I tried many ways to copy an object without referencing it, and it's seems to me all of them shallow copy not deep.
Let's say I have sent an Object of type java List<List<Integer>> using parameters from main to a method.
void reverseMatrix(List<List<Integer> > grid)
{
}
so I defined List> tempGrid= new ArrayList<>(); inside this method above, and I want to copy all the values(by value not by reference.)
I tried Iterator as below :
Iterator<List<Integer>> iterator = grid.iterator();
while(iterator.hasNext()){
tempGrid.add(iterator.next());
}
after applying the code above it, still, change the parent object(grid) not tempGrid.
If I applied this code java tempGrid.get(0).set(0,1);
it will change grid too.
I saw some subject about iterate by values.
List<Integer> temp1 = new ArrayList<>();
for(List<Integer> list: grid){
for(Integer numb: list) {
temp1.add(numb);
}
tempGrid.add(temp1);
}
I tried to use Collections.copy, but give me error, that des size not fit to source.
I expect to get a new object not get effected by operations on the old object.
Integer objects are immutable so you don't need to deep copy them.
Both the outer and inner lists are mutable, so they need to be copied. Assuming we don't need to retain the list type, e.g. ArrayList vs LinkedList, we can simply use the ArrayList(Collection) constructor to copy the inner lists.
Using code that doesn't require Java 8:
static void deepCopy(List<List<Integer>> input) {
List<List<Integer>> output = new ArrayList<>(input.size());
for (List<Integer> inner : input)
output.add(new ArrayList<>(inner));
return output;
}
I have a method that takes vararg Array of strings
void count(long delta, String... tags);
I have a predefined array of tags for the most cases
String[] tags = { "foo_tag:Foo",
"bar_tag:Bar",
"baz_tag:Baz"
};
and only one tag to be added to predefined tags in each call "project_id:12345"
So the call of count should look like this:
count(delta, "foo_tag:Foo", "bar_tag:Bar", "baz_tag:Baz", "project_id:12345");
How can I simply create a new array containing my existing one plus additional element just in place of calling the method?
Something like this hypothetical Arrays.append method:
count(delta, Arrays.append(tags, "project_id:12345"));
This is storing statistics operation, not a business logic, so I want this operation to be as fast as possible.
Currently, I have helper method appendTag, but it doesn't look elegant as for me
private String[] appendTag(String[] tags, String s)
{
String[] result = new String[tags.length + 1];
System.arraycopy(tags, 0, result, 0, tags.length);
result[result.length-1] = s;
return result;
}
In java, arrays have a fixed size so it won't be possible to extend an array by appending new elements to it.
You will need to create a new array with a larger size and copy the first one elements into it, then add new elements to it, but it's not dynamic yet.
What I can suggest is to use a Collection maybe an ArrayList you will profit from its built-in methods like .add()
There is no easy way to expand an array by one element and add something new. But if you were working with a list instead, you could easily add a new element and then convert it to an array when calling the method:
String[] tags = { "foo_tag:Foo",
"bar_tag:Bar",
"baz_tag:Baz"
};
List<String> tagList = new ArrayList<String>(Arrays.asList(tags));
tagList.add("project_id:12345");
count(delta, tagList.toArray(new String[0]));
If you think you will have a long term need for this, then perhaps consider changing the implementation of count() to use a list instead of an array. You could also overload this method and expose a version which accepts list instead of array.
Why this question is not a possible duplication of How Arrays.asList(int[]) can return List<int[]>?.
That question doesn't really answer my particular situation as I am trying to figure out if there is a discrepancy in my use of Arrays.copyOf.
CASE 1: Supposed deep copy of the array
// Creating a integer array, populating its values
int[] src = new int[2];
src[0] = 2;
src[1] = 3;
// Create a copy of the array
int [] dst= Arrays.copyOf(src,src.length);
Assert.assertArrayEquals(src, dst);
// Now change one element in the original
dst[0] = 4;
// Following line throws an exception, (which is expected) if the copy is a deep one
Assert.assertArrayEquals(src, dst);
CASE 2:
Here is where things seem to be weird:
What I am trying to do with the below method (lifted verbatim from a book) is to create an immutable list view of a copy of the input array arguments. That way, if the input array changes, the contents of the returned list don't change.
