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ArrayList initialized to a size, but index is out of bounds and size is zero. Why?

So I am initializing an ArrayList of ArrayLists in order to have a set of resizable Arrays to hold the names of classes as strings for a Design Analyzer homework assignment. The ArrayLists are initialized to a size of 7 (For the test file that I am using), yet when I perform a get on element 1, I am getting an IndexOutOfBounds exception. Upon checking the size of the ArrayList causing the problem (providers), the size is zero. I am having a hard time understanding why the ArrayList is size zero, despite the fact that I have initialized it to be the size of my cls ArrayList (which is 7 in my test case). The exception is being thrown when I attempt the get(i) on providers in my if statement, but why? Any help would be appreciated.
public class DesignAnalyzer {
//private Hashtable classSet = new Hashtable();
//ArrayList<Integer> counters = new ArrayList<>();
private static ArrayList<ArrayList<String>> providers;
private static ArrayList<ArrayList<String>> clients;
//private static ArrayList<String>[]
public static void analyze(ArrayList<Class<?>> cls, String path){
Package homePkg = Package.getPackage(path.substring(path.lastIndexOf("\\")+1));
ArrayList<ArrayList<String>> providers = new ArrayList<ArrayList<String>>(cls.size());
ArrayList<ArrayList<String>> clients = new ArrayList<ArrayList<String>>(cls.size());
providers.ensureCapacity(cls.size());
clients.ensureCapacity(cls.size());
for(int i = 0; i < cls.size(); ++i){
providers.set(i, new ArrayList<String>());
clients.set(i, new ArrayList<String>());
}
getProviders(cls, homePkg);
getClients(cls);
//providers.clear();
}
private static void getProviders(ArrayList<Class<?>> cls, Package pkg){
for(int i = 0; i < cls.size(); ++i){
Class<?> spr = cls.get(i).getSuperclass();
int temp = providers.size(); //should be 7 in test case, but coming back as zero
if(spr != null && spr.getPackage().toString().equals(pkg.toString()) &&
!providers.get(i).contains(spr.toString())) // exception being thrown here at i = 1 b/c providers.size is zero...
providers.get(i).add(spr.toString());

The number that you pass to ArrayList's constructor is the initial capacity of the ArrayList, not the initial size. The initial size is 0, unless you are passing to the constructor (not the same constructor that takes the capacity parameter) another Collection used to populate your ArrayList.
You should never call providers.get(i) before checking that providers.size() > i.
P.S.
I see that you have code that initializes the providers ArrayList in your analyze method. However, you initialize a local variable and not the static class member of the same name. This means that getProviders should actually throw a NullPointerException and not IndexOutOfBoundsException (unless you initialize the providers static member in some code you didn't include).
Try to change your analyze method to :
public static void analyze(ArrayList<Class<?>> cls, String path){
Package homePkg = Package.getPackage(path.substring(path.lastIndexOf("\\")+1));
providers = new ArrayList<ArrayList<String>>(cls.size());
clients = new ArrayList<ArrayList<String>>(cls.size());
....

Accordingly to the documentation, such a constructor does what follows:
Constructs an empty list with the specified initial capacity.
So, it's perfectly reasonable that accessing it ends in an out of bound exception, for it's empty.
Something similar happens for ensureCapacity, that I see you are using in your code. It simply does what follows:
Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
I guess you have misunderstood what capacity means for a container like ArrayList and what's actually the size. While the latter actually indicates how many items are there, the former is related to the internal representation of the container. As you know, such a container grows dynamically on demand, but you can slightly optimize it if you know which is the intended final size, as an example when used in algorithms bounded in terms of visited items.

If clone( ) for ArrayList is broken why can't they fix it?

After reading multiple posts (here's one) about how clone( ) from the ArrayList class is broken, I was wondering no one has re-implemented it so that it isn't broken where people can be more comfortable using the method.
Is there something I'm missing here for a reason why no one has re-implemented it so that it is not broken?

