I'm trying to iterate over an ArrayList of ArrayLists - but somehow everything fails on me and I don't understand the error message.
The error is:
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.util.ArrayList
I've tried using a regular for(int i; i < lists.length; i++) but get the same error. All I want to do is check if any of the ArrayLists in "lists" contains the integer "v".
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
boolean b = false;
for (ArrayList<Integer> list : lists) {
if (list.contains(v)) {
b = true;
} else {
b = false;
}
}
return b;
}
The actual line that causes the error is the "for (ArrayList list"...
Edited: For clarity I edited in the code with more declarative generics (which works just as little as the first code I posted unfortunately).
Edit2: Ok so it's somehow not the method itself that causes the problem so upon request here's the rest of the code that populates these lists. The code is not done but I got caught with this problem while finishing it.
public static void main(String[] args) {
Graph g = DataSource.load();
ArrayList<ArrayList<Integer>> lists = new ArrayList<ArrayList<Integer>>();
for(int i = 0; i < g.numberOfVertices(); i++) {
if(!(listsContains(lists, i))) { // add list if node is unlisted (since after first iteration one entire network is found)
listsCreate(lists, i);
}
Iterator it = g.adj(i).iterator(); // create iterator for current node's edges
if (!(it.hasNext())) { // node has no edges
listsCreate(lists, i);
} else { // node has edges, iterate through them
while(it.hasNext()) {
Edge current = (Edge) it.next();
if(!(listsContains(lists, current.to))) { // unlisted node
int index = listsIndexOf(lists, current.from);
findNetwork(g, lists.get(index), current.to);
} else {
continue; // node already listed
}
}
}
}
System.out.println("Number of connected graphs: " + lists.size());
} // Main
You did not specify the inner ArrayList's components' type. And from your log I can tell that it contains Integers:
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
for (ArrayList<Integer> list : lists) {
if (list.contains(v))
return true;
}
return false; // No inner arrayList contains 'v'
}
EDIT:
or using Java 8 :
public static boolean listsContains(ArrayList<ArrayList<Integer>> lists, int v) {
return lists.stream().anyMatch(list -> list.contains(v));
}
Because you test:
list.contains(v)
list is of type ArrayList without inside type
v is int
replace your ArrayList by ArrayList< Integer >
Related
I'm having a hard time thinking about how should I implement the checking for duplicates while the string array with length of 5 is initially empty. Before adding an element in the array, I have to check first if it already exists in the array but because the array is initially empty (which means the five elements are null) it prompts an error, I think that is because I'm trying to compare the element (that I'm trying to add in the array) to null.
What I want to do is check if the the length of the array is less than the limit, check if the element that I want to add has no duplicate in the array. If it doesn't have a duplicate, then I'll add it in array, if it has a duplicate then I won't add it then I'll print a prompt message.
I am working on a project with multiple classes, here's the snippet of my code:
public class Collections {
Guardian[] guardians;
int count;
final static int MAX_GUARDIANS = 5;
public Collection () {
guardians = new Guardian[Collection.MAX_GUARDIANS];
}
public void addGuardians (Guardian guardian) {
if (this.count < MAX_GUARDIANS) {
for (int i = 0; i < guardians.length; i++) {
if (guardians[i].equals(guardian)) {
System.out.println("The guardian is already in the list!\n");
} else {
this.guardians[this.count++] = guardian;
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
}
}
} else {
System.out.println("Maximum number of guardians in the list has been reached!\n");
}
}
}
Is it possible to compare the element that I'm planning to add to null?
So when you want to search for a duplicate, you have to search the whole array first. Then if there's no duplicates, add an element after the for loop.
for (int i = 0; i < count; i++) {
if (guardians[i].equals(guardian)) {
System.out.println("The guardian is already in the list!\n");
return; // <-- add this to EXIT when find a match
}
}
// now that you've searched the whole list,
// you can add a new element
guardians[count++] = guardian;
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
You can try using a HashSet<String> to keep track of duplicates, and keep track of unique strings in an array.
