What is the difference between iterating map in the below ways? - java

I have a array of String which has few words and I'm trying to find the most frequent (number of occurrences) words of k from the given array. I also have a constraint that consider the most frequent words are of frequency 2 then I need to consider the one which comes first in alphabetical order then the next ones.
I created a TreeMap to hold the words vs it's frequency count. Now the keys (ie' words) in the map would be sorted alphabetically. Now I was trying to sort the map based on it's values with the following snippet.
public static <K, V extends Comparable<V>> Map<K, V>
sortByValues(final Map<K, V> map) {
Comparator<K> valueComparator =
new Comparator<K>() {
public int compare(K k1, K k2) {
int compare =
map.get(k2).compareTo(map.get(k1));
if (compare == 0)
return 1;
else
return compare;
}
};
Map<K, V> sortedByValues =
new TreeMap<K, V>(valueComparator);
sortedByValues.putAll(map);
return sortedByValues;
}
Now I'm iterating the map in the following ways,
Method1
Map sortedMap = sortByValues(map);
ArrayList<String> result = new ArrayList<String>();
for (Map.Entry<String, Integer> entry : sortedMap.entrySet()) {
if (k-- <= 0) {
return result;
}
result.add(entry.getKey());
}
Method2
Map sortedMap = sortByValues(map);
ArrayList<String> result = new ArrayList<String>();
// Get a set of the entries on the sorted map
Set set = sortedMap.entrySet();
// Get an iterator
Iterator i = set.iterator();
// Display elements
while(i.hasNext()) {
if (k-- <= 0) {
return result;
}
Map.Entry me = (Map.Entry)i.next();
result.add((String) me.getKey());
}
Consider the following content in TreeMap initially,
// Put elements to the map
treemap.put("love", 2);
treemap.put("i", 2);
treemap.put("coding", 1);
Method1 returns ["coding","i"] as output whereas Method2 way of iterating gives me ["i","love"].
Doubts
I'm confused with the two ways of iterating the map?
If I sort the TreeMap which is already sorted based on keys, if i sort again based by values will the sorting by keys be retained for equal value elements?

Related

What is the main difference between LinkedList, HashSet and HashMap?

Well, you don't mind I will write my "test" project;
First of all I had created class that implements AbstractMap.
public class TestClass <K, V> extends AbstractMap <K, V>
TestClass has the private LinkedList that take as a parameter (It is another class that implements Map.Entry:
private int size = 1000;
private LinkedList <InfoClass <K, V>> [] array = new LinkedList [size];
After, I had created the method that checks and replaces duplicates:
public V put (K key, V value){ // Void doesn't work, therefore we need to return any value;
V temp = null;
boolean found = false;
int index = Math.abs(key.hashCode()) % size;
if (array[index] == null)
array[index] = new LinkedList <InfoClass <K, V>> (); // If the specified member of array is null, create new member;
LinkedList <InfoClass <K, V>> list = array[index];
InfoClass <K, V> info = new InfoClass <K, V> (key, value);
ListIterator <InfoClass <K, V>> it = list.listIterator();
while (it.hasNext()){
InfoClass<K, V> temp2 = it.next(); // Create a temp instance;
if (temp2.getKey().equals(key)){ // If there is a duplicate of value, just replace by new one;
found = true;
temp = temp2.getValue();
it.set(info);
break;
}
}
if (!found) // if there is not a duplicate, add new one;
array[index].add(info);
return temp;
}
Next method returns a value, otherwise this returns null if a member of array doesn't exist:
public V get (Object key){ // The parameter K doesn't work;
int index = Math.abs(key.hashCode()) % size;
if (array[index] == null)
return null;
else
for (InfoClass <K, V> info : array[index])
if (info.getKey().equals(key))
return info.getValue();
return null;
}
This method forms a set of AbstractMap:
public Set<java.util.Map.Entry<K, V>> entrySet() {
Set <Map.Entry<K, V>> sets = new HashSet <Map.Entry<K, V>> ();
for (LinkedList <InfoClass<K, V>> temp1 : array){
if (temp1 == null)
continue;
else{
for (InfoClass<K,V> temp2 : temp1)
sets.add(temp2);
}
}
return sets;
}
Ok, create new object in main method:
public static void main (String [] args){
TestClass <Integer, String> TC = new TestClass <Integer, String> ();
TC.putAll(CollectionDataMap.newCollection(new Group.Ints(), new Group.Name(), 10));
System.out.println(TC);
System.out.println(TC.get(1));
TC.put(1, "Hello this world");
System.out.println(TC);
}
Hope I explained correctly. I have a question, what is difference between LinkedList and LinkedHashMap (HashMap) if they work the same way?
Thank you very much!
LinkedList can contain the same element multiple times if the same element is added multiple times.
HashSet can only contain the same object once even if you add it multiple times, but it does not retain insertion order in the set.
LinkedHashSet can only contain the same object once even if you add it multiple times, but it also retains insertion order.
HashMap maps a value to a key, and the keys are stored in a set (so it can be in the set only once). HashMap doesn't preserve insertion order for the key set, while LinkedHashMap does retain insertion order for the keys.