#SafeVarargs
public static <T> List<T> list(T... t) {
return Collections.unmodifiableList(new ArrayList<>(Arrays.asList(Arrays.copyOf(t, t.length))));
}
int[] arr2 = new int[2];
arr2[0] = 2;
arr2[1] = 3;
// Create an unmodifiable list
List<int[]> list2 = list(arr2);
list2.stream().forEach(s -> System.out.println(Arrays.toString(s)));
// Prints [2, 3] as expected
arr2[0] = 3;
list2.stream().forEach(s -> System.out.println(Arrays.toString(s)));
// Prints [3, 3] which doesn't make sense to me... I would have thought it would print [2, 3] and not be affected by my changing the value of the element.
The contradiction that I see is that in one case (Case 1), Arrays.copyOf seems to be a deep copy, whereas in the other case (Case 2), it seems like a shallow one. The changes to the original array seem to have written through to the list, even though I have copied the array in creating my unmodifiable list.
Would someone be able to help me resolve this discrepancy?
First of all, your list method performs an unnecessary step, you don't need the copyOf operation, so here goes:
#SafeVarargs
public static <T> List<T> list(T... t) {
return Collections.unmodifiableList(
new ArrayList<>(Arrays.asList(t))
);
}
The ArrayList constructor already copies the incoming list, so you're safe there.
Next, when you are calling your list() method with an int[], that array is considered to be a single element of type int[], because the type erasure of your T... is Object..., and int is primitive. There is no way you can make your method do a deep copy inside the list without either changing the parameter types or doing an instanceOf check and performing the copy manually inside the method. I'd say the wisest thing to do is probably to move the Arrays.copyOf() call outside the method:
List<int[]> list2 = list(Arrays.copyOf(arr2));
Is it possible to add elements to a collection while iterating over it?
More specifically, I would like to iterate over a collection, and if an element satisfies a certain condition I want to add some other elements to the collection, and make sure that these added elements are iterated over as well. (I realise that this could lead to an unterminating loop, but I'm pretty sure it won't in my case.)
The Java Tutorial from Sun suggests this is not possible: "Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress."
So if I can't do what I want to do using iterators, what do you suggest I do?
How about building a Queue with the elements you want to iterate over; when you want to add elements, enqueue them at the end of the queue, and keep removing elements until the queue is empty. This is how a breadth-first search usually works.
There are two issues here:
The first issue is, adding to an Collection after an Iterator is returned. As mentioned, there is no defined behavior when the underlying Collection is modified, as noted in the documentation for Iterator.remove:
... The behavior of an iterator is
unspecified if the underlying
collection is modified while the
iteration is in progress in any way
other than by calling this method.
The second issue is, even if an Iterator could be obtained, and then return to the same element the Iterator was at, there is no guarantee about the order of the iteratation, as noted in the Collection.iterator method documentation:
... There are no guarantees concerning the
order in which the elements are
returned (unless this collection is an
instance of some class that provides a
guarantee).
For example, let's say we have the list [1, 2, 3, 4].
Let's say 5 was added when the Iterator was at 3, and somehow, we get an Iterator that can resume the iteration from 4. However, there is no guarentee that 5 will come after 4. The iteration order may be [5, 1, 2, 3, 4] -- then the iterator will still miss the element 5.
As there is no guarantee to the behavior, one cannot assume that things will happen in a certain way.
One alternative could be to have a separate Collection to which the newly created elements can be added to, and then iterating over those elements:
Collection<String> list = Arrays.asList(new String[]{"Hello", "World!"});
Collection<String> additionalList = new ArrayList<String>();
for (String s : list) {
// Found a need to add a new element to iterate over,
// so add it to another list that will be iterated later:
additionalList.add(s);
}
for (String s : additionalList) {
// Iterate over the elements that needs to be iterated over:
System.out.println(s);
}
Edit
Elaborating on Avi's answer, it is possible to queue up the elements that we want to iterate over into a queue, and remove the elements while the queue has elements. This will allow the "iteration" over the new elements in addition to the original elements.
Let's look at how it would work.
