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How to print elements from generic array list, using iterator in Java

I'm working in a school project, where I want to implement the Iterator design pattern. I want to use generic arrays.
Container.java
public interface Container {
Iterator getIterator();
}
Iterator.java
public interface Iterator <T> {
boolean hasNext();
T next();
}
TransactionRepository.java
public class TransactionRepository<T> implements Container {
public TransactionRepository(){
userTransactions = new ArrayList<>();
}
public List<T> userTransactions;
#Override
public Iterator <T> getIterator() {
return new UserTransactions();
}
private T t;
public void add(T t) {
this.t = t;
}
public T get() {
return t;
}
private class UserTransactions implements Iterator <T> {
int index;
#Override
public boolean hasNext() {
return index < userTransactions.size();
}
#Override
public T next() {
if(this.hasNext())
return userTransactions.get(index);
return null;
}
}
}
In my other class, I add the elements to the list by first creating the TransactionRepository object like this: TransactionRepository<String> companyName = new TransactionRepository<String>();.
Then I add elements to the array with the add method companyName.add("CompanyName");. After that I want to print the array using Iterator, but It just won't print the elements. I have tried multiple variations, but none of them worked.
Iterator <String> stringIterator = companyName.getIterator();
while (stringIterator.hasNext()) {
System.out.println("Name : " + companyName.get());
}

With the current implementation List<T> userTransactions is never updated.
In this case userTransactions.size() in hasNext() method will always return 0 so the result of method will be false.
Moreover, you should use stringIterator.next() instead of companyName.get(). Since you implement your own iterator you don't want to use get() method at all.
There is also a need to update index counter variable after calling next() method.
#Override
public T next() {
if (this.hasNext())
return userTransactions.get(index++);
return null;
}
Change modifier on userTransactions to private final as it should be referenced just with iterator.
Code with proposed improvements:
public class TransactionRepository<T> implements Container {
public TransactionRepository() {
userTransactions = new ArrayList<>();
}
public List<T> userTransactions;
#Override
public Iterator<T> getIterator() {
return new UserTransactions();
}
public void add(T t) {
userTransactions.add(t);
}
private class UserTransactions implements Iterator<T> {
int index;
#Override
public boolean hasNext() {
return index < userTransactions.size();
}
#Override
public T next() {
if (this.hasNext()) {
return userTransactions.get(index++);
}
return null;
}
}
}

It seems that you are never adding elements to your userTransactions List on the add method

You add() method doesnt add anything to your list , it's just like a setter of the attribute t , you should use it to add elements to the list instead
public void add(T t) {
userTransactions.add(t);
}
There is also another problem , the index , your next() method gets the index element while you didnt initialise your index variable , i recommand you to do it in this way :
int index = 0 ;
...
public T next() {
if(this.hasNext())
int temp = index;
index++;
return userTransactions.get(temp);
return null;
}

Right way to make my own collection in java

I'm trying to do my own collection class in java.
I need to find data by a key and also be able to iterate on it and get a element by his index, so i decide to make an encapsulation of the hashtable and the arraylist.
This is my code:
public class GeoCollection <T extends Geographic> implements Iterable<T>, Iterator<T>{
private Hashtable<String,T> data_table;
private ArrayList<T> data;
private int cursor = 0;
public GeoCollection(){
data = new ArrayList<>();
data_table = new Hashtable<>();
}
public void add(String key,T data){
this.data.add(data);
data_table.put(key,data);
}
public T get(int index){
if(index >= data.size())
throw new IndexOutOfBoundsException();
return data.get(index);
}
public T get(String v){
return data_table.get(v);
}
public T next() {
if( cursor == data.size())
throw new NoSuchElementException();
cursor++;
return data.get(cursor-1);
}
public T first(){
cursor = 0;
return data.get(cursor);
}
public boolean hasNext(){
return cursor < data.size();
}
public boolean remove(Person p) {
return data.remove(p);
}
//se implemeta el iterator
#Override
public Iterator<T> iterator() {
cursor = 0;
return this;
}
}
There's any need to implement list interface or something like that? Because i don't know if encapsulating the hashtable and the arraylist and implementing the basic operations in enough to call "Collection" this class.
I will be very thankful for any advice or correction of this code.
Thanks.

i think the best way to do this is implementing Collection
implements java.util.Collection<E>
that way you have to implement every method in the collection interface bt
making your class extending AbstractCollection extends AbstractCollection is much more easy since it does
all the nessessary things for us and only thing you have to worry about is
iterator() and size()