The clone() itself is not broken: it's implemented well according to the specification of the clone() method. The problematic place is this specification itself. As it was already discussed in the linked question, there are several drawbacks, namely the necessity to do the unchecked cast, the potential problems which arise if your ArrayList is overridden and the necessity to know that the input List is actually an ArrayList. A good practice for any code requiring the ArrayList is to accept any List (or even Collection) implementation: this way your code becomes more flexible.
As it was already noted, there's a better alternative: using
ArrayList<Type> copy = new ArrayList<>(source);
It's universal: it will work for any source collection and the result will be exactly ArrayList (not derived class). And you should not worry about the performance. According to the implementation it's about the same. See the clone() method code:
public Object clone() {
try {
#SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
So it makes a shallow copy (fast operation of comstant complexity as shallow size is constant and small), then makes single array copy and replaces the mod count. Let's check the constructor:
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
It calls the toArray of original collection, uses its result as the internal array for itself and updates the size. It copies the array only in the case if original collection incorrectly returned typed array from the toArray method. What happens if the input collection is also ArrayList? Check the ArrayList.toArray:
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
See, it's exactly the same operation as in clone() method. Thus in both cases (clone and copy-constructor) you have single Arrays.copyOf(elementData, size) call and small constant overhead.

Which is good practice - Modifying a List in the method, or returning a new List in the method?

Example code:
modifyMyList(myList);
public void modifyMyList(List someList){
someList.add(someObject);
}
or:
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
someList.add(someObject)
return someList;
}
There is also a 3rd option I believe: You can create a new List in modifyMyList method and return this new List...
( 3rd option is here, I was too lazy but someone already added it in the answers: )
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}
Is there any reason why I should choose one over another? What should be considered in such case?

I have a (self imposed) rule which is "Never mutate a method parameter in a public method". So, in a private method, it's ok to mutate a parameter (I even try to avoid this case too). But when calling a public method, the parameters should never be mutated and should be considered immutable.
I think that mutating method arguments is a bit hacky and can lead to bugs that are harder to see.
I have been known to make exceptions to this rule but I need a really good reason.

Actually there is no functional difference.
You'll come to know the difference when you want the returned list
List someNewList = someInstnace.modifyMyList(list);

The second is probably confusing as it implies a new value is being created and returned - and it isn't.
An exception would be if the method was part of a 'fluent' API, where the method was an instance method and was modifying its instance, and then returning the instance to allow method chaining: the Java StringBuilder class is an example of this.
In general, however, I wouldn't use either.
I'd go for your third option: I write a method that creates and returns a new list with the appropriate change. This is a bit artificial in the case of your example, as the example is really just reproducing List.add(), but...
/** Creates a copy of the list, with val appended. */
public static <T> List<T> modifyMyList(List<T> list, T val) {
List<T> xs = new ArrayList<T>(list);
xs.add(val);
return xs;
}
Aside: I wouldn't, as suggested by Saket return an immutable list. His argument for immutability and parallelism is valid. But most of the time Java programmers expect to be able to modify a collection, except in special circumstances. By making you method return an immutable collection, you limit it's reusability to such circumstances. (The caller can always make the list immutable if they want to: they know the returned value is a copy and won't be touched by anything else.) Put another way: Java is not Clojure. Also, if parallelism is a concern, look at Java 8 and streams (the new kind - not I/O streams).
Here's a different example:
/** Returns a copy of a list sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
List<T> xs = new ArrayList<T>();
for(T x : it)
if(x!=null) xs.add(x);
return xs;
}
Note that I've genercized the method and made it more widely applicable to taking an Iterable instead of a list. In real code, I'd have two overloaded versions, one taking an Iterable and one an Iterator. (The first would be implemented by calling the second, with the iterable's iterator.) Also, I've made it static as there was no reason for your method to be an instance method (it does not depend on state from the instance).
Sometimes, though, if I'm writing library code, and if it is not clear whether a mutating or non-mutating implementation is more generally useful, I create both. Here's a fuller example:
/** Returns a copy of the elements from an Iterable, as a List, sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
return compact(it.iterator());
}
public static <T> List<T> compact(Iterator<T> iter) {
List<T> xs = new ArrayList<T>();
while(iter.hasNext()) {
T x = iter.next();
if(x!=null) xs.add(x);
}
return xs;
}
/** In-place, mutating version of compact(). */
public static <T> void compactIn(Iterable<T> it) {
// Note: for a 'fluent' version of this API, have this return 'it'.
compactIn(it.iterator());
}
public static <T> void compactIn(Iterator<T> iter) {
while(iter.hasNext()) {
T x = iter.next();
if(x==null) iter.remove();
}
}
If this was in a real API I'd check the arguments for null and throw IllegalArgumentException. (NOT NullPointerException - though it is often used for this purpose. NullPointerException happens for other reasons as well, e.g. buggy code. IllegalArgumentException is better for invalid parameters.)
(There'd also be more Javadoc than actual code too!)

