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priority queue cannot sort automatically when its value has been changed

For example,
class App{
int k;
public App(int k)
{
this.k = k;
}
}
Main Code here:
App one = new App(2);
App Two = new App(3);
PriorityQueue<App> p = new PriorityQueue<>(2,new Comparator<App>() {
#Override
public int compare(App o1, App o2) {
if(o1.k < o2.k) return -1;
return 1;
}
});
p.add(two);
p.add(one);
Obviously, one is at the head of the queue. (p.peek().k is 2)
However, after:
one.k = 9;
two.k = 8;
the one still is at the head of the queue (p.peek.k is 9)!! Priority queue cannot sort automatically when its value has been changed.
Is there a method that can sort the queue when its value is changed?
Hope someone can help.

PriorityQueue and other collections working with comparable elements (e.g. TreeSet) are not meant for mutable objects. They only work if the ordering of the elements does not change (either because you don't change them in that way, or they are immutable and hence cannot be mutated at all).
So what you do, you shouldn't. But if you still do, PriorityQueue does not provide a way to redo the ordering.
Your 2 options:
remove all elements and add them again
create a new PriorityQueue, add all elements and use that
On a side note, your Comparator is not even correct, it should return 0 if 2 values are equal. Try using o1.k - o2.k or Integer.compare(o1.k, o2.k).

Data structure to check for pairs?

Say I have objects A,B,C,D. They can contain references to one another, for example, A might reference B and C, and C might reference A. I want to create segments but dont want to create them twice, so I don't want segment A C and segment C A, just 1 of them. So I want to keep a list of created segments, ex: A C, and check if I already have an A C or C A and skip it if so.
Is there a data structure that can do this?
Thanks
if(list.contains(a,b)
{
//dont add
}

you may introduce something like
class PairKey<T extends Comparable<T>> {
final T fst, snd;
public PairKey(T a, T b) {
if (a.compareTo(b) <=0 ) {
fst = a;
snd = b;
} else {
fst = b;
snd = a;
}
}
#Override
public int hashCode() {
return a.hashCode() & 37 & b.hashCode();
}
#Override
public boolean equals(Object other) {
if (other == this) return true;
if (!(other instanceOf PairKey)) return false;
PairKey<T> obj = (PairKey<T>) other;
return (obj.fst.equals(fst) && obj.snd.equals(snd));
}
}
then you may put edges into HashSet < PairKey < ? extends Comparable> > and then check if the given pair is already there.
You will need to make your vertexes comparable, so it will be possible to treat PairKey(A,B) equal to PairKey(B,A)
And then HashSet will do the rest for you, e.g you will be able to query
pairs.contains(new PairKey(A,B));
and if pairs contain either PairKey(A,B) or PairKey(B,A) - it will return true.
hashCode implementation might be slightly different, may be IDE will generate something more sophisticated.
Hope that helps.

I would use an object called Pair that would look something like this:
class Pair
{
Node start;
Node end;
public Pair(Node start, Node end)
{
this.start=start;
this.end=end;
}
public Pair reverse()
{
return new Pair(end,start);
}
}
Now you can do something like this:
if(pairs.contains(currentPair) || pairs.contains(currentPair.reverse())
{
continue;
} else{
pairs.add(currentPair);
}
As pointed out in the comments, you will need to implement equals and hashcode. However, doing the check in equals to make it match the reversal of the segment is a bad practice in a pure OO since. By implementing equals in the fashion, described within the comments, would bind Pair to your application only and remove the portability of it.

You can use a set of sets of objects.
Set<Set<MyObjectType>> segments = new HashSet<Set<MyObjectType>>();
Then you can add two-element sets representing pairs of MyObject. Since sets are unordered, if segments contains a set with A and B, attempting to add a set containing B and A will treat it as already present in segments.
Set<MyObjectType> segment = new HashSet<MyObjectType>();
segment.add(A); // A and B are instances of MyObjectType
segment.add(B);
segments.add(segment);
segment = new HashSet<MyObjectType>();
segment.add(B);
segment.add(A);
segments.add(segment);
System.out.println("Number of segments: " + segments.size()); // prints 1

Your problem is related with graph theory.
What you can try is to remove that internal list and create a Incidence Martrix, that all you objects share.
The final solution mostly depend of the task goal and available structure. So is hard to choose best solution for you problem with the description you have provided.