Writing an iterator for my own array list class in Java

I am writing an iterator method for my own array list class, however, when I try to test the class it says foreach loop is not applicable to MyArrayList. can anyone help me out with where I have gone wrong?
The class itself uses arrays of objects and the necessary methods to act like an arraylist (add, remove, get etc.)
Here is the class constructor and my iterator class:
public class MyArrayList {
public Object[] arrayList = new Object[5];
int length1 = 5;
public MyArrayList() {
}
public MyArrayList(Object[] arrayList) {
this.arrayList = arrayList;
length1 = arrayList.length;
}
public ArrayListIterator iterator(){
return new ArrayListIterator(this);
}
class ArrayListIterator<MyArrayList> implements Iterator<Object> {
private Object[] arrayListIterable;
private int count = 0;
public ArrayListIterator(Object[] x){
arrayListIterable = x;
}
public ArrayListIterator(MyArrayList myArrayList) {
}
public boolean hasNext(){
if(count < arrayList.length){
return true;
}else{
return false;
}
}
public Object next(){
int x = count;
count++;
return arrayListIterable[x];
}
}

Your MyArrayList class must implement Iterable<T> interface.
Check the javadoc for Iterable:
Implementing this interface allows an object to be the target of the "for-each loop" statement.

Is it possible to merge iterators in Java?

Is it possible to merge iterators in Java? I have two iterators and I want to combine/merge them so that I could iterate though their elements in one go (in same loop) rather than two steps. Is that possible?
Note that the number of elements in the two lists can be different therefore one loop over both lists is not the solution.
Iterator<User> pUsers = userService.getPrimaryUsersInGroup(group.getId());
Iterator<User> sUsers = userService.getSecondaryUsersInGroup(group.getId());
while(pUsers.hasNext()) {
User user = pUsers.next();
.....
}
while(sUsers.hasNext()) {
User user = sUsers.next();
.....
}

Guava (formerly Google Collections) has Iterators.concat.

Also the Apache Commons Collection have several classes for manipulating Iterators, like the IteratorChain, that wraps a number of Iterators.

You could create your own implementation of the Iterator interface which iterates over the iterators:
public class IteratorOfIterators implements Iterator {
private final List<Iterator> iterators;
public IteratorOfIterators(List<Iterator> iterators) {
this.iterators = iterators;
}
public IteratorOfIterators(Iterator... iterators) {
this.iterators = Arrays.asList(iterators);
}
public boolean hasNext() { /* implementation */ }
public Object next() { /* implementation */ }
public void remove() { /* implementation */ }
}
(I've not added generics to the Iterator for brevity.) The implementation is not too hard, but isn't the most trivial, you need to keep track of which Iterator you are currently iterating over, and calling next() you'll need to iterate as far as you can through the iterators until you find a hasNext() that returns true, or you may hit the end of the last iterator.
I'm not aware of any implementation that already exists for this.
Update:
I've up-voted Andrew Duffy's answer - no need to re-invent the wheel. I really need to look into Guava in more depth.
I've added another constructor for a variable number of arguments - almost getting off topic, as how the class is constructed here isn't really of interest, just the concept of how it works.

I haven't written Java code in a while, and this got me curious to whether I've still "got it".
First try:
import java.util.Iterator;
import java.util.Arrays; /* For sample code */
public class IteratorIterator<T> implements Iterator<T> {
private final Iterator<T> is[];
private int current;
public IteratorIterator(Iterator<T>... iterators)
{
is = iterators;
current = 0;
}
public boolean hasNext() {
while ( current < is.length && !is[current].hasNext() )
current++;
return current < is.length;
}
public T next() {
while ( current < is.length && !is[current].hasNext() )
current++;
return is[current].next();
}
public void remove() { /* not implemented */ }
/* Sample use */
public static void main(String... args)
{
Iterator<Integer> a = Arrays.asList(1,2,3,4).iterator();
Iterator<Integer> b = Arrays.asList(10,11,12).iterator();
Iterator<Integer> c = Arrays.asList(99, 98, 97).iterator();
Iterator<Integer> ii = new IteratorIterator<Integer>(a,b,c);
while ( ii.hasNext() )
System.out.println(ii.next());
}
}
You could of course use more Collection classes rather than a pure array + index counter, but this actually feels a bit cleaner than the alternative. Or am I just biased from writing mostly C these days?
Anyway, there you go. The answer to you question is "yes, probably".