Conceptually, if we have the following elements in the queue:
[1, 2, 3, 4]
And, when we remove 1, we decide to add 42, the queue will be as the following:
[2, 3, 4, 42]
As the queue is a FIFO (first-in, first-out) data structure, this ordering is typical. (As noted in the documentation for the Queue interface, this is not a necessity of a Queue. Take the case of PriorityQueue which orders the elements by their natural ordering, so that's not FIFO.)
The following is an example using a LinkedList (which is a Queue) in order to go through all the elements along with additional elements added during the dequeing. Similar to the example above, the element 42 is added when the element 2 is removed:
Queue<Integer> queue = new LinkedList<Integer>();
queue.add(1);
queue.add(2);
queue.add(3);
queue.add(4);
while (!queue.isEmpty()) {
Integer i = queue.remove();
if (i == 2)
queue.add(42);
System.out.println(i);
}
The result is the following:
1
2
3
4
42
As hoped, the element 42 which was added when we hit 2 appeared.
You may also want to look at some of the more specialised types, like ListIterator, NavigableSet and (if you're interested in maps) NavigableMap.
Actually it is rather easy. Just think for the optimal way.
I beleive the optimal way is:
for (int i=0; i<list.size(); i++) {
Level obj = list.get(i);
//Here execute yr code that may add / or may not add new element(s)
//...
i=list.indexOf(obj);
}
The following example works perfectly in the most logical case - when you dont need to iterate the added new elements before the iteration element. About the added elements after the iteration element - there you might want not to iterate them either. In this case you should simply add/or extend yr object with a flag that will mark them not to iterate them.
Use ListIterator as follows:
List<String> l = new ArrayList<>();
l.add("Foo");
ListIterator<String> iter = l.listIterator(l.size());
while(iter.hasPrevious()){
String prev=iter.previous();
if(true /*You condition here*/){
iter.add("Bah");
iter.add("Etc");
}
}
The key is to iterate in reverse order - then the added elements appear on the next iteration.
I know its been quite old. But thought of its of any use to anyone else. Recently I came across this similar problem where I need a queue that is modifiable during iteration. I used listIterator to implement the same much in the same lines as of what Avi suggested -> Avi's Answer. See if this would suit for your need.
ModifyWhileIterateQueue.java
import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;
public class ModifyWhileIterateQueue<T> {
ListIterator<T> listIterator;
int frontIndex;
List<T> list;
public ModifyWhileIterateQueue() {
frontIndex = 0;
list = new ArrayList<T>();
listIterator = list.listIterator();
}
public boolean hasUnservicedItems () {
return frontIndex < list.size();
}
public T deQueue() {
if (frontIndex >= list.size()) {
return null;
}
return list.get(frontIndex++);
}
public void enQueue(T t) {
listIterator.add(t);
}
public List<T> getUnservicedItems() {
return list.subList(frontIndex, list.size());
}
public List<T> getAllItems() {
return list;
}
}
ModifyWhileIterateQueueTest.java
#Test
public final void testModifyWhileIterate() {
ModifyWhileIterateQueue<String> queue = new ModifyWhileIterateQueue<String>();
queue.enQueue("one");
queue.enQueue("two");
queue.enQueue("three");
for (int i=0; i< queue.getAllItems().size(); i++) {
if (i==1) {
queue.enQueue("four");
}
}
assertEquals(true, queue.hasUnservicedItems());
assertEquals ("[one, two, three, four]", ""+ queue.getUnservicedItems());
assertEquals ("[one, two, three, four]", ""+queue.getAllItems());
assertEquals("one", queue.deQueue());
}
Using iterators...no, I don't think so. You'll have to hack together something like this:
Collection< String > collection = new ArrayList< String >( Arrays.asList( "foo", "bar", "baz" ) );
int i = 0;
while ( i < collection.size() ) {
String curItem = collection.toArray( new String[ collection.size() ] )[ i ];
if ( curItem.equals( "foo" ) ) {
collection.add( "added-item-1" );
}
if ( curItem.equals( "added-item-1" ) ) {
collection.add( "added-item-2" );
}
i++;
}
System.out.println( collection );
Which yeilds:
[foo, bar, baz, added-item-1, added-item-2]
Besides the solution of using an additional list and calling addAll to insert the new items after the iteration (as e.g. the solution by user Nat), you can also use concurrent collections like the CopyOnWriteArrayList.
