Develop Reference

Iterating through sets

I am writing a program that will receive a list of words. After that, it will store the repeated words and the non repeated into two different lists. My code is the following:
public class Runner
{
public static void run (Set<String> words)
{
Set<String> uniques= new HashSet<String>();
Set<String> dupes= new HashSet<String>();
Iterator<String> w = words.iterator();
while (w.hasNext())
{
if (!(uniques.add(w.next())))
{
dupes.add(w.next());
}
else
{
uniques.add(w.next());
}
}
System.out.println ("Uniques: "+ uniques);
System.out.println ("Dupes: "+ dupes);
}
}
However, the output for the following:
right, left, up, left, down
is:
Uniques: [left, right, up, down]
Dupes: []
and my desired would be:
Uniques: [right, left, up, down]
Dupes: [ left ]
I want to achieve this using sets. I know it would be way easier to just an ArrayList but I am trying to understand sets.

The reason for your problem is that the argument words is a Set<String>. A set by definition will not contain duplicates. The argument words should be a List<String>. The code also makes the mistake of calling w.next() twice. A call to the next() will cause the iterator to advance.
public static void run(List<String> words) {
Set<String> uniques= new HashSet<String>();
Set<String> dupes= new HashSet<String>();
Iterator<String> w = words.iterator();
while(w.hasNext()) {
String word = w.next();
if(!uniques.add(word)) {
dupes.add(word);
}
}
}

You are doing uniques.add(w.next()) twice. Why?
Also, don't keep calling w.next() - this makes the iteration happen. Call it once and keep a local reference.
Use:
String next = w.next();
if(uniques.contains(next)) {
// it's a dupe
} else {
// it's not a dupe
}

How should I iterate through a HashSet, removing certain elements, and altering others?

I have the following method which is passed a HashSet<String> of words from IMDB reviews.
private static void reduceVocab(HashSet<String> vocab) {
for (Iterator<String> i = vocab.iterator(); i.hasNext();) {
String element = i.next();
element = element.replaceAll("[^a-zA-Z0-9]", ""); // Need to replace this
if (element.length() <= 3) {
i.remove();
}
}
}
I want to perform a couple of actions to reduce the size of the HashSet by removing Strings that are too short and removing non-alphanumeric characters. Is there any way to perform what I'm trying to do with element.replaceAll()?

You cannot add to a HashSet while iterating over it. This makes what you are trying to do slightly awkward. The line
element = element.replaceAll("[^a-zA-Z0-9]", "");
gives a new string, but the new string won't be in the set.
You can do it like this:
private static void reduceVocab(HashSet<String> vocab) {
Set<String> copy = new HashSet<>();
for (String str : vocab) {
str = str.replaceAll("[^a-zA-Z0-9]", "");
if (str.length() > 3)
copy.add(str);
}
vocab.clear();
vocab.addAll(copy);
}

Writing a method with ArrayList of strings as parameters

I am trying to write a method that takes an ArrayList of Strings as a parameter and that places a string of four asterisks in front of every string of length 4.
However, in my code, I am getting an error in the way I constructed my method.
Here is my mark length class
import java.util.ArrayList;
public class Marklength {
void marklength4(ArrayList <String> themarklength){
for(String n : themarklength){
if(n.length() ==4){
themarklength.add("****");
}
}
System.out.println(themarklength);
}
}
And the following is my main class:
import java.util.ArrayList;
public class MarklengthTestDrive {
public static void main(String[] args){
ArrayList <String> words = new ArrayList<String>();
words.add("Kane");
words.add("Cane");
words.add("Fame");
words.add("Dame");
words.add("Lame");
words.add("Same");
Marklength ish = new Marklength();
ish.marklength4(words);
}
}
Essentially in this case, it should run so it adds an arraylist with a string of "****" placed before every previous element of the array list because the lengths of the strings are all 4.
BTW
This consists of adding another element
I am not sure where I went wrong. Possibly in my for loop?
I got the following error:
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at Marklength.marklength4(Marklength.java:7)
at MarklengthTestDrive.main(MarklengthTestDrive.java:18)
Thank you very much. Help is appreciated.