The first and second solution are very similar, The advantage of the second is to permit chaining. The question of "is it a good practise" is subjected to debate as we can see here:
Method Chaining in Java
So the real question is between the first solution with mutable list and the third with a unmutable list, and again, there is not a unique response, it is the same debate between returning String, which are immutable and using Stringbuffer, which are mutable but permits better performance.
If you need reliablility of your API , and if you don't have performance issues use immutable (the third solution). Use it if your lists are always small.
If you need only performance use a mutable list (the first solution)

I will recommend creating a new list in the method and returning an immutable list. That way your code will work even when you are passed in an Immutable list. It is generally a good practice to create immutable objects as we generally move towards functional programming and try to scale across multiple processor architectures.
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}

As I said in my other answer, I don't think you should mutate the list parameter. But there are times where you also don't want to take a copy of the original list and mutate the copy.
The original list might be large so the copy is expensive
You want the copy to be kept up-to-date with any updates to the original list.
In these scenarios, you could create a MergedList which is a view over two (or perhaps more) lists
import java.util.*;
public class MergedList<T> extends AbstractList<T> {
private final List<T> list1;
private final List<T> list2;
public MergedList(List<T> list1, List<T> list2) {
this.list1 = list1;
this.list2 = list2;
}
#Override
public Iterator<T> iterator() {
return new Iterator<T>() {
Iterator<T> it1 = list1.iterator();
Iterator<T> it2 = list1.iterator();
#Override
public boolean hasNext() {
return it1.hasNext() || it2.hasNext();
}
#Override
public T next() {
return it1.hasNext() ? it1.next() : it2.next();
}
};
}
#Override
public T get(int index) {
int size1 = list1.size();
return index < size1 ? list1.get(index) : list2.get(index - size1);
}
#Override
public int size() {
return list1.size() + list2.size();
}
}
The you could do
public List<String> modifyMyList(List<String> someList){
return new MergedList(someList, List.of("foo", "bar", "baz"));
}

Both ways will work because in this case java works with the reference of the List but i prefer the secound way because this solution works for pass by value too, not only for pass by reference.

Functionally both are same.
However when you expose your method as an API, second method may give an impression that it returns a new modified list other than the original passed list.
While the first method would make it clear (of-course based on method naming convention) that it will modify the original list (Same object).
Also, the second method returns a list, so ideally the caller should check for a null return value even if the passed list is non null (The method can potentially return a null instead of modified list).
Considering this I generally prefer to use method one over second.

creating array without declaring the size - java

i've digging around about the same issue but i couldn't find the same as i had
i want to create an array without declaring the size because i don't know how it will be !
to clear the issue i'd like to give you the code that i'm looking up for
public class t
{
private int x[];
private int counter=0;
public void add(int num)
{
this.x[this.counter] = num;
this.counter++;
}
}
as you see the user could use the add function to add element to the array 10000 times or only once so it's unknown size

Using Java.util.ArrayList or LinkedList is the usual way of doing this. With arrays that's not possible as I know.
Example:
List<Float> unindexedVectors = new ArrayList<Float>();
unindexedVectors.add(2.22f);
unindexedVectors.get(2);

You might be looking for a List? Either LinkedList or ArrayList are good classes to take a look at. You can then call toArray() to get the list as an array.