The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException.
With this special collection (usually used for concurrent access) it is possible to manipulate the underlying list while iterating over it. However, the iterator will not reflect the changes.
Is this better than the other solution? Probably not, I don't know the overhead introduced by the Copy-On-Write approach.
public static void main(String[] args)
{
// This array list simulates source of your candidates for processing
ArrayList<String> source = new ArrayList<String>();
// This is the list where you actually keep all unprocessed candidates
LinkedList<String> list = new LinkedList<String>();
// Here we add few elements into our simulated source of candidates
// just to have something to work with
source.add("first element");
source.add("second element");
source.add("third element");
source.add("fourth element");
source.add("The Fifth Element"); // aka Milla Jovovich
// Add first candidate for processing into our main list
list.addLast(source.get(0));
// This is just here so we don't have to have helper index variable
// to go through source elements
source.remove(0);
// We will do this until there are no more candidates for processing
while(!list.isEmpty())
{
// This is how we get next element for processing from our list
// of candidates. Here our candidate is String, in your case it
// will be whatever you work with.
String element = list.pollFirst();
// This is where we process the element, just print it out in this case
System.out.println(element);
// This is simulation of process of adding new candidates for processing
// into our list during this iteration.
if(source.size() > 0) // When simulated source of candidates dries out, we stop
{
// Here you will somehow get your new candidate for processing
// In this case we just get it from our simulation source of candidates.
String newCandidate = source.get(0);
// This is the way to add new elements to your list of candidates for processing
list.addLast(newCandidate);
// In this example we add one candidate per while loop iteration and
// zero candidates when source list dries out. In real life you may happen
// to add more than one candidate here:
// list.addLast(newCandidate2);
// list.addLast(newCandidate3);
// etc.
// This is here so we don't have to use helper index variable for iteration
// through source.
source.remove(0);
}
}
}
For examle we have two lists:
public static void main(String[] args) {
ArrayList a = new ArrayList(Arrays.asList(new String[]{"a1", "a2", "a3","a4", "a5"}));
ArrayList b = new ArrayList(Arrays.asList(new String[]{"b1", "b2", "b3","b4", "b5"}));
merge(a, b);
a.stream().map( x -> x + " ").forEach(System.out::print);
}
public static void merge(List a, List b){
for (Iterator itb = b.iterator(); itb.hasNext(); ){
for (ListIterator it = a.listIterator() ; it.hasNext() ; ){
it.next();
it.add(itb.next());
}
}
}
a1 b1 a2 b2 a3 b3 a4 b4 a5 b5
I prefer to process collections functionally rather than mutate them in place. That avoids this kind of problem altogether, as well as aliasing issues and other tricky sources of bugs.
So, I would implement it like:
List<Thing> expand(List<Thing> inputs) {
List<Thing> expanded = new ArrayList<Thing>();
for (Thing thing : inputs) {
expanded.add(thing);
if (needsSomeMoreThings(thing)) {
addMoreThingsTo(expanded);
}
}
return expanded;
}
IMHO the safer way would be to create a new collection, to iterate over your given collection, adding each element in the new collection, and adding extra elements as needed in the new collection as well, finally returning the new collection.
Given a list List<Object> which you want to iterate over, the easy-peasy way is:
while (!list.isEmpty()){
Object obj = list.get(0);
// do whatever you need to
// possibly list.add(new Object obj1);
list.remove(0);
}
So, you iterate through a list, always taking the first element and then removing it. This way you can append new elements to the list while iterating.
Forget about iterators, they don't work for adding, only for removing. My answer applies to lists only, so don't punish me for not solving the problem for collections. Stick to the basics:
List<ZeObj> myList = new ArrayList<ZeObj>();
// populate the list with whatever
........
int noItems = myList.size();
for (int i = 0; i < noItems; i++) {
ZeObj currItem = myList.get(i);
// when you want to add, simply add the new item at last and
// increment the stop condition
if (currItem.asksForMore()) {
myList.add(new ZeObj());
noItems++;
}
}
I tired ListIterator but it didn't help my case, where you have to use the list while adding to it. Here's what works for me:
Use LinkedList.