How to take separate data from LinkedHashMap<String, Double> in Java

I used LinkedHashMap<String, Double> . I want to take separate values from it. If it is Array we can use .get[2] ,.get[5] etc. for take 2nd and 5th value. But for LinkedHashMap<String, Double> how to do it. I used following code. But it print all the values contained in LinkedHashMap<String, Double>. I need to take separately.
Set set = mylist.entrySet();
Iterator i = set.iterator();
while(i.hasNext()) {
Map.Entry me1 = (Map.Entry)i.next();
System.out.print(me1.getKey());
System.out.println(me1.getValue());
You may use the LinkedHashMap#get(Object key) method
Which will return the value corresponding to the key parameter. Since your keys are String, you can not use an int to retrieve them.
Example
If your LinkedHashMap contains ["key", 2.5], calling
System.out.println(lnkHashMap.get("key"));
will print
2.5
Addition
If you're using java-8, there is a workaround using a Stream object.
Double result = hashmap.values()
.stream()
.skip(2)
.findFirst()
.get();
This will skip the two first values and get to the third one directly and return it.
If not, here is a solution
public <T> T getValueByIndex (Map<? extends Object, T> map, int index){
Iterator<T> it = map.values().iterator();
T temp = null;
for (int i = 0 ; i < index ; i++){
if (it.hasNext()){
temp = it.next();
} else {
throw new IndexOutOfBoundsException();
}
}
return temp;
}
It could be the case that you are using the wrong data structure for your purpose.
If you look closely to the LinkedHashMap API you will notice that it is indeed a Map and the only way to access a previously stored value is by providing its key.
But if you really think you need to access the ith value of the LinkedHashMap according to its insertion-order (or access-order) you can use a simple utility method like the following:
Java 8 Solution
private static <K, V> Optional<V> getByInsertionOrder(LinkedHashMap<K, V> linkedHashMap, int index) {
return linkedHashMap.values().stream()
.skip(index)
.findFirst();
}
Java 7 Soution
private static <K, V> V getByInsertionOrder(LinkedHashMap<K, V> linkedHashMap, int index) {
if (index < 0 || index >= linkedHashMap.size()) {
throw new IndexOutOfBoundsException();
}
Iterator<Entry<K, V>> iterator = linkedHashMap.entrySet().iterator();
for (int i = 0; i < index; i++) {
iterator.next();
}
return iterator.next().getValue();
}

Using Comparator to sort HashMap in java? [duplicate]