public class IteratorJoin<T> implements Iterator<T> {
private final Iterator<T> first, next;
public IteratorJoin(Iterator<T> first, Iterator<T> next) {
this.first = first;
this.next = next;
}
#Override
public boolean hasNext() {
return first.hasNext() || next.hasNext();
}
#Override
public T next() {
if (first.hasNext())
return first.next();
return next.next();
}
}

Starting with Java 8 and later this can be done without external dependencies using Stream API. This also allows concatenation of iterator with other types of streams.
Streams.concat(StreamSupport.stream(<iter1>, false), StreamSupport.stream(<iter2>, false));

move your loop to a method and pass the iterator to method.
void methodX(Iterator x) {
while (x.hasNext()) {
....
}
}

an iterator comes FROM a collection or a set.
why not use the method already available
Collection.addAll(Collection c);
and then create your iterator from the last object.
this way, your iterator will iterate all the contents of both collection.

You can use my version of an extendable iterator. It uses a double-ended queue of iterators which to me makes sense:
import java.util.Deque;
import java.util.Iterator;
import java.util.concurrent.ConcurrentLinkedDeque;
public class ExtendableIterator<T> implements Iterator<T> {
public Deque<Iterator<T>> its = new ConcurrentLinkedDeque<Iterator<T>>();
public ExtendableIterator() {
}
public ExtendableIterator(Iterator<T> it) {
this();
this.extend(it);
}
#Override
public boolean hasNext() {
// this is true since we never hold empty iterators
return !its.isEmpty() && its.peekLast().hasNext();
}
#Override
public T next() {
T next = its.peekFirst().next();
if (!its.peekFirst().hasNext()) {
its.removeFirst();
}
return next;
}
public void extend(Iterator<T> it) {
if (it.hasNext()) {
its.addLast(it);
}
}
}

The Merged Iterator:
import static java.util.Arrays.asList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.NoSuchElementException;
public class ConcatIterator<T> implements Iterator<T> {
private final List<Iterable<T>> iterables;
private Iterator<T> current;
#SafeVarargs
public ConcatIterator(final Iterable<T>... iterables) {
this.iterables = new LinkedList<>(asList(iterables));
}
#Override
public boolean hasNext() {
checkNext();
return current != null && current.hasNext();
}
#Override
public T next() {
checkNext();
if (current == null || !current.hasNext()) throw new NoSuchElementException();
return current.next();
}
#Override
public void remove() {
if (current == null) throw new IllegalStateException();
current.remove();
}
private void checkNext() {
while ((current == null || !current.hasNext()) && !iterables.isEmpty()) {
current = iterables.remove(0).iterator();
}
}
}
The concat method to create an Iterable:
#SafeVarargs
public static <T> Iterable<T> concat(final Iterable<T>... iterables) {
return () -> new ConcatIterator<>(iterables);
}
Simple JUnit test:
#Test
public void testConcat() throws Exception {
final Iterable<Integer> it1 = asList(1, 2, 3);
final Iterable<Integer> it2 = asList(4, 5);
int j = 1;
for (final int i : concat(it1, it2)) {
assertEquals(j, i);
j++;
}
}

I would refactor the original design from:
Iterator<User> pUsers = userService.getPrimaryUsersInGroup(group.getId());
Iterator<User> sUsers = userService.getSecondaryUsersInGroup(group.getId());
To something like:
Iterator<User> users = userService.getUsersInGroup(group.getId(), User.PRIMARY, User.SECONDARY, ...);

You can try ConcatIterator from Cactoos:
Iterator<String> names = new ConcatIterator<>(
Arrays.asList("Sarah", "Mary").iterator(),
Arrays.asList("Jeff", "Johnny").iterator(),
);
Also check ConcatIterable, which concatenates Iterables.

In the Apache Commons Collections there is public static <E> Iterator<E> org.apache.commons.collections4.IteratorUtils.chainedIterator(Collection<Iterator<? extends E>> iterators) that says
Gets an iterator that iterates through a collections of Iterators one after another.
which should be what you want.
import java.util.Arrays;
import java.util.Iterator;
import org.apache.commons.collections4.IteratorUtils;
//also works: import org.apache.commons.collections.IteratorUtils;
class Scratch {
public static void main( String[] args ) {
final Iterator<String> combinedIterator = IteratorUtils.chainedIterator(
Arrays.asList( "a", "b", "c" ).iterator(),
Arrays.asList( "1", "2", "3" ).iterator()
);
while( combinedIterator.hasNext() ){
System.out.println( combinedIterator.next() );
}
// "abc123" will have been printed out
}
}

every Iterator object holds own memory location (adress), so you can't simply "merge" them. except if you extend iterator class and write your own implementation there.
If you are dealing with the same number of objects in both iterators an alternative solution would be to process two iterators in one loop like this :
while (iterator1.hasNext() && iterator2.hasNext()) {
// code
}

Is there a fixed sized queue which removes excessive elements?