Let's think about this piece of code, and pretend like you don't get that exception:
import java.util.ArrayList;
public class Marklength {
void marklength4(ArrayList <String> themarklength){
for(String n : themarklength){
if(n.length() ==4){
themarklength.add("****");
}
}
System.out.println(themarklength);
}
}
Ok, so what happens if your list just contains item.
You hit the line if(n.length() ==4){, which is true because you are looking at item, so you go execute its block.
Next you hit the line themarklength.add("****");. Your list now has the element **** at the end of it.
The loop continues, and you get the next item in the list, which happens to be the one you just added, ****.
The next line you hit is if(n.length() ==4){. This is true, so you execute its block.
You go to the line themarklength.add("****");, and add **** to the end of the list.
Do we see a bad pattern here? Yes, yes we do.
The Java runtime environment also knows that this is bad, which is why it prevents something called Concurrent Modification. In your case, this means you cannot modify a list while you are iterating over it, which is what that for loop does.
My best guess as to what you are trying to do is something like this:
import java.util.ArrayList;
public class Marklength {
ArrayList<String> marklength4(ArrayList <String> themarklength){
ArrayList<String> markedStrings = new ArrayList<String>(themarklength.size());
for(String n : themarklength){
if(n.length() ==4){
markedStrings.add("****");
}
markedStrings.add(n);
}
System.out.println(themarklength);
return markedStrings;
}
}
And then:
import java.util.ArrayList;
public class MarklengthTestDrive {
public static void main(String[] args){
ArrayList <String> words = new ArrayList<String>();
words.add("Kane");
words.add("Cane");
words.add("Fame");
words.add("Dame");
words.add("Lame");
words.add("Same");
Marklength ish = new Marklength();
words = ish.marklength4(words);
}
}

This...
if(n.length() ==4){
themarklength.add("****");
}
Is simply trying to add "****" to the end of the list. This fails because the Iterator used by the for-each loop won't allow changes to occur to the underlying List while it's been iterated.
You could create a copy of the List first...
List<String> values = new ArrayList<String>(themarklength);
Or convert it to an array of String
String[] values = themarklength.toArray(new String[themarklength.size()]);
And uses these as you iteration points...
for (String value : values) {
Next, you need to be able to insert a new element into the ArrayList at a specific point. To do this, you will need to know the original index of the value you are working with...
if (value.length() == 4) {
int index = themarklength.indexOf(value);
And then add a new value at the required location...
themarklength.add(index, "****");
This will add the "****" at the index point, pushing all the other entries down
Updated
As has, correctly, been pointed out to me, the use of themarklength.indexOf(value) won't take into account the use case where the themarklength list contains two elements of the same value, which would return the wrong index.
I also wasn't focusing on performance as a major requirement for the providing a possible solution.
Updated...
As pointed out by JohnGarnder and AnthonyAccioly, you could use for-loop instead of a for-each which would allow you to dispense with the themarklength.indexOf(value)
This will remove the risk of duplicate values messing up the index location and improve the overall performance, as you don't need to create a second iterator...
// This assumes you're using the ArrayList as the copy...
for (int index = 0; index < themarklength.size(); index++) {
String value = themarklength.get(index);
if (value.length() == 4) {
themarklength.add(index, "****");
index++;
But which you use is up to you...

The problem is that in your method, you didn't modify each string in the arraylist, but only adds 4 stars to the list. So the correct way to do this is, you need to modify each element of the arraylist and replace the old string with the new one:
void marklength4(ArrayList<String> themarklength){
int index = 0;
for(String n : themarklength){
if(n.length() ==4){
n = "****" + n;
}
themarklength.set(index++, n);
}
System.out.println(themarklength);
}
If this is not what you want but you want to add a new string "**" before each element in the arraylist, then you can use listIterator method in the ArrayList to add new additional element before EACH string if the length is 4.
ListIterator<String> it = themarklength.listIterator();
while(it.hasNext()) {
String name = it.next();
if(name.length() == 4) {
it.previous();
it.add("****");
it.next();
}
}
The difference is: ListIterator allows you to modify the list when iterating through it and also allows you to go backward in the list.