Declare a hashset:
HashSet<String> set = new HashSet<String>();
// Or:
// Set<String> set = new HashSet<String>();
Check if a string is in a set with:
if(set.contains("Hello")) {
// String is in the set
}
Add a string to a set with:
set.add("Hello");
Use a list instead of an array to be able to check if it contains that guardian already.
public class Collections {
List<Guardian> guardians;
int count;
final static int MAX_GUARDIANS = 5;
public Collections () {
guardians = new LinkedList<>();
}
public void addGuardians (Guardian guardian) {
if (guardians.size() >= MAX_GUARDIANS) {
System.out.println("Maximum number of guardians in the list has been reached!\n");
return;
}
if (guardians.contains(guardian)) {
System.out.println("The guardian is already in the list!\n");
} else {
guardians.add(guardian);
System.out.println("Guardian "+guardian.getName()+" was added to the list!");
}
}
}
public class MyArrayList<T> implements MyList<T>{
int num; //number of things in the list
T[] vals; //to store the contents
#SuppressWarnings("unchecked")
public MyArrayList() {
num = 0;
vals = (T[]) new Object[3];
}
public T getUnique(){
T distinct = null;
int count = 0;
for (int i=0; i<vals.length; i++){
distinct = vals[i];
for (int j = 0; j<vals.length; j++){
if (vals[j] == vals[i]){
count++;
}
if (count == 1){
return distinct;
}
}
}
if (distinct == null){
throw new IllegalArgumentException();
}
return distinct;
}
I am trying to work on a get Unique Method. A method getUnique that takes no arguments and returns the first value in the list that appears only once. (For example, calling the method on the list [1,2,3,1,2,4] would return 3 since 1 and
2 both appear more than once.) If the list is empty or all its values appear more than once, the method throws a NoSuchElementException
I have added some FIXME's to your code:
public T getUnique(){
T distinct = null;
int count = 0; // FIXME: move this initialization inside the i loop
for (int i=0; i<vals.length; i++){
distinct = vals[i];
for (int j = 0; j<vals.length; j++){
if (vals[j] == vals[i]){ // FIXME: use .equals() not ==
count++;
}
if (count == 1){ // FIXME: move this check outside the j loop
return distinct;
}
}
}
if (distinct == null){ //FIXME: no check needed, just throw it
throw new IllegalArgumentException();
}
return distinct; //FIXME: no valid return can reach this point
}
Patrick Parker's advice will fix your code, but I wanted to provide a cleaner and faster solution to the problem of finding a unique element in a list. This algorithm runs in time O(n) instead of O(n^2).
public static <T> Optional<T> getUnique(List<T> ls) {
// Create a map whose keys are elements of the list and whose values are
// lists of their occurences. E.g. [1,2,3,1,2,4] becomes {1->[1, 1],
// 2->[2, 2], 3->[3], 4->[4]}. Then elements.get(x).size() tells us how
// many times x occured in ls.
Map<T, List<T>> elements = ls.stream()
.collect(Collectors.groupingBy(x -> x));
// Find the first element that occurs exactly one time in ls.
return ls.stream().filter(x -> elements.get(x).size() == 1)
.findFirst();
}
You might call it like this:
Integer[] vals = {1,2,3,1,2,4};
System.out.println(getUnique(Arrays.asList(vals))
.orElseThrow(NoSuchElementException::new));
This code uses Java 8 streams and Optional. Below is another implementation of the same algorithm that doesn't use Java 8 language features; if you've never encountered streams, you may find it more understandable.
private static <T> T getUnique(List<T> arr) {
Map<T, Integer> numOccurrences = new HashMap<>();
for (T item : arr) {
numOccurrences.put(item, 1 + numOccurrences.getOrDefault(item, 0));
}
for (T item : arr) {
if (numOccurrences.get(item) == 1) {
return item;
}
}
throw new NoSuchElementException();
}
So I have this code:
public class SortedIntList extends IntList
{
private int[] newlist;
public SortedIntList(int size)
{
super(size);
newlist = new int[size];
}
public void add(int value)
{
for(int i = 0; i < list.length; i++)
{
int count = 0,
current = list[i];
if(current < value)
{
newlist[count] = current;
count++;
}
else
{
newlist[count] = value;
count++;
}
}
}
}
Yet, when I run the test, nothing prints out. I have the system.out.print in another class in the same source.
Where am I going wrong?