I want to write a comparator that will let me sort a TreeMap by value instead of the default natural ordering.
I tried something like this, but can't find out what went wrong:
import java.util.*;
class treeMap {
public static void main(String[] args) {
System.out.println("the main");
byValue cmp = new byValue();
Map<String, Integer> map = new TreeMap<String, Integer>(cmp);
map.put("de",10);
map.put("ab", 20);
map.put("a",5);
for (Map.Entry<String,Integer> pair: map.entrySet()) {
System.out.println(pair.getKey()+":"+pair.getValue());
}
}
}
class byValue implements Comparator<Map.Entry<String,Integer>> {
public int compare(Map.Entry<String,Integer> e1, Map.Entry<String,Integer> e2) {
if (e1.getValue() < e2.getValue()){
return 1;
} else if (e1.getValue() == e2.getValue()) {
return 0;
} else {
return -1;
}
}
}
I guess what am I asking is: Can I get a Map.Entry passed to the comparator?
You can't have the TreeMap itself sort on the values, since that defies the SortedMap specification:
A Map that further provides a total ordering on its keys.
However, using an external collection, you can always sort Map.entrySet() however you wish, either by keys, values, or even a combination(!!) of the two.
Here's a generic method that returns a SortedSet of Map.Entry, given a Map whose values are Comparable:
static <K,V extends Comparable<? super V>>
SortedSet<Map.Entry<K,V>> entriesSortedByValues(Map<K,V> map) {
SortedSet<Map.Entry<K,V>> sortedEntries = new TreeSet<Map.Entry<K,V>>(
new Comparator<Map.Entry<K,V>>() {
#Override public int compare(Map.Entry<K,V> e1, Map.Entry<K,V> e2) {
int res = e1.getValue().compareTo(e2.getValue());
return res != 0 ? res : 1;
}
}
);
sortedEntries.addAll(map.entrySet());
return sortedEntries;
}
Now you can do the following:
Map<String,Integer> map = new TreeMap<String,Integer>();
map.put("A", 3);
map.put("B", 2);
map.put("C", 1);
System.out.println(map);
// prints "{A=3, B=2, C=1}"
System.out.println(entriesSortedByValues(map));
// prints "[C=1, B=2, A=3]"
Note that funky stuff will happen if you try to modify either the SortedSet itself, or the Map.Entry within, because this is no longer a "view" of the original map like entrySet() is.
Generally speaking, the need to sort a map's entries by its values is atypical.
Note on == for Integer
Your original comparator compares Integer using ==. This is almost always wrong, since == with Integer operands is a reference equality, not value equality.
System.out.println(new Integer(0) == new Integer(0)); // prints "false"!!!
Related questions
When comparing two Integers in Java does auto-unboxing occur? (NO!!!)
Is it guaranteed that new Integer(i) == i in Java? (YES!!!)
polygenelubricants answer is almost perfect. It has one important bug though. It will not handle map entries where the values are the same.
This code:...
Map<String, Integer> nonSortedMap = new HashMap<String, Integer>();
nonSortedMap.put("ape", 1);
nonSortedMap.put("pig", 3);
nonSortedMap.put("cow", 1);
nonSortedMap.put("frog", 2);
for (Entry<String, Integer> entry : entriesSortedByValues(nonSortedMap)) {
System.out.println(entry.getKey()+":"+entry.getValue());
}
Would output:
ape:1
frog:2
pig:3
Note how our cow dissapeared as it shared the value "1" with our ape :O!
This modification of the code solves that issue:
static <K,V extends Comparable<? super V>> SortedSet<Map.Entry<K,V>> entriesSortedByValues(Map<K,V> map) {
SortedSet<Map.Entry<K,V>> sortedEntries = new TreeSet<Map.Entry<K,V>>(
new Comparator<Map.Entry<K,V>>() {
#Override public int compare(Map.Entry<K,V> e1, Map.Entry<K,V> e2) {
int res = e1.getValue().compareTo(e2.getValue());
return res != 0 ? res : 1; // Special fix to preserve items with equal values
}
}
);
sortedEntries.addAll(map.entrySet());
return sortedEntries;
}
In Java 8:
LinkedHashMap<Integer, String> sortedMap = map.entrySet().stream()
.sorted(Map.Entry.comparingByValue(/* Optional: Comparator.reverseOrder() */))
.collect(Collectors.toMap(Map.Entry::getKey,
Map.Entry::getValue,
(e1, e2) -> e1, LinkedHashMap::new));
A TreeMap is always sorted by the keys, anything else is impossible. A Comparator merely allows you to control how the keys are sorted.
If you want the sorted values, you have to extract them into a List and sort that.
This can't be done by using a Comparator, as it will always get the key of the map to compare. TreeMap can only sort by the key.
Olof's answer is good, but it needs one more thing before it's perfect. In the comments below his answer, dacwe (correctly) points out that his implementation violates the Compare/Equals contract for Sets. If you try to call contains or remove on an entry that's clearly in the set, the set won't recognize it because of the code that allows entries with equal values to be placed in the set. So, in order to fix this, we need to test for equality between the keys:
static <K,V extends Comparable<? super V>> SortedSet<Map.Entry<K,V>> entriesSortedByValues(Map<K,V> map) {
SortedSet<Map.Entry<K,V>> sortedEntries = new TreeSet<Map.Entry<K,V>>(
new Comparator<Map.Entry<K,V>>() {
#Override public int compare(Map.Entry<K,V> e1, Map.Entry<K,V> e2) {
int res = e1.getValue().compareTo(e2.getValue());