I would use a ListIterator instead of a for each, listiterator.add likely do exactly what you want.
public void marklength4(List<String> themarklength){
final ListIterator<String> lit =
themarklength.listIterator(themarklength.size());
boolean shouldInsert = false;
while(lit.hasPrevious()) {
if (shouldInsert) {
lit.add("****");
lit.previous();
shouldInsert = false;
}
final String n = lit.previous();
shouldInsert = (n.length() == 4);
}
if (shouldInsert) {
lit.add("****");
}
}
Working example

Oh I remember this lovely error from the good old days. The problem is that your ArrayList isn't completely populated by the time the array element is to be accessed. Think of it, you create the object and then immediately start looping it. The object hence, has to populate itself with the values as the loop is going to be running.
The simple way to solve this is to pre-populate your ArrayList.
public class MarklengthTestDrive {
public static void main(String[] args){
ArrayList <String> words = new ArrayList<String>() {{
words.add("Kane");
words.add("Cane");
words.add("Fame");
words.add("Dame");
words.add("Lame");
words.add("Same");
}};
}
}
Do tell me if that fixes it. You can also use a static initializer.

make temporary arraylist, modify this list and copy its content at the end to the original list
import java.util.ArrayList;
public class MarkLength {
void marklength4(ArrayList <String> themarklength){
ArrayList<String> temp = new ArrayList<String>();
for(String n : themarklength){
if(n.length() ==4){
temp.add(n);
temp.add("****");
}
}
themarklength.clear();
themarklength.addAll(temp);
System.out.println(themarklength);
}
}

Infinite Loop Iterating Through Hashtable

I'm trying find the most popular word in an array using Hashtables. For some reason the while loop is looping infinitely. I've debugged and the element never changes from the first one it gets. Any ideas on why this is happening?
Here is my code:
import java.util.Hashtable;
public class MyClass {
public String mostPopularString (String []words) {
if (words == null)
return null;
if (words.length == 0)
return null;
Hashtable<String, Integer> wordsHash = new Hashtable<String, Integer>();
for (String thisWord : words)
{
if (wordsHash.containsKey(thisWord))
{
wordsHash.put(thisWord, wordsHash.get(thisWord) + 1);
}
else
{
wordsHash.put(thisWord, 1);
}
}
Integer mostPopularCount = 0;
String mostPopularWord = null;
boolean tie = false;
while (wordsHash.keys().hasMoreElements())
{
String currentWord = (String) wordsHash.keys().nextElement();
if (wordsHash.get(currentWord) > mostPopularCount)
{
mostPopularCount = wordsHash.get(currentWord);
mostPopularWord = currentWord;
tie = false;
}
else if (wordsHash.get(currentWord) == mostPopularCount)
{
tie = true;
}
}
if (tie)
return null;
else
return mostPopularWord;
}
}

You're calling wordsHash.keys() on each iteration of the loop, which gives you a fresh Enumeration<String> on each iteration - you're then calling it again inside the loop.
You want to call it once, and then iterate over the single Enumeration<String>:
Enumeration<String> iterator = wordsHash.keys();
while (iterator.hasMoreElements())
{
String currentWord = iterator.nextElement();
...
}
Note that as you're also getting the value for each element, you'd be better off iterating over the entrySet() rather than the keys().
You'd also be better off using HashMap instead of Hashtable, as then you could just use an enhanced for loop...

The problem is in line
while (wordsHash.keys().hasMoreElements())
each time through the loop, you are getting a new copy of the enumeration. You'll want to get the keyset once, and iterate over that.
It would probably be easier to use an enhanced for Loop here as well
for (Map.Entry<String,Integer> entry : wordsHash.entrySet()) {
String currentWord = entry.getKey();
Integer currentCount = entry.getValue();
//more code here
}
This should provide the behavior you want, while being simpler and easier to read.

The problem is that whenever you call wordsHash.keys(), it returns a new enumeration:
while (wordsHash.keys().hasMoreElements()) // <=== HERE
{
String currentWord = (String) wordsHash.keys().nextElement(); // <=== AND HERE
What you need to do is create a single enumeration and use it throughout the loop.
P.S. Why are you using Hashtable and not HashMap?

Every call to .keys() returns a new enumeration, with a new internal pointer for iterating:
Hashtable table = new Hashtable();
table.put("a", "a");
table.put("b", "b");
boolean b = table.keys() == table.keys();
System.out.println(b); // false
// the two calls to `.keys()` returned different instances of Enumeration
So assign your keys enumeration to a variable:
Enumeration keys = wordsHash.keys();
while (keys.hasMoreElements())
{
String currentWord = (String) keys.nextElement();
}

Change your code to:
Enumeration<String> keys = wordsHash.keys();
while (keys.hasMoreElements()) {
String currentWord = keys.nextElement();
So that a new enumeration pointing to the first key of the HashTable is not created every time that you enter the loop.