As others have said, use ArrayList. Here's how:
public class t
{
private List<Integer> x = new ArrayList<Integer>();
public void add(int num)
{
this.x.add(num);
}
}
As you can see, your add method just calls the ArrayList's add method. This is only useful if your variable is private (which it is).

Once the array size is fixed while running the program ,it's size can't be changed further.
So better go for ArrayList while dealing with dynamic arrays.

How about this
private Object element[] = new Object[] {};

I think what you really want is an ArrayList or Vector. Arrays in Java are not like those in Javascript.

How do I perform an action on each element of a List and return the result (without affecting the original of course)?

How do I write a static method in Java that will take a List, perform an action on each element, and return the result (without affecting the original of course)?
For example, if I want to add 2 to each element what goes in the ... here? The concrete return type must be the same, e.g. if my List is a LinkedList with values 1,2,3 I should get back a LinkedList with values 3,4,5. Similarly for ArrayList, Vector, Stack etc, which are all Lists.
I can see how to do this using multiple if (lst instanceof LinkedList) ... etc... any better way?
import java.util.List;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) {
...
return result;
}
}

There are already many answers, but I'd like to show you a different way to think of this problem.
The operation you want to perform is known as map in the world of functional programming. It is something we do really all the time in functional languages.
Let M<A> be some kind of container (in your case, M would be List, and A would be Integer; however, the container can be lots of other things). Suppose you have a function that transforms As into Bs, that is, f: A -> B. Let's write this function as of type F<A, B>, to use a notation closer to Java. Note that you can have A = B, as in the example you give (in which A = B = Integer).
Then, the operation map is defined as follows:
M<B> map(M<A>, F<A, B>)
That is, the operation will return a M<B>, presumably by applying F<A, B> to each A in M<A>.
In practice...
There's a brilliant library developed by Google, called Guava, which brings lot's of functional idioms to Java.
In Guava, the map operation is called transform, and it can operate on any Iterable. It has also more specific implementations that work directly on lists, sets, etc.
Using Guava, the code you want to write would look like this:
static List<Integer> add2(List<Integer> ns) {
return Lists.transform(ns, new Function<Integer, Integer>() {
#Override Integer apply(Integer n) { return n + 2; }
}
}
Simple as that.
This code won't touch the original list, it will simply provide a new list that calculates its values as needed (that is, the values of the newly created list won't be calculated unless needed -- it's called a lazy operation).
As a final consideration, it is not possible for you to be absolutely sure that you will be able to return exactly the same implementation of List. And as many others pointed out, unless there's a very specific reason for this, you shouldn't really care. That's why List is an interface, you don't care about the implementation.

Fundamentally, the List interface doesn't make any guarantees that you'll have a way to duplicate it.
You may have some luck with various techniques:
Using clone() on the passed in List, although it may throw, or (since it is protected in Object) simply not be accessible
Use reflection to look for a public no-argument constructor on the passed-in List
Try to serialize and deserialize it in order to perform a "deep clone"
Create some sort of factory and build in knowledge of how to duplicate each different kind of List your code may encounter (What if it's a wrapper created by unmodifiableList(), or some oddball custom implementation backed by a RandomAccessFile?)
If all else fails, either throw, or return an ArrayList or a Vector for lack of better options

You could use reflection to look for a public zero-arg constructor on the result of lst.getClass() and then invoke() it to obtain the List into which you'll place your results. The Java Collections Framework recommends that any derivative of Collection offer a zero-arg constructor. That way, your results we be of the same runtime class as the argument.

Here is a variant which does neither copies nor modifies the original list. Instead, it wraps the original list by another object.
public List<Integer> add2(final List<Integer> lst) {
return new AbstractList<Integer>() {
public int size() {
return lst.size();
}
public Integer get(int index) {
return 2 + lst.get(index);
}
};
}
The returned list is not modifiable, but will change whenever the original list changes.
(This implements the iterator based on index access, thus it will be slow for a linked list. Then better implement it based on AbstractSequentialList.)
Of course, the resulting list will obviously not be of the same class as the original list.
Use this solution only if you really only need a read-only two added view of your original list, not if you want a modified copy with similar properties.