if (e1.getKey().equals(e2.getKey())) {
return res; // Code will now handle equality properly
} else {
return res != 0 ? res : 1; // While still adding all entries
}
}
}
);
sortedEntries.addAll(map.entrySet());
return sortedEntries;
}
"Note that the ordering maintained by a sorted set (whether or not an explicit comparator is provided) must be consistent with equals if the sorted set is to correctly implement the Set interface... the Set interface is defined in terms of the equals operation, but a sorted set performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the sorted set, equal."
(http://docs.oracle.com/javase/6/docs/api/java/util/SortedSet.html)
Since we originally overlooked equality in order to force the set to add equal valued entries, now we have to test for equality in the keys in order for the set to actually return the entry you're looking for. This is kinda messy and definitely not how sets were intended to be used - but it works.
I know this post specifically asks for sorting a TreeMap by values, but for those of us that don't really care about implementation but do want a solution that keeps the collection sorted as elements are added, I would appreciate feedback on this TreeSet-based solution. For one, elements are not easily retrieved by key, but for the use case I had at hand (finding the n keys with the lowest values), this was not a requirement.
TreeSet<Map.Entry<Integer, Double>> set = new TreeSet<>(new Comparator<Map.Entry<Integer, Double>>()
{
#Override
public int compare(Map.Entry<Integer, Double> o1, Map.Entry<Integer, Double> o2)
{
int valueComparison = o1.getValue().compareTo(o2.getValue());
return valueComparison == 0 ? o1.getKey().compareTo(o2.getKey()) : valueComparison;
}
});
int key = 5;
double value = 1.0;
set.add(new AbstractMap.SimpleEntry<>(key, value));
A lot of people hear adviced to use List and i prefer to use it as well
here are two methods you need to sort the entries of the Map according to their values.
static final Comparator<Entry<?, Double>> DOUBLE_VALUE_COMPARATOR =
new Comparator<Entry<?, Double>>() {
#Override
public int compare(Entry<?, Double> o1, Entry<?, Double> o2) {
return o1.getValue().compareTo(o2.getValue());
}
};
static final List<Entry<?, Double>> sortHashMapByDoubleValue(HashMap temp)
{
Set<Entry<?, Double>> entryOfMap = temp.entrySet();
List<Entry<?, Double>> entries = new ArrayList<Entry<?, Double>>(entryOfMap);
Collections.sort(entries, DOUBLE_VALUE_COMPARATOR);
return entries;
}
import java.util.*;
public class Main {
public static void main(String[] args) {
TreeMap<String, Integer> initTree = new TreeMap();
initTree.put("D", 0);
initTree.put("C", -3);
initTree.put("A", 43);
initTree.put("B", 32);
System.out.println("Sorted by keys:");
System.out.println(initTree);
List list = new ArrayList(initTree.entrySet());
Collections.sort(list, new Comparator<Map.Entry<String, Integer>>() {
#Override
public int compare(Map.Entry<String, Integer> e1, Map.Entry<String, Integer> e2) {
return e1.getValue().compareTo(e2.getValue());
}
});
System.out.println("Sorted by values:");
System.out.println(list);
}
}
//convert HashMap into List
List<Entry<String, Integer>> list = new LinkedList<Entry<String, Integer>>(map.entrySet());
Collections.sort(list, (o1, o2) -> o1.getValue().compareTo(o2.getValue()));
If you want to use a Hash map you can add a condition in Comparator to check by values first & if values are equal perform a sort on keys
HashMap<String , Integer> polpularity = new HashMap<>();
List<String> collect = popularity.entrySet().stream().sorted((t2, t1) -> {
if (t2.getValue() > t1.getValue()) {
return -1;
} else if (t2.getValue() < t1.getValue()) {
return +1;
} else {
return t2.getKey().compareTo(t1.getKey());
}
}).map(entry -> entry.getKey()).collect(Collectors.toList());
If you don't want to take care of the latter condition then use a Treemap which will offer you sorting by itself, this can be done in an elegant single line of code:
TreeMap<String, Integer> popularity = new TreeMap<>();
List<String> collect = popularity.entrySet().stream().sorted(Collections.reverseOrder(Map.Entry.comparingByValue())).map(entry -> entry.getKey()).collect(Collectors.toList());
TreeMap is always sorted by the keys.
If you want TreeMap to be sorted by the values, so you can simply construct it also.
Example:
// the original TreeMap which is sorted by key
Map<String, Integer> map = new TreeMap<>();
map.put("de",10);
map.put("ab", 20);
map.put("a",5);
// expected output:
// {a=5, ab=20, de=10}
System.out.println(map);
// now we will constrcut a new TreeSet which is sorted by values
// [original TreeMap values will be the keys for this new TreeMap]
TreeMap<Integer, String> newTreeMapSortedByValue = new TreeMap();
treeMapmap.forEach((k,v) -> newTreeMapSortedByValue.put(v, k));
// expected output:
// {5=a, 10=de, 20=ab}
System.out.println(newTreeMapSortedByValue);
Only 1 Line Of Code Solution
Normal Order
map.entrySet().stream().sorted(Map.Entry.comparingByValue()).forEach(x->{});
Reverse Order
map.entrySet().stream().sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())).forEachOrdered(x -> {});