Nothing is modifying the wordsHash. That means that if wordsHash.keys().hasMoreElements() is true once, it'll continue to be true for the rest of the program. This causes an infinite loop. You either need to remove the keys as you go along or you should just use a for

you get a new Iterable ofer all keys each loop iteration: wordsHash.keys() as long as there is at least one key in it the while loop never ends.
Replace:
while (wordsHash.keys().hasMoreElements()){
String currentWord = (String) wordsHash.keys().nextElement();
by
for (String currentWord: wordsHash.keys()){

Also, unrelated to your Enumeration issue, this is probably a defect:
else if (wordsHash.get(currentWord) == mostPopularCount)
That's a reference comparison of a java.lang.Integer to another java.lang.Integer. It is not a comparison of the actual values they represent. It is working for "small" numbers because auto-boxing uses cached references, but will eventually break. You probably want:
else if (wordsHash.get(currentWord) == mostPopularCount.intValue())

what is causing my NoSuchElementException for ArrayList

I know that there are lots of threads on NoSuchElementException in Java here but I still cannot figure out what is going on here
I am trying to come up with a solution for Transitive Dependencies Kata 18 which is posted at http://codekata.pragprog.com/2007/01/kata_eighteen_t.html
dependencies_for method is supposed to take in a char item and compute all dependencies for the item. The exception occurs when I try to add an element to finalDependencies ArrayList
This is the place where my NullPointerException occurs. I have traced all of these data structures and none of them have a Null value. I don't understand what is causing my exception here. Please see my code:
public class Test_Dependencies
{
public static void main(String[] args) {
Dependencies Dep = new Dependencies();
Dep.add_direct('A', "B C");
Dep.add_direct('B', "C D");
Dep.dependencies_for('A');
}
}
public class Dependencies {
HashMap dependenciesList;
public Dependencies()
{
HashMap<Character, ArrayList> dependenciesList = new HashMap<Character, ArrayList>();
}
public void add_direct(char mainItem, String dependentItems)
{
// code that works here
}
public String dependencies_for(char item)
{
ArrayList finalDependencies = new ArrayList<Character>();
Character key = new Character(item);
//get initial dependencies for the item and add them
ArrayList processingDependencies = dependenciesList.get(key);
Iterator itr = processingDependencies.iterator();
while(itr.hasNext())
{
if(finalDependencies.contains(itr.next()) == false && itr.next() != key)
{
// NoSuchElement exception here
finalDependencies.add(itr.next());
// look again at each item in dependenciesList. If it is in the list then add it to processingDependencies
if(dependenciesList.containsKey(itr.next()) && !processingDependencies.contains(itr.next()))
{
processingDependencies.add(itr.next());
}
}
}
// turn finalDependencies into a string
itr = finalDependencies.iterator();
String allDependencies = "";
while(itr.hasNext())
{
allDependencies = allDependencies + " " + itr.next();
}
return allDependencies;
}
}
I am a bit perprlexed because processingDependencies and finalDependencies ArrayLists are not null. And processingDependencies arraylist contains an item

You are calling twice. The first call is "protected" by a matching hasNext Call. The second is not. Save the result of next into a temporary variable and use that, instead of using the value directly, since every call to next will try to advance the iterator first. In the good case, you get an exception. In the bad case, things seem to work, but your program is dealing with the wrong value.

You can't do this:
while(itr.hasNext())
{
if(finalDependencies.contains(itr.next()) == false && itr.next() != key)
{
// NoSuchElement exception here
finalDependencies.add(itr.next());
// stuff removed
}
}
You must verify that iter.hasNext() is true prior to each call of itr.next(). What happens when you reach the last item in itr, but then call itr.next() three times?
Answer: NoSuchElementException. Check out Iterator

The problem is here:
HashMap dependenciesList;
public Dependencies()
{
HashMap<Character, ArrayList> dependenciesList = new HashMap<Character, ArrayList>();
}
You declare a hashmap called dependenciesList. You then try to instantiate that list, but what you actually do is create a local variable named the same thing. They are two separate variables. Then you try to use the one that hasn't been instantiated here:
ArrayList processingDependencies = dependenciesList.get(key);
What you need to do is instantiate the first dependenciesList instead of creating a new one
(I'm not a pro at java, but something like dependenciesList = new HashMap....() instead of HashMap<..> dependenciesList = new HashMap...() )