Use java.util.Set/ java.util.HashSet and keep adding the references you find e.g.
Set set1 = new HashSet();
set1.add(A), set1.Add(C), set1.Add(C)
You can add this finding in an external set, as finalSet.add(set1)
Set<Set> finalSet = new HashSet<Set>();
finalSet.add(set1);
This will filter out the duplicates automatically and in the end, you will be left with A & C only.

Java: Retrieving an element from a HashSet

Why cannot I retrieve an element from a HashSet?
Consider my HashSet containing a list of MyHashObjects with their hashCode() and equals() methods overridden correctly. I was hoping to construct a MyHashObject myself, and set the relevant hash code properties to certain values.
I can query the HashSet to see if there "equivalent" objects in the set using the contains() method. So even though contains() returns true for the two objects, they may not be == true.
How come then there isn’t any get() method similar to how the contains() works?
What is the thinking behind this API decision?

If you know what element you want to retrieve, then you already have the element. The only question for a Set to answer, given an element, is whether it contains() it or not.
If you want to iterator over the elements, just use a Set.iterator().
It sounds like what you're trying to do is designate a canonical element for an equivalence class of elements. You can use a Map<MyObject,MyObject> to do this. See this Stack Overflow question or this one for a discussion.
If you are really determined to find an element that .equals() your original element with the constraint that you must use the HashSet, I think you're stuck with iterating over it and checking equals() yourself. The API doesn't let you grab something by its hash code. So you could do:
MyObject findIfPresent(MyObject source, HashSet<MyObject> set)
{
if (set.contains(source)) {
for (MyObject obj : set) {
if (obj.equals(source))
return obj;
}
}
return null;
}
It is brute-force and O(n) ugly, but if that's what you need to do...

You can use HashMap<MyHashObject, MyHashObject> instead of HashSet<MyHashObject>.
Calling containsKey() on your "reconstructed" MyHashObject will first hashCode() - check the collection, and if a duplicate hashcode is hit, finally equals() - check your "reconstructed" against the original, at which you can retrieve the original using get()
Complexity is O(1) but the downside is you will likely have to override both equals() and hashCode() methods.

It sounds like you're essentially trying to use the hash code as a key in a map (which is what HashSets do behind the scenes). You could just do it explicitly, by declaring HashMap<Integer, MyHashObject>.
There is no get for HashSets because typically the object you would supply to the get method as a parameter is the same object you would get back.

If you know the order of elements in your Set, you can retrieve them by converting the Set to an Array. Something like this:
Set mySet = MyStorageObject.getMyStringSet();
Object[] myArr = mySet.toArray();
String value1 = myArr[0].toString();
String value2 = myArr[1].toString();

The idea that you need to get the reference to the object that is contained inside a Set object is common. It can be archived by 2 ways:
Use HashSet as you wanted, then:
public Object getObjectReference(HashSet<Xobject> set, Xobject obj) {
if (set.contains(obj)) {
for (Xobject o : set) {
if (obj.equals(o))
return o;
}
}
return null;
}
For this approach to work, you need to override both hashCode() and equals(Object o) methods
In the worst scenario we have O(n)
Second approach is to use TreeSet
public Object getObjectReference(TreeSet<Xobject> set, Xobject obj) {
if (set.contains(obj)) {
return set.floor(obj);
}
return null;
}
This approach gives O(log(n)), more efficient.
You don't need to override hashCode for this approach but you have to implement Comparable interface. ( define function compareTo(Object o)).