Sorted map won't allow removal

I searched earlier for a way to sort TreeMaps according to the values, was able to find a function that does that, but now it seems something's wrong with the remove() method, tried various different ways to remove using a key but its not working, and to tell the truth I don't fully understand the sorting function, can someone explain what is the problem?
The Code:
public static void main(String[] args) {
TreeMap<String,Integer> theTree = new TreeMap<String,Integer>();
theTree.put("A", 5);
theTree.put("B", 3);
theTree.put("F", 1);
theTree.put("D", 5);
theTree.put("E", 6);
theTree.put("C", 7);
System.out.println(theTree.toString());
Map<String,Integer> sortedTree = new TreeMap<String,Integer>();
sortedTree = sortByValues(theTree);
System.out.println(sortedTree.toString());
for (Map.Entry<String, Integer> entry : sortedTree.entrySet())
{
if(entry.getKey().equals("A"))
sortedTree.remove(entry.getKey());
}
System.out.println(sortedTree.toString());
theTree = new TreeMap<String,Integer>(sortedTree);
theTree.remove("B");
System.out.println(theTree.toString());
}
public static <K, V extends Comparable<V>> Map<K, V> sortByValues(final Map<K, V> map) {
Comparator<K> valueComparator = new Comparator<K>() {
public int compare(K k1, K k2) {
int compare = map.get(k2).compareTo(map.get(k1));
if (compare == 0) return 1;
else return compare;
}
};
Map<K, V> sortedByValues = new TreeMap<K, V>(valueComparator);
sortedByValues.putAll(map);
return sortedByValues;
}
The Output:
{A=5, B=3, C=7, D=5, E=6, F=1}
{C=7, E=6, A=5, D=5, B=3, F=1}
{C=7, E=6, A=5, D=5, B=3, F=1}
{A=5, C=7, D=5, E=6, F=1}
The A is still in the map
In your Comparator, you are removing any chance of it returning 0 (if (compare == 0)return 1) Thus when you go to remove, there is never any chance for your inputted value to equal a value in the Set and nothing will be removed. If there is no particular reason why you are removing the chance for them to be deemed equal, just remove that line from your comparator.
You can delete only via Iterator during map traversal:
Map<String,Integer> sortedTree = new TreeMap<String,Integer>();
System.out.println(sortedTree.toString());
Iterator<Entry<String,Integer>> itr = sortedTree.entrySet().iterator();
while(itr.hasNext())
{
Entry<String,Integer> entry = itr.next();
if(entry.getKey().equals("A"))
itr.remove();
}
map.remove would work fine while not under iteration else removal would throw ConcurrentModificationException if iteration is not happening via Iterator. That's the case with many other collections too.
This is what I meant as per TreeMap javadoc:
The iterators returned by the iterator method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the
iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a
ConcurrentModificationException.

