Can we use iterator with HashMap?

In Java HashMap, we can iterate through its keys, values, and key/value mappings.

Table of Contents

How can we iterate a HashMap in Java?

There is a numerous number of ways to iterate over HashMap of which 5 are listed as below:

Iterate through a HashMap EntrySet using Iterators.
Iterate through HashMap KeySet using Iterator.
Iterate HashMap using for-each loop.
Iterating through a HashMap using Lambda Expressions.
Loop through a HashMap using Stream API.

How iterator works in HashMap?

HashMap stores the data in (Key, Value) pairs, and you can access them by an index of another type. HashMap class implements Map interface which allows us to store key.
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Different Ways of Traversal

Using an Iterator.
Using enhanced for Loop (for-each loop)
Using forEach() Method.

How do you make a HashMap iterator?

Java – HashMap Iterator example

Create a HashMap and populate it with key-value pairs.
Get the Set of key-value pairs by calling entrySet() method.
Obtain the iterator for entry set.
Display the key & pairs using getKey() and getValue() methods of Map. Entry interface.

What is the best way to iterate over HashMap?

Lets start with different ways to iterating over HashMap first:

Using enrtySet() in for each loop. for (Map.Entry<String,Integer> entry : testMap.entrySet()) {
Using keySet() in for each loop. for (String key : testMap.keySet()) {
Using enrtySet() and iterator.
Using keySet() and iterator.

Does HashMap allow duplicate keys?

HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.

What is iterator in Java?

An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet. It is called an “iterator” because “iterating” is the technical term for looping. To use an Iterator, you must import it from the java. util package.

Which Map is faster in Java?

HashMap

HashMap will generally be fastest, since it has the best cache behavior ( HashMap iterates directly over the backing array, whereas TreeMap and LinkedHashMap iterate over linked data structures).

Can a HashMap have duplicate keys?

Why HashMap is faster than HashSet?

The reason that HashMap is faster than HashSet is that the HashMap uses the unique keys to access the values. It stores each value with a corresponding key and we can retrieve these values faster using keys during iteration. While HashSet is completely based on objects and therefore retrieval of values is slower.

Can we store null key in HashMap?

HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.

Why do we use iterator?

Iterator in Java is an object used to cycle through arguments or elements present in a collection. It is derived from the technical term “iterating,” which means looping through. Generally, an iterator in Java is used to loop through any collection of objects.

Where do we use iterator?

Iterator in Java is used to traverse each and every element in the collection. Using it, traverse, obtain each element or you can even remove. ListIterator extends Iterator to allow bidirectional traversal of a list, and the modification of elements. The iterator() method is provided by every Collection class.

Can HashMap have null values?

Does HashMap allow duplicates?

Does HashMap allow null key?

HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value. HashMap is generally preferred over HashTable if thread synchronization is not needed.

Can we have duplicate keys in HashMap?

Can a map have duplicates?

Points to remember: Map doesn’t allow duplicate keys, but it allows duplicate values. HashMap and LinkedHashMap allows null keys and null values but TreeMap doesn’t allow any null key or value.

What is iterator () in Java?

An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet. It is called an “iterator” because “iterating” is the technical term for looping.

What is the benefit of iterator in Java?

Advantages of Iterator in Java
Iterator in Java supports both read as well as remove operations. If you are using for loop you cannot update(add/remove) the Collection whereas with the help of an iterator you can easily update Collection. It is a Universal Cursor for the Collection API.

What is the benefit of iterator?

Use of an iterator simplifies the code and makes it general. Benefits of using this iterator API include: treats variables of all types, sizes, and shapes uniformly, whether they fit in memory or not, even if a single row won’t fit in memory. makes recursion unnecessary in handling arrays of arbitrary dimensionality.

Why HashMap is faster than hash table?

HashMap is not synchronized, therefore it’s faster and uses less memory than Hashtable. Generally, unsynchronized objects are faster than synchronized ones in a single threaded application.

Can HashMap have null key?

Why iterator is used?

Is iterator faster than for loop?

Iterator and for-each loop are faster than simple for loop for collections with no random access, while in collections which allows random access there is no performance change with for-each loop/for loop/iterator.

Can we use iterator with HashMap?