I need a queue with a fixed size. When I add an element and the queue is full, it should automatically remove the oldest element.
Is there an existing implementation for this in Java?

Actually the LinkedHashMap does exactly what you want. You need to override the removeEldestEntry method.
Example for a queue with max 10 elements:
queue = new LinkedHashMap<Integer, String>()
{
#Override
protected boolean removeEldestEntry(Map.Entry<Integer, String> eldest)
{
return this.size() > 10;
}
};
If the "removeEldestEntry" returns true, the eldest entry is removed from the map.

Yes, Two
From my own duplicate question with this correct answer, I learned of two:
EvictingQueue in Google Guava
CircularFifoQueue in Apache Commons
I made productive use of the Guava EvictingQueue, worked well.
To instantiate an EvictingQueue call the static factory method create and specify your maximum size.
EvictingQueue< Person > people = com.google.common.collect.EvictingQueue.create( 100 ) ; // Set maximum size to 100.

I just implemented a fixed size queue this way:
public class LimitedSizeQueue<K> extends ArrayList<K> {
private int maxSize;
public LimitedSizeQueue(int size){
this.maxSize = size;
}
public boolean add(K k){
boolean r = super.add(k);
if (size() > maxSize){
removeRange(0, size() - maxSize);
}
return r;
}
public K getYoungest() {
return get(size() - 1);
}
public K getOldest() {
return get(0);
}
}

There is no existing implementation in the Java Language and Runtime. All Queues extend AbstractQueue, and its doc clearly states that adding an element to a full queue always ends with an exception. It would be best ( and quite simple ) to wrap a Queue into a class of your own for having the functionality you need.
Once again, because all queues are children of AbstractQueue, simply use that as your internal data type and you should have a flexible implementation running in virtually no time :-)
UPDATE:
As outlined below, there are two open implementations available (this answer is quite old, folks!), see this answer for details.

This is what I did with Queue wrapped with LinkedList, It is fixed sized which I give in here is 2;
public static Queue<String> pageQueue;
pageQueue = new LinkedList<String>(){
private static final long serialVersionUID = -6707803882461262867L;
public boolean add(String object) {
boolean result;
if(this.size() < 2)
result = super.add(object);
else
{
super.removeFirst();
result = super.add(object);
}
return result;
}
};
....
TMarket.pageQueue.add("ScreenOne");
....
TMarket.pageQueue.add("ScreenTwo");
.....

public class CircularQueue<E> extends LinkedList<E> {
private final int capacity;
public CircularQueue(int capacity){
this.capacity = capacity;
}
#Override
public boolean add(E e) {
if(size() >= capacity)
removeFirst();
return super.add(e);
}
}
Usage and test result:
public static void main(String[] args) {
CircularQueue<String> queue = new CircularQueue<>(3);
queue.add("a");
queue.add("b");
queue.add("c");
System.out.println(queue.toString()); //[a, b, c]
String first = queue.pollFirst(); //a
System.out.println(queue.toString()); //[b,c]
queue.add("d");
queue.add("e");
queue.add("f");
System.out.println(queue.toString()); //[d, e, f]
}

I think what you're describing is a circular queue. Here is an example and here is a better one