Sorting HashMap<?,?> by key

hello
I need to implement a method that receives a HashMap and sorts (mergeSort) it's values by key (without using TreeMap, SortedMap or Collections.Sort or use any sort solutions from JAVA Packages).
my problem is dealing with the wildcard Types...
this is my implementation (that returns compilation errors because of wildcards use)
public HashMap<?, ?> mergeSort(HashMap<?, ?> map) {
if (map.size() < 1) {
return map;
}
// rounds downwards
int middle = map.size() / 2;
int location = 0;
HashMap<?,?> mapLeft = new HashMap<?, ?>();
HashMap<?,?> mapRight = new HashMap<?, ?>();
// splitting map
for (Iterator<?> keyIter = map.keySet().iterator(); keyIter.hasNext();) {
if (location < middle) {
mapLeft.put(keyIter, map.get(keyIter));
} else {
mapRight.put(keyIter, map.get(keyIter));
}
location++;
}
// recursive call
mapLeft = mergeSort(mapLeft);
mapRight = mergeSort(mapRight);
return merge(mapLeft, mapRight);
}
public HashMap<?, ?> merge(HashMap<?, ?> mapLeft, HashMap<?, ?> mapRight) {
HashMap<?, ?> result = new HashMap<?, ?>();
Iterator<?> keyLeftIter = mapLeft.keySet().iterator();
Iterator<?> keyRightIter = mapRight.keySet().iterator();
String keyLeft;
String keyRight;
while (keyLeftIter.hasNext()) {
keyLeft = keyLeftIter.next();
while (keyRightIter.hasNext()) {
keyRight = keyRightIter.next();
if (keyLeft.compareTo(keyRight) < 0) {
result.put(keyLeft, mapLeft.get(keyLeft));
keyLeft = keyLeftIter.next();
} else {
result.put(keyRight, mapRight.get(keyRight));
keyRight = keyRightIter.next();
}
}
}
return result;
}
I appreciate your help!
If all you have to do is meet a method contract, you can do this.
public HashMap<?, ?> mergeSort(HashMap<?, ?> map) {
return new LinkedHashMap(new TreeMap(map));
}
This will sort the keys and return a subclass of HashMap. The design of this method is broken, but sometimes you can't change things.
If you are sorting a map, you should be using a SortedMap like TreeMap. hashmap doesn't retain an order so using it for a merge sort is not possible. Using a merge sort for a TreeMap is redundant.
You cannot assume that ? is a Comparable. You can write something like.
public static <K extends Comparable<K>, V> SortedMap<K,V> sort(Map<K,V> map) {
return new TreeMap<K, V>(map);
}
As you can see this is shorter and simpler than your approach. Is this homework? Do you reall need to use a merge sort?
The problem you have is that you cannot return a HashMap as it doesn't keep the order, adn you cannot return a TreeMap because it will sort the keys for you making anything else you redundant. For this task you can only return a LinkedHashMap as it does retain order, without doing the sorting for you.
here is an example using LinkedHashMap. Note it doesn't create copies of Maps as it goes, it creates a single array and merge sorts portions of it until its completely sorted.
Note: I use TreeMap as a SortedMap to show its sorted correctly. ;)
public static void main(String... args) throws IOException {
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for(int i=0;i<100;i++)
map.put((int)(Math.random()*1000), i);
System.out.println("Unsorted "+map);