EDIT: Print code from comment:
public class ListTest
{
public static void main(String[] args)
{
SortedIntList myList = new SortedIntList(10);
myList.add(100);
myList.add(50);
myList.add(200);
myList.add(25);
System.out.println(myList);
}
}
EDIT2: Superclass from comment below
public class IntList
{
protected int[] list;
protected int numElements = 0;
public IntList(int size)
{
list = new int[size];
}
public void add(int value)
{
if (numElements == list.length)
System.out.println("Can't add, list is full");
else {
list[numElements] = value; numElements++;
}
}
public String toString()
{
String returnString = "";
for (int i=0; i<numElements; i++)
returnString += i + ": " + list[i] + "\n";
return returnString;
}
}
Let's walk through the logic of how you want it to work here:
first you make a new sorted list passing 10 to the constructor, which make an integer array of size 10.
now you call your add method passing 100 into it. the method sets position 0 to 100
now you add 50, the method sets 50 in position 0 and 100 in position 1
now you add 200, which gets placed at position 2
and you add 25. which gets set to position 0, and everything else gets shuffled on down
then your method will print out everything in this list.
So here are your problems:
For the first add, you compare current, which is initialized at 0, to 50. 0 will always be less than 50, so 50 never gets set into the array. This is true for all elements.
EDIT: Seeing the super class this is how you should look to fix your code:
public class SortedIntList extends IntList
{
private int[] newlist;
private int listSize;
public SortedIntList(int size)
{
super(size);
// I removed the newList bit becuase the superclass has a list we are using
listSize = 0; // this keeps track of the number of elements in the list
}
public void add(int value)
{
int placeholder;
if (listSize == 0)
{
list[0] = value; // sets first element eqal to the value
listSize++; // incriments the size, since we added a value
return; // breaks out of the method
}
for(int i = 0; i < listSize; i++)
{
if (list[i] > value) // checks if the current place is greater than value
{
placeholder = list[i]; // these three lines swap the value with the value in the array, and then sets up a comparison to continue
list[i] = value;
value = placeholder;
}
}
list[i] = value; // we are done checking the existing array, so the remaining value gets added to the end
listSize++; // we added an element so this needs to increase;
}
public String toString()
{
String returnString = "";
for (int i=0; i<listSize; i++)
returnString += i + ": " + list[i] + "\n";
return returnString;
}
}
Now that I see the superclass, the reason why it never prints anything is clear. numElements is always zero. You never increment it because you never call the superclass version of the add method.
This means that the loop in the toString method is not iterated at all, and toString always just returns empty string.
Note
This is superseded by my later answer. I have left it here, rather than deleting it, in case the information in it is useful to you.
Two problems.
(1) You define list in the superclass, and presumably that's what you print out; but you add elements to newList, which is a different field.
(2) You only add as many elements to your new list as there are in your old list. So you'll always have one element too few. In particular, when you first try to add an element, your list has zero elements both before and after the add.
You should probably have just a single list, not two of them. Also, you should break that for loop into two separate loops - one loop to add the elements before the value that you're inserting, and a second loop to add the elements after it.
I'm trying to generalize the code to find all subsets of a given string(elements that are repeated will be treated as distinct) into one that would work for any list.
public class Subsets{
private static <T> void RecursiveSubsets(List<List<T>> list, ArrayList<T> soFar, List<T> rest)
{
if(rest.isEmpty())
{
list.add(soFar);
}
else
{
List<T> remaining;
if(rest.size() == 1)
{
remaining = new ArrayList<T>();
}
else
{
remaining = rest.subList(1, rest.size() - 1);
}
//include the element
ArrayList<T> includeFirst = new ArrayList<T>(soFar);
includeFirst.add(rest.get(0));
RecursiveSubsets(list, includeFirst, remaining);
//exclude the element
RecursiveSubsets(list, soFar, remaining);
}
}
public static <T> List<List<T>> getAllSubsets(List<T> set)
{
List<List<T>> subsets = new ArrayList<List<T>>();
RecursiveSubsets(subsets,new ArrayList<T>(),set);
return subsets;
}
public static void main(String [] args)
{
List<Integer> ints = new ArrayList<Integer>(){
{
add(0);add(1);add(2);add(3);
}
};
List<List<Integer>> allSubsets = getAllSubsets(ints);
System.out.println("Total Subsets returned : " + allSubsets.size());
for(int i=0; i<allSubsets.size(); ++i)
{
for(int j=0; j<allSubsets.get(i).size(); ++j)
{
System.out.print(allSubsets.get(i).get(j) + " ");
}
System.out.println();
}
}
}
After a few attempts I was able to get this to compile but this is what I get as output.
Even if I have more integers, it still returns this. I'm not able to figure out what I have missed and need help finding it.