One of the easiest ways is to convert to Array:
for(int i = 0; i < set.size(); i++) {
System.out.println(set.toArray()[i]);
}

If I know for sure in my application that the object is not used in search in any of the list or hash data structure and not used equals method elsewhere except the one used indirectly in hash data structure while adding. Is it advisable to update the existing object in set in equals method. Refer the below code. If I add the this bean to HashSet, I can do group aggregation on the matching object on key (id). By this way I am able to achieve aggregation functions such as sum, max, min, ... as well. If not advisable, please feel free to share me your thoughts.
public class MyBean {
String id,
name;
double amountSpent;
#Override
public int hashCode() {
return id.hashCode();
}
#Override
public boolean equals(Object obj) {
if(obj!=null && obj instanceof MyBean ) {
MyBean tmpObj = (MyBean) obj;
if(tmpObj.id!=null && tmpObj.id.equals(this.id)) {
tmpObj.amountSpent += this.amountSpent;
return true;
}
}
return false;
}
}

First of all, convert your set to an array. Then, get the item by indexing the array.
Set uniqueItem = new HashSet();
uniqueItem.add("0");
uniqueItem.add("1");
uniqueItem.add("0");
Object[] arrayItem = uniqueItem.toArray();
for(int i = 0; i < uniqueItem.size(); i++) {
System.out.println("Item " + i + " " + arrayItem[i].toString());
}

If you could use List as a data structure to store your data, instead of using Map to store the result in the value of the Map, you can use following snippet and store the result in the same object.
Here is a Node class:
private class Node {
public int row, col, distance;
public Node(int row, int col, int distance) {
this.row = row;
this.col = col;
this.distance = distance;
}
public boolean equals(Object o) {
return (o instanceof Node &&
row == ((Node) o).row &&
col == ((Node) o).col);
}
}
If you store your result in distance variable and the items in the list are checked based on their coordinates, you can use the following to change the distance to a new one with the help of lastIndexOf method as long as you only need to store one element for each data:
List<Node> nodeList;
nodeList = new ArrayList<>(Arrays.asList(new Node(1, 2, 1), new Node(3, 4, 5)));
Node tempNode = new Node(1, 2, 10);
if(nodeList.contains(tempNode))
nodeList.get(nodeList.lastIndexOf(tempNode)).distance += tempNode.distance;
It is basically reimplementing Set whose items can be accessed and changed.

If you want to have a reference to the real object using the same performance as HashSet, I think the best way is to use HashMap.
Example (in Kotlin, but similar in Java) of finding an object, changing some field in it if it exists, or adding it in case it doesn't exist:
val map = HashMap<DbData, DbData>()
val dbData = map[objectToFind]
if(dbData!=null){
++dbData.someIntField
}
else {
map[dbData] = dbData
}

Java ConcurrentMap - Position of Key in Map

I have an interesting problem I would like some help with. I have implemented a couple of queues for two separate conditions, one based on FIFO and the other natural order of a key (ConcurrentMap). That is you can image both queues have the same data just ordered differently. The question I have (and I am looking for an efficient way of doing this) if I find the key in the ConcurrentMap based on some criteria, what is the best way of finding the "position" of the key in the FIFO map. Essentially I would like to know whether it is the firstkey (which is easy), or say it is the 10th key.
Any help would be greatly appreciated.

There is no API for accessing the order in a FIFO map. The only way you can do it is iterate over keySet(), values() or entrySet() and count.

I believe something like the code below will do the job. I've left the implementation of element --> key as an abstract method. Note the counter being used to assign increasing numbers to elements. Also note that if add(...) is being called by multiple threads, the elements in the FIFO are only loosely ordered. That forces the fancy max(...) and min(...) logic. Its also why the position is approximate. First and last are special cases. First can be indicated clearly. Last is tricky because the current implementation returns a real index.
Since this is an approximate location, I would suggest you consider making the API return a float between 0.0 and 1.0 to indicate relative position in the queue.
If your code needs to support removal using some means other than pop(...), you will need to use approximate size, and change the return to ((id - min) / (max - min)) * size, with all the appropriate int / float casting & rounding.
public abstract class ApproximateLocation<K extends Comparable<K>, T> {
protected abstract K orderingKey(T element);
private final ConcurrentMap<K, Wrapper<T>> _map = new ConcurrentSkipListMap<K, Wrapper<T>>();
private final Deque<Wrapper<T>> _fifo = new LinkedBlockingDeque<Wrapper<T>>();
private final AtomicInteger _counter = new AtomicInteger();
public void add(T element) {
K key = orderingKey(element);
Wrapper<T> wrapper = new Wrapper<T>(_counter.getAndIncrement(), element);
_fifo.add(wrapper);
_map.put(key, wrapper);
}
public T pop() {
Wrapper<T> wrapper = _fifo.pop();
_map.remove(orderingKey(wrapper.value));
return wrapper.value;
}
public int approximateLocation(T element) {
Wrapper<T> wrapper = _map.get(orderingKey(element));
Wrapper<T> first = _fifo.peekFirst();
Wrapper<T> last = _fifo.peekLast();
if (wrapper == null || first == null || last == null) {
// element is not in composite structure; fifo has not been written to yet because of concurrency
return -1;
}
int min = Math.min(wrapper.id, Math.min(first.id, last.id));
int max = Math.max(wrapper.id, Math.max(first.id, last.id));
if (wrapper == first || max == min) {
return 0;
}
if (wrapper == last) {
return max - min;
}
return wrapper.id - min;
}
private static class Wrapper<T> {
final int id;
final T value;
Wrapper(int id, T value) {
this.id = id;
this.value = value;
}
}
}

