Access and modify the next entry set while iterating a map in Java

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While working on a problem statement, I need to access the next Entry of a Map and based on a condition, I need to modify the next entry value.

I have been considering the solution provided here: How to get the previous key/value and the next key/value in Maps where I can access the higherEntry using NavigableMap but it throws:

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.AbstractMap$SimpleImmutableEntry.setValue(AbstractMap.java:797)

which is totally understood. Here is the code snippet

for (Map.Entry<Character, Integer> current : myMap.entrySet()) {

    Map.Entry<Character, Integer> next = myMap.higherEntry(current.getKey());

    if (current.getValue() == next.getValue()) {
        // Code block
    } else if (current.getValue() > next.getValue()) {
        // Code block
    } else {
        next.setValue(1); // Line that throws Exception
    }
}

How can I Iterate through a Map, access and modify the next Entry when required?

Instead of using an enhanced for loop, you can use a normal for loop and keep track of the index. This is tricky because you need a way to access the next entry and it depends on if you are using a ordered implementation of Map (like LinkedHashMap). If you are not using LinkedHashMap or some similar ordered map this will not be consistent and the entries could get out of order. Assuming you are using an ordered map, you can first convert the entry set to an array and go from there. This would look like Map.Entry<Character, Integer>[] entries = myMap.entrySet().stream().toArray(Map.Entry[]::new); Then, you can use a for loop along with this array.

for (int i = 0; i < entries.length-1; i++) {
    Map.Entry<Character, Integer> current = entries[i];
    Map.Entry<Character, Integer> next = entries[i+1];
    //code that does stuff
}

Different Ways to Iterate Through a Map in Java, Entry object and by looping over them, you can easily get access to key and value object. Well, when it comes to choosing between different ways to iterate over Map in Java, modifying Map, then iterating over entry set using Java 1.5 foreach loop In next section, we will see code using Iterator, which can help you for� Using Map.entry<K,V>method. Map.Entry<K,V> is an interface. It returns a collection view of the map, whose elements are of this class. A map.entrySet() method returns a Set view of the mapping contained in the map. The changes in the map are reflected in the Set also and vice-versa. It also supports element removal, which removes the

TreeMap.higherEntry returns an immutable entry, and that explains the UnsupportedOperationException that you're seeing:

//exportEntry is being called by TreeMap.higherEntry(K)
static <K,V> Map.Entry<K,V> exportEntry(TreeMap.Entry<K,V> e) {
    return (e == null) ? null :
        new AbstractMap.SimpleImmutableEntry<>(e);
}

One possible solution/workaround is to collect your updated entries into a map and then modify your original map after the loop:

Map<Character, Integer> updates = new HashMap<>();

for (Map.Entry<Character, Integer> current : myMap.entrySet()) {

    Map.Entry<Character, Integer> next = myMap.higherEntry(current.getKey());

    if(null != next) {
        if (current.getValue() == next.getValue()) {
        } else if (current.getValue() > next.getValue()) {
        } else {
            updates.put(next.getKey(), 1); // entry to be updated
        }
    }
}

myMap.putAll(updates);

Note that I've added a null check as myMap.higherEntry may return null.

Best Way to Iterate Over Each Entry of HashMap in Java, When only the keys from a map are needed in a loop, iterating the keySet to iterate the entrySet , which will give access to both the key and value, instead. Here, we're converting our map to a set of entries and then iterating through them using the classical for-each approach. We can access a key of each entry by calling getKey() and we can access a value of each entry by calling getValue(). 3. Iterator and EntrySet

        Map.Entry<String, Integer> next = myMap.higherEntry(current.getKey());

        if(next != null)
        {
            if (current.getValue() == next.getValue())
            {
                //Code block
            }
            else if (current.getValue() > next.getValue()) {
                //Code block
            }

            else {
                myMap.put(current.getKey(), 1); 

            }
        }

Java: "entrySet()" should be iterated when both the key and value , Learn different ways of iterating through the entries of a Map in Java. Last modified: July 7, 2020 I just announced the new Learn Spring course, focused on the entrySet API returns a collection-view of the map, whose elements entry by calling getKey() and we can access a value of each entry by calling getValue(). HashMap member function entrySet() returns a set of Entry<K,V> in the HashMap and its backed by HashMap i.e. any items remover from the Entry Set will be removed from HashMap too. So lets Iterate over the HashMap using Entry Set Iterator and remove all the elements whose value is 10 while Iterating i.e.