LinkedList<String> l = new LinkedList<String>();
l.addLast("A");
while(!l.isEmpty()){
String str = l.removeFirst();
if(/* Condition for adding new element*/)
l.addLast("<New Element>");
else
System.out.println(str);
}
This could give an exception or run into infinite loops. However, as you have mentioned
I'm pretty sure it won't in my case
checking corner cases in such code is your responsibility.
This is what I usually do, with collections like sets:
Set<T> adds = new HashSet<T>, dels = new HashSet<T>;
for ( T e: target )
if ( <has to be removed> ) dels.add ( e );
else if ( <has to be added> ) adds.add ( <new element> )
target.removeAll ( dels );
target.addAll ( adds );
This creates some extra-memory (the pointers for intermediate sets, but no duplicated elements happen) and extra-steps (iterating again over changes), however usually that's not a big deal and it might be better than working with an initial collection copy.
Even though we cannot add items to the same list during iteration, we can use Java 8's flatMap, to add new elements to a stream. This can be done on a condition. After this the added item can be processed.
Here is a Java example which shows how to add to the ongoing stream an object depending on a condition which is then processed with a condition:
List<Integer> intList = new ArrayList<>();
intList.add(1);
intList.add(2);
intList.add(3);
intList = intList.stream().flatMap(i -> {
if (i == 2) return Stream.of(i, i * 10); // condition for adding the extra items
return Stream.of(i);
}).map(i -> i + 1)
.collect(Collectors.toList());
System.out.println(intList);
The output of the toy example is:
[2, 3, 21, 4]
In general, it's not safe, though for some collections it may be. The obvious alternative is to use some kind of for loop. But you didn't say what collection you're using, so that may or may not be possible.
I have a TreeSet in Java and I have my own comparator function for this tree set. Now I am traversing this tree set using descendingIterator() method and changing the elements. So does this update the actual tree set as well wrt to the way it is sorted with my custom comparator? Or do I need to remove the element and put back the updated element?
You need to remove the element and add it back. The position of the element in the tree is decided when the element is inserted, by comparing it with other elements. If you change the object so that the comparison to other elements changes, you must remove the element first, then change it, then re-add it.
Note that removing the element while iterating will only work using the iterator's remove method. And you won't be able to add it during the iteration without getting a ConcurrentModificationException, AFAIK. So store it in a list of elements to be re-added to the set once the iteration has ended.
If you modify any part of the object that is a part of the "key" (as defined by your custom comparator) you need to remove and re-insert the object for the tree set to "learn" about the change. You should not be doing it while you are iterating, either: a good approach is to collect items that need changing in one loop, and then modify and re-insert them in another loop.
As a general rule of thumb, it isn't advisable to "modify" any value types added to Java containers which rely on equality, hash code etc. given that none of the known standard containers perform auto-balancing or adjustment in response to the change of values (which makes sense).
Along with Set, this rule is equally valid for Map types. If you are iterating over a map and modify the "key" in-place, things go bad. This is the reason why it is recommended to have immutable types as your Map keys (think of String, Integer etc.) Your case can be demonstrated by a simple example:
public class Test {
public static void main(final String[] args) {
Mutable m1 = new Mutable(1);
Mutable m2 = new Mutable(2);
Mutable m3 = new Mutable(3);
Mutable m4 = new Mutable(4);
TreeSet<Mutable> ts = new TreeSet<Mutable>(new Cmp());
ts.add(m1); ts.add(m2); ts.add(m3); ts.add(m4);
System.out.println(ts);
for (Iterator<Mutable> iter = ts.iterator(); iter.hasNext(); ) {
Mutable m = iter.next();
if (m.i == 1 || m.i == 3) {
m.i = m.i + 10;
}
}
System.out.println(ts);
}
}
class Mutable {
public int i;
public Mutable(int i) {
this.i = i;
}
public String toString() {
return "Mutable[" + i + "]";
}
}
class Cmp implements Comparator<Mutable> {
#Override public int compare(Mutable o1, Mutable o2) {
return Integer.valueOf(o1.i).compareTo(Integer.valueOf(o2.i));
}
}
Output:
[Mutable[1], Mutable[2], Mutable[3], Mutable[4]]
[Mutable[11], Mutable[2], Mutable[13], Mutable[4]]