If you can use a ConcurrentNavigableMap, the size of the headMap gives you exactly what you want.

What is the fastest way to compare two sets in Java?

I am trying to optimize a piece of code which compares elements of list.
Eg.
public void compare(Set<Record> firstSet, Set<Record> secondSet){
for(Record firstRecord : firstSet){
for(Record secondRecord : secondSet){
// comparing logic
}
}
}
Please take into account that the number of records in sets will be high.
Thanks
Shekhar

firstSet.equals(secondSet)
It really depends on what you want to do in the comparison logic... ie what happens if you find an element in one set not in the other? Your method has a void return type so I assume you'll do the necessary work in this method.
More fine-grained control if you need it:
if (!firstSet.containsAll(secondSet)) {
// do something if needs be
}
if (!secondSet.containsAll(firstSet)) {
// do something if needs be
}
If you need to get the elements that are in one set and not the other.
EDIT: set.removeAll(otherSet) returns a boolean, not a set. To use removeAll(), you'll have to copy the set then use it.
Set one = new HashSet<>(firstSet);
Set two = new HashSet<>(secondSet);
one.removeAll(secondSet);
two.removeAll(firstSet);
If the contents of one and two are both empty, then you know that the two sets were equal. If not, then you've got the elements that made the sets unequal.
You mentioned that the number of records might be high. If the underlying implementation is a HashSet then the fetching of each record is done in O(1) time, so you can't really get much better than that. TreeSet is O(log n).

If you simply want to know if the sets are equal, the equals method on AbstractSet is implemented roughly as below:
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection c = (Collection) o;
if (c.size() != size())
return false;
return containsAll(c);
}
Note how it optimizes the common cases where:
the two objects are the same
the other object is not a set at all, and
the two sets' sizes are different.
After that, containsAll(...) will return false as soon as it finds an element in the other set that is not also in this set. But if all elements are present in both sets, it will need to test all of them.
The worst case performance therefore occurs when the two sets are equal but not the same objects. That cost is typically O(N) or O(NlogN) depending on the implementation of this.containsAll(c).
And you get close-to-worst case performance if the sets are large and only differ in a tiny percentage of the elements.
UPDATE
If you are willing to invest time in a custom set implementation, there is an approach that can improve the "almost the same" case.
The idea is that you need to pre-calculate and cache a hash for the entire set so that you could get the set's current hashcode value in O(1). Then you can compare the hashcode for the two sets as an acceleration.
How could you implement a hashcode like that? Well if the set hashcode was:
zero for an empty set, and
the XOR of all of the element hashcodes for a non-empty set,
then you could cheaply update the set's cached hashcode each time you added or removed an element. In both cases, you simply XOR the element's hashcode with the current set hashcode.
Of course, this assumes that element hashcodes are stable while the elements are members of sets. It also assumes that the element classes hashcode function gives a good spread. That is because when the two set hashcodes are the same you still have to fall back to the O(N) comparison of all elements.
You could take this idea a bit further ... at least in theory.
WARNING - This is highly speculative. A "thought experiment" if you like.
Suppose that your set element class has a method to return a crypto checksums for the element. Now implement the set's checksums by XORing the checksums returned for the elements.
What does this buy us?
Well, if we assume that nothing underhand is going on, the probability that any two unequal set elements have the same N-bit checksums is 2-N. And the probability 2 unequal sets have the same N-bit checksums is also 2-N. So my idea is that you can implement equals as:
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection c = (Collection) o;
if (c.size() != size())
return false;
return checksums.equals(c.checksums);
}
Under the assumptions above, this will only give you the wrong answer once in 2-N time. If you make N large enough (e.g. 512 bits) the probability of a wrong answer becomes negligible (e.g. roughly 10-150).
The downside is that computing the crypto checksums for elements is very expensive, especially as the number of bits increases. So you really need an effective mechanism for memoizing the checksums. And that could be problematic.
And the other downside is that a non-zero probability of error may be unacceptable no matter how small the probability is. (But if that is the case ... how do you deal with the case where a cosmic ray flips a critical bit? Or if it simultaneously flips the same bit in two instances of a redundant system?)