This class does the job using composition instead of inheritance (other answers here) which removes the possibility of certain side-effects (as covered by Josh Bloch in Essential Java). Trimming of the underlying LinkedList occurs on the methods add,addAll and offer.
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Queue;
public class LimitedQueue<T> implements Queue<T>, Iterable<T> {
private final int limit;
private final LinkedList<T> list = new LinkedList<T>();
public LimitedQueue(int limit) {
this.limit = limit;
}
private boolean trim() {
boolean changed = list.size() > limit;
while (list.size() > limit) {
list.remove();
}
return changed;
}
#Override
public boolean add(T o) {
boolean changed = list.add(o);
boolean trimmed = trim();
return changed || trimmed;
}
#Override
public int size() {
return list.size();
}
#Override
public boolean isEmpty() {
return list.isEmpty();
}
#Override
public boolean contains(Object o) {
return list.contains(o);
}
#Override
public Iterator<T> iterator() {
return list.iterator();
}
#Override
public Object[] toArray() {
return list.toArray();
}
#Override
public <T> T[] toArray(T[] a) {
return list.toArray(a);
}
#Override
public boolean remove(Object o) {
return list.remove(o);
}
#Override
public boolean containsAll(Collection<?> c) {
return list.containsAll(c);
}
#Override
public boolean addAll(Collection<? extends T> c) {
boolean changed = list.addAll(c);
boolean trimmed = trim();
return changed || trimmed;
}
#Override
public boolean removeAll(Collection<?> c) {
return list.removeAll(c);
}
#Override
public boolean retainAll(Collection<?> c) {
return list.retainAll(c);
}
#Override
public void clear() {
list.clear();
}
#Override
public boolean offer(T e) {
boolean changed = list.offer(e);
boolean trimmed = trim();
return changed || trimmed;
}
#Override
public T remove() {
return list.remove();
}
#Override
public T poll() {
return list.poll();
}
#Override
public T element() {
return list.element();
}
#Override
public T peek() {
return list.peek();
}
}

Sounds like an ordinary List where the add method contains an extra snippet which truncates the list if it gets too long.
If that is too simple, then you probably need to edit your problem description.

Also see this SO question, or ArrayBlockingQueue (be careful about blocking, this might be unwanted in your case).

It is not quite clear what requirements you have that led you to ask this question. If you need a fixed size data structure, you might also want to look at different caching policies. However, since you have a queue, my best guess is that you're looking for some type of router functionality. In that case, I would go with a ring buffer: an array that has a first and last index. Whenever an element is added, you just increment the last element index, and when an element is removed, increment the first element index. In both cases, addition is performed modulo the array size, and make sure to increment the other index when needed, that is, when the queue is full or empty.
Also, if it is a router-type application, you might also want to experiment with an algorithm such as Random Early Dropping (RED), which drops elements from the queue randomly even before it gets filled up. In some cases, RED has been found to have better overall performance than the simple method of allowing the queue to fill up before dropping.

Actually you can write your own impl based on LinkedList, it is quite straight forward, just override the add method and do the staff.

I think the best matching answer is from this other question.
Apache commons collections 4 has a CircularFifoQueue which is what you are looking for. Quoting the javadoc:
CircularFifoQueue is a first-in first-out queue with a fixed size that replaces its oldest element if full.

A Simple solution, below is a Queue of "String"
LinkedHashMap<Integer, String> queue;
int queueKeysCounter;
queue.put(queueKeysCounter++, "My String");
queueKeysCounter %= QUEUE_SIZE;
Note that this will not maintain the Order of the items in the Queue, but it will replace the oldest entry.

As it's advised in OOPs that we should prefer Composition over Inheritance
Here my solution keeping that in mind.
package com.choiceview;
import java.util.ArrayDeque;
class Ideone {
public static void main(String[] args) {
LimitedArrayDeque<Integer> q = new LimitedArrayDeque<>(3);
q.add(1);
q.add(2);
q.add(3);
System.out.println(q);
q.add(4);
// First entry ie 1 got pushed out
System.out.println(q);
}
}
class LimitedArrayDeque<T> {
private int maxSize;
private ArrayDeque<T> queue;
private LimitedArrayDeque() {
}
public LimitedArrayDeque(int maxSize) {
this.maxSize = maxSize;
queue = new ArrayDeque<T>(maxSize);
}
public void add(T t) {
if (queue.size() == maxSize) {
queue.removeFirst();
}
queue.add(t);
}
public boolean remove(T t) {
return queue.remove(t);
}
public boolean contains(T t) {
return queue.contains(t);
}
#Override
public String toString() {
return queue.toString();
}
}

Ok, I'll throw out my version too. :-) This is build to be very performant - for when that matters. It's not based on LinkedList - and is thread safe (should be at least). FIFO
static class FixedSizeCircularReference<T> {
T[] entries
FixedSizeCircularReference(int size) {
this.entries = new Object[size] as T[]
this.size = size
}
int cur = 0
int size
synchronized void add(T entry) {
entries[cur++] = entry
if (cur >= size) {
cur = 0
}
}
List<T> asList() {
int c = cur
int s = size
T[] e = entries.collect() as T[]
List<T> list = new ArrayList<>()
int oldest = (c == s - 1) ? 0 : c
for (int i = 0; i < e.length; i++) {
def entry = e[oldest + i < s ? oldest + i : oldest + i - s]
if (entry) list.add(entry)
}
return list
}
}