The whole point of using an interface, in this case List, is to abstract the fact that the implementation is hidden behind the interface.
Your intention is clear to me, however: the Clonable interface supports creating a new instance with the same state. This interface might not be defined on your List.
Often it's a good idea to rethink this situation: why do you need to clone the List in this place, this class? Shouldn't your list-creator be responsible for cloning the list? Or shouldn't the caller, who knows the type, make sure he passes in a clone of his list?
Probably, if you look for the semantics as you defined it, you can implement all your supported Lists:
static Vector<Integer> addTwo(Vector<Integer> vector) {
Vector<Integer> copy = null; // TODO: copy the vector
return addTwo_mutable(copy);
}
static ArrayList<Integer> addTwo(ArrayList<Integer> aList) {
ArrayList<Integer> copy = null; // TODO: copy the array list
return addTwo_mutable(copy);
}
static LinkedList<Integer> addTwo(LinkedList<Integer> lList) {
LinkedList<Integer> copy = null; // TODO: copy the linked list
return addTwo_mutable(copy);
}
private <T extends List<Integer>> static T addTwo_mutable(T list) {
return list; // TODO: implement
}
Even, when you don't support a data-type, you'll get a nice compiler error that the specified method does not exists.
(code not tested)

Just to show you that what you want to do is not possible in the general case, consider the following class:
final class MyList extends ArrayList<Integer> {
private MyList() {
super.add(1);
super.add(2);
super.add(3);
}
private static class SingletonHolder {
private static final MyList instance = new MyList();
}
public static MyList getInstance() {
return SingletonHolder.instance;
}
}
It is a singleton (also, a lazy, thread-safe singleton by the way), it's only instance can be obtained from MyList.getInstance(). You cannot use reflection reliably (because the constructor is private; for you to use reflection, you'd have to rely on proprietary, non-standard, non-portable APIs, or on code that could break due to a SecurityManager). So, there's no way for you to return a new instance of this list, with different values.
It's final as well, so that you cannot return a child of it.
Also, it would be possible to override every method of ArrayList that would modify the list, so that it would be really an immutable singleton.
Now, why would you want to return the exact same implementation of List?

OK well someone mentioned reflection. It seems to be an elegant solution:
import java.util.*;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) throws Exception {
List<Integer> result = lst.getClass().newInstance();
for (Integer i : lst) result.add(i + 2);
return result;
}
}
Concise, but it thows an checked exception, which is not nice.
Also, wouldn't it be nicer if we could use the method on concrete types as well, e.g. if a is an ArrayList with values 1, 2, 3, we could call add2(a) and get an ArrayList back? So in an improved version, we could make the signature generic:
static <T extends List<Integer>> T add2 (T lst) {
T res;
try {
res = (T) lst.getClass().newInstance();
} catch (InstantiationException e) {
throw new IllegalArgumentException(e);
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
for (Integer i : lst) res.add(i + 2);
return res;
}
I think throwing a runtime exception is the least worst option if a list without a nullary construcor is passed in. I don't see a way to ensure that it does. (Java 8 type annotations to the rescue maybe?) Returning null would be kind of useless.
The downside of using this signature is that we can't return an ArrayList etc as the default, as we could have done as an alternative to throwing an exception, since the return type is guaranteed to be the same type as that passed in. However, if the user actually wants an ArrayList (or some other default type) back, he can make an ArrayList copy and use the method on that.
If anyone with API design experience reads this, I would be interested to know your thoughts on which is the preferable option: 1) returning a List that needs to be explicity cast back into the original type, but enabling a return of a different concrete type, or 2) ensuring the return type is the same (using generics), but risking exceptions if (for example) a singleton object without a nullary constructor is passed in?