There is a method in Guava Sets which can help here:
public static <E> boolean equals(Set<? extends E> set1, Set<? extends E> set2){
return Sets.symmetricDifference(set1,set2).isEmpty();
}

There's an O(N) solution for very specific cases where:
the sets are both sorted
both sorted in the same order
The following code assumes that both sets are based on the records comparable. A similar method could be based on on a Comparator.
public class SortedSetComparitor <Foo extends Comparable<Foo>>
implements Comparator<SortedSet<Foo>> {
#Override
public int compare( SortedSet<Foo> arg0, SortedSet<Foo> arg1 ) {
Iterator<Foo> otherRecords = arg1.iterator();
for (Foo thisRecord : arg0) {
// Shorter sets sort first.
if (!otherRecords.hasNext()) return 1;
int comparison = thisRecord.compareTo(otherRecords.next());
if (comparison != 0) return comparison;
}
// Shorter sets sort first
if (otherRecords.hasNext()) return -1;
else return 0;
}
}

You have the following solution from https://www.mkyong.com/java/java-how-to-compare-two-sets/
public static boolean equals(Set<?> set1, Set<?> set2){
if(set1 == null || set2 ==null){
return false;
}
if(set1.size() != set2.size()){
return false;
}
return set1.containsAll(set2);
}
Or if you prefer to use a single return statement:
public static boolean equals(Set<?> set1, Set<?> set2){
return set1 != null
&& set2 != null
&& set1.size() == set2.size()
&& set1.containsAll(set2);
}

If you are using Guava library it's possible to do:
SetView<Record> added = Sets.difference(secondSet, firstSet);
SetView<Record> removed = Sets.difference(firstSet, secondSet);
And then make a conclusion based on these.

I would put the secondSet in a HashMap before the comparison. This way you will reduce the second list's search time to n(1). Like this:
HashMap<Integer,Record> hm = new HashMap<Integer,Record>(secondSet.size());
int i = 0;
for(Record secondRecord : secondSet){
hm.put(i,secondRecord);
i++;
}
for(Record firstRecord : firstSet){
for(int i=0; i<secondSet.size(); i++){
//use hm for comparison
}
}

public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Set<String> a = this;
Set<String> b = o;
Set<String> thedifference_a_b = new HashSet<String>(a);
thedifference_a_b.removeAll(b);
if(thedifference_a_b.isEmpty() == false) return false;
Set<String> thedifference_b_a = new HashSet<String>(b);
thedifference_b_a.removeAll(a);
if(thedifference_b_a.isEmpty() == false) return false;
return true;
}

I think method reference with equals method can be used. We assume that the object type without a shadow of a doubt has its own comparison method. Plain and simple example is here,
Set<String> set = new HashSet<>();
set.addAll(Arrays.asList("leo","bale","hanks"));
Set<String> set2 = new HashSet<>();
set2.addAll(Arrays.asList("hanks","leo","bale"));
Predicate<Set> pred = set::equals;
boolean result = pred.test(set2);
System.out.println(result); // true