The method higherEntry returns an immutable entry, but you code also lacks the handling of the case that there is no higher entry. On the other hand, the next entry returned by the iterator would be the right entry anyway (if there is one), so this can be done using an Iterator manually, like this:

Iterator<Map.Entry<Character, Integer>> it = myMap.entrySet().iterator();
if(it.hasNext()) {
    for(Map.Entry<Character, Integer> current=it.next(), next; it.hasNext(); current=next) {
        next = it.next();
        // don't compare Integer objects via ==, use equals
        if (current.getValue().equals(next.getValue())) {
            // Code block
        } else if (current.getValue() > next.getValue()) {
            // Code block
        } else {
            next.setValue(1);
        }
    }
}

A "not so clean" but reasonably simple alternative would be

Map.Entry<Character, Integer> current = null;
for(Map.Entry<Character, Integer> next: myMap.entrySet()) {
    if(current != null) {
        if (current.getValue().equals(next.getValue())) {
            // Code block
        } else if (current.getValue() > next.getValue()) {
            // Code block
        } else {
            next.setValue(1);
        }
    }
    current = next;
}

Iterate over a Map in Java, HashMap iteration in Java tutorial shows how to iterate over a HashMap in Java. last modified July 6, 2020 Map.Entry represents a key-value pair in HashMap . HashMap's entrySet() returns a Set view of the In the next example, we iterate over a key set with an iterator while using HashMap's keySet() method, which� Since all maps in Java implement Map interface, following techniques will work for any map implementation (HashMap, TreeMap, LinkedHashMap, Hashtable, etc.) Iterating over Map.entrySet() using For-Each loop : Map.entrySet() method returns a collection-view(Set<Map.Entry<K, V>>) of the mappings contained in this map. So we can iterate over key

Java HashMap iteration, Java. There are several ways of iterating over a Map in Java. Lets go over the most or values from the map, you can iterate over keySet or values instead of entrySet . while (entries.hasNext()) { Map.Entry<Integer, Integer> entry = entries .next(); View Background Image � Chrome Update Notifier � and a bunch of others. The entrySet() method is used to get a Set view of the mappings contained in this map. Declaration. Following is the declaration for java.util.HashMap.entrySet() method. public Set<Map.Entry<K,V>> entrySet() Parameters. NA. Return Value. The method call returns a set view of the mappings contained in this map. Exception. NA. Example

How to Iterate Over a Map in Java, The Java Map interface is a collection containing key + value pairs. Last update: 2020-04-06 with respect to the order of the elements when iterating the Map , and the time (big O You can then access and iterate keys and values without casting them. of a Java Map is via an entry Iterator obtained from the entry Set . Java Map Interface. A map contains values on the basis of key, i.e. key and value pair. Each key and value pair is known as an entry. A Map contains unique keys. A Map is useful if you have to search, update or delete elements on the basis of a key. Java Map Hierarchy

Java Map, Apart from navigation method e.g. hasNext() and next(), Iterator also in conjunction with iteration + any other operation which modifies the structure of Collection. Btw, you would need a Pluralsight membership to access this course, Map.Entry; import java.util.Set; /* * Java Program to remove key value� - How to loop a Map in Java. It is cheap, but not free. I gives you extra complexity to your code. You will feel it every time, when you will have to process 100 messages per second.

Comments
  • Why not just loop through the list of keys, then use the keys to retrieve the values for comparison, then use Map.put() to overwrite it? The implementation of the Map you are using uses the SimpleImmutableEntry implementation for the entries, which doesn't allow you to use setValue().