananas);
There are 3 classes created for the list
PriceBase: Base class that returns the sort key (the price).
Purchase: subclass that contains transaction data.
PriceKey: subclass used for the range search.
When I initially implemented this with TreeSet it worked except in the case where the prices are the same. The trick is to define the compareTo() so that it is polymorphic:
If we are comparing Purchase to PriceKey then only compare the price.
If we are comparing Purchase to Purchase then compare the price and the name if the prices are the same.
For example here are the compareTo() functions for the PriceBase and Purchase classes.
// in PriceBase
#Override
public int compareTo(PriceBase _other) {
return Double.compare(this.getPrice(), _other.getPrice());
}
// in Purchase
#Override
public int compareTo(PriceBase _other) {
// compare by price
int _compare = super.compareTo(_other);
if(_compare != 0) {
// prices are not equal
return _compare;
}
if(_other instanceof Purchase == false) {
throw new RuntimeException("Right compare must be a Purchase");
}
// compare by item name
Purchase _otherPurchase = (Purchase)_other;
return this.getName().compareTo(_otherChild.getName());
}
This trick allows the TreeSet to sort the purchases by price but still do a real comparison when one needs to be uniquely identified.
In summary I needed an object index to support a range scan where the key is a continuous value like double and add/remove is efficient.
I understand there are many other ways to solve this problem but I wanted to avoid writing my own tree class. My solution seems like a hack and I am surprised that I can't find anything else. if you know of a better way then please comment.
I'm not sure how to word this correctly, but I'm told to write a method that would return the largest course object (the course with the most students). If there are two courses that have the same number of students, it would return both.
The second part of the problem is what troubles me, because I'm not allowed to make another ArrayList other than the ones he specified (which is already used). Is there a way to keep track of two+ objects without using an list/hash?
This is what I've done so far, but it only returns one course object.
public Course largestEnrollment(){
int size = 0;
Course p = null;
for (Integer c : courseList.keySet()){
if (courseList.get(c).getClassList().size() > size){
p = courseList.get(c);
size = courseList.get(c).getClassList().size();
}
return p;
}
return null;
}
Return an array of Course objects:
public Course[] largestEnrollment(){
You'll need to decide how to manipulate the array inside your for loop.
Sort the ArrayList based on size. Then you can return a sub-list of the largest courses.
If you don't have so many Course (e.g. <1k), you could implement Comparable or write a Comparator for your Course object. So that you could just from the map get all Values(Course) in collection, then sort the collection, just from the end of the sorted collection take those elements with same values(size).
I mentioned the size of the collection because sort make the O(n) problem into O(nlogn). But if the size is small, it is a convenient way to go.
Anyway, you have to change the method return type to a collection or an array.
Sort then return a sublist:
public List<Course> largestEnrollment(List<Course> courses) {
Collections.sort(courses, new Comparator<Course>() {
#Override
public int compare(Course o1, Course o2) {
return o1.getClassList().size() - o2.getClassList().size();
}
});
for (int indexOfLargest = 1; indexOfLargest < courses.size(); indexOfLargest ++) {
if (courses.get(indexOfLargest - 1).getClassList().size() > courses.get(indexOfLargest).getClassList().size())
return courses.subList(0, indexOfLargest);
}
return courses;
}
I have a method which add element to an arraylist
My task is to Modify the addProduct method so that a new product
cannot be added to the product list with the same ID
as an existing one.
Since both number and string are in the same word "item" and stored on the same index, I don't know how I can just get the number. I need the number to test to see if the number already exist
Any suggestion on how I should do this?
The way I add to the arraylist is like this below:
(new Product(132, "Clock Radio"))
public void addProduct(Product item)
{
stock.add(item);
}
I would greatly recommend you to go for Set inside the addProduct() method.
From the Javadocs,
SET
A collection that contains no duplicate elements. More formally, sets
contain no pair of elements e1 and e2 such that e1.equals(e2), and at
most one null element.
Implement like this,
public static boolean checkDuplicate(ArrayList list) {
HashSet set = new HashSet();
for (int i = 0; i < list.size(); i++) {
boolean val = set.add(list.get(i));
if (val == false) {
return val;
}
}
return true;
}
public void addProduct(Product item){
for (Product p: stock)
if (p.getId() == item.getId())
return;
stock.add(item);
}
I would use a java.util.Set. You would need to implement the equals() and hashcode() methods of the Product class based on the two fields passed into the constructor.
Try using a HashMap with the ID as the Key and the Item as the Value. In an HashMap you cant duplicate Items with the same Key, so your problem is solved at the bottom of your programming. :)
Create a ProductList class that has an ArrayList field and a integer set to keep track of ID's that have been added. When you add an item, check if the set already contains the item's ID. If it doesn't, add the item to the ArrayList. So this basically wraps around an ArrayList quite nicely. Here's how I would do it:
public class ProductList{
...
private ArrayList<Product> list = new ArrayList<Product>();
private Set<Integer> ids = new Set<Integer>();
...
public void addProduct(Product product){
if(!ids.contains(product.getID())){
list.add(product);
ids.add(product.getID());
}
}
public Product removeProduct(Product product){
if(!list.contains(product)) return null;
ids.remove(product.getID());
return list.remove(product);
}
...
}
You can then just use
ProductList stock = new ProductList();
and stock.addProduct(Product item); in your other class.
If you think you'll be using your list quite extensively, creating practical constructors to integrate with your data fields will be very useful as well.
This is a very good approach from an abstraction point of view, however it's probably not the most efficient way of doing it.