$ java Subsets
Total Subsets returned : 4
0 1
0
1
Your program is actually almost correct, and you just have the sublist logic a bit wrong.
The javadoc for List.sublist says
Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.
The word "exclusive" here is critical.
If you just change
remaining = rest.subList(1, rest.size() - 1);
to
remaining = rest.subList(1, rest.size());
your code works.
The logic of this (in pseudocode) is typically:
List<List<T>> subsets( List<T> list ){
if( list is empty ) return a list containing the empty list;
// else:
subsetsWithout = subsets( list w/o 0th element );
result.addAll(subsetsWithout);
for( subset in subsetsWithout )
result.add( subset + list[0] )
return result;
}
It looks like what you're doing is different, and the fact that you're trying to return things through the function parameters is making it more confusing.
The lists are sorted the way they are supposed to but when I try to merge the two lists together in my makeUnion it prints out the list is empty. can anyone help me and tell my why? in main when I try SortedLinkedList merge = sortedNames1.makeUnion(sortedNames2) I get "Empty list".
public class SortedLinkedList<T extends Comparable<? super T>>
extends LinkedList<T>
{
private LinkedList<T> list; //the sorted list
//the constructor
public SortedLinkedList(LinkedList<T> in)
{
if(in.isEmpty())
{
System.out.println("Empty list");
}
if(in.size() < 2)
{
return;
}
else
{
list = new LinkedList<T>();
for(int i = 1; i < in.size(); i++)
{
T temp = in.get(i);
int j = i;
while(j > 0 && in.get(j - 1).compareTo(temp) > 0)
{
in.set(j, in.get(j-1));
j--;
}
in.set(j, temp);
}
for(T elements : in)
{
list.add(elements);
}
}
}
//return the union of the sorted linked lists this and other
public SortedLinkedList<T> makeUnion( SortedLinkedList<T> other)
{
SortedLinkedList<T> first = new SortedLinkedList<T>(other);
SortedLinkedList<T> second = new SortedLinkedList<T>(list);
SortedLinkedList<T> UnionList = null;
int i = 0;
int j = 0;
while(i<first.size() && j<second.size())
{
if(first.get(i).compareTo(second.get(j)) <= 0)
{
UnionList.add(first.get(i));
i++;
}
else
{
UnionList.add(second.get(j));
j++;
}
}
if(i == first.size())
{
for(int k = j; k<second.size(); k++)
{
UnionList.add(second.get(k));
}
}
else if(j == second.size())
{
for(int x = i; x<first.size(); x++)
{
UnionList.add(first.get(x));
}
}
return UnionList;
}
//print the items int list
public void print()
{
ListIterator itr = list.listIterator();
while(itr.hasNext())
{
System.out.println(itr.next());
}
}
}
SortedLinkedList<T> UnionList = null;
You can't call UnionList.add() if UnionList is null. You will need to allocate a new list before you can add things to it.
Actually, I think your original problem might be that SortedLinkedList both extends LinkedList and also contains an instance of a LinkedList. You should choose one or the other, but not both. Your code sometimes accesses one list, and sometimes the other, so one list appears empty because you've added items to the other list.
You don't initialize UnionList before you start using it.
SortedLinkedList<T> UnionList = null;
should read
SortedLinkedList<T> UnionList = new SortedLinkedList<T>();
As a bonus, ListIterator ought to be ListIterator<T> so that the right toString() method is used. As it is, you'll be calling Object.toString().
Because you used inheritance instead of delegation. You inherit LinkedList, and the only thing you do is define a constructor which adds the content of an unsorted list to a new one, in the appropriate order. But you don't override the size method, so this method is inherited from LinkedList, which doesn't care about your internal sorted list and thus always returns 0.
Extending a collection is, most of the time, a bad idea. In this case, it's a particularly bad idea because it's impossible to have a sorted LinkedList that respects the LinkedList API. Suppose your list contains A, B and C, and you call addFirst("Z") on it. Where will you put Z, if at the beginning, your list is not sorted anymore. If at the end, you don't respect the contract of addFirst.
Just use linked lists (instead of extending them), and sort them. You could just do :
LinkedList list = new LinkedList(someUnsortedList);
Collections.sort(list); // now the list is sorted
list.addAll(someOtherList);
Collections.sort(list); // now both lists are merged, and the resulting list is sorted.