System.out.println("Sorted "+sort(map));
final String sortedToString = sort(map).toString();
final String treeMapToString = new TreeMap<Integer, Integer>(map).toString();
if (!sortedToString.equals(treeMapToString))
System.out.println(sortedToString+" != \n"+treeMapToString);
}
public static <K extends Comparable<K>, V> Map<K, V> sort(Map<K, V> map) {
return mergeSort(map);
}
// a very bad design idea, but needed for compatibility.
public static <K extends Comparable<K>, V> HashMap<K, V> mergeSort(Map<K, V> map) {
Map.Entry<K, V>[] entries = map.entrySet().toArray(new Map.Entry[map.size()]);
mergeSort0(entries, 0, entries.length);
HashMap<K, V> ret = new LinkedHashMap<K, V>();
for (Map.Entry<K, V> entry : entries)
ret.put(entry.getKey(), entry.getValue());
return ret;
}
private static <K extends Comparable<K>, V> void mergeSort0(Map.Entry<K, V>[] entries, int start, int end) {
int len = end - start;
if (len < 2) return;
int mid = (end + start) >>> 1;
mergeSort0(entries, start, mid);
mergeSort0(entries, mid, end);
// merge [start, mid) and [mid, end) to [start, end)
for(int p = start, l=start, r=mid; p < end && l < r && r < end; p++) {
int cmp = entries[l].getKey().compareTo(entries[r].getKey());
if (cmp <= 0) {
l++;
// the entry is in the right place already
} else if (p != r) {
// we need to insert the entry from the right
Map.Entry<K,V> e= entries[r];
// shift up.
System.arraycopy(entries, p, entries, p+1, r - p);
l++;
// move down.
entries[p] = e;
r++;
}
}
}
prints
Unsorted {687=13, 551=0, 2=15, 984=3, 608=6, 714=16, 744=1, 272=5, 854=9, 96=2, 918=18, 829=8, 109=14, 346=7, 522=4, 626=19, 495=12, 695=17, 247=11, 725=10}
Sorted {2=15, 96=2, 109=14, 247=11, 272=5, 346=7, 495=12, 522=4, 551=0, 608=6, 626=19, 687=13, 695=17, 714=16, 725=10, 744=1, 829=8, 854=9, 918=18, 984=3}
Like other commenters I would suggest reading up on the subject of generics in Java. What you did in merge is using wildcards on the result HashMap
HashMap<?, ?> result = new HashMap<?, ?>();
When you put wildcards on it, you are basically saying "I will only be reading from this". Later on you trying to push something in
result.put(keyLeft, mapLeft.get(keyLeft));
The compiler will say "Hey, you just told me you would only read and now you want to put something in... FAIL
Then it generates your compile time errors.
Solution
Don't put wildcards on collections you will modify.
Why are you always using ?. Give the kids a name like Key or Value.
Edit: You should work through this tutorial fist: Lesson: Generics
Here is a method that sorts a Map by its keys. It makes use of the Collections.sort(List, Comparator) method.
static Map sortByKey(Map map) {
List list = new LinkedList(map.entrySet());
Collections.sort(list, new Comparator() {
public int compare(Object o1, Object o2) {
return ((Comparable) ((Map.Entry) (o1)).getKey())
.compareTo(((Map.Entry) (o2)).getKey());
}
});
Map result = new LinkedHashMap();
for (Iterator it = list.iterator(); it.hasNext();) {
Map.Entry entry = (Map.Entry)it.next();
result.put(entry.getKey(), entry.getValue());
}
return result;
}

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