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How to Iterate Any Map in Java?

Last updated:
13th Jun, 2023
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How to Iterate Any Map in Java?

In recent days, Java is one of the most popular programming languages. It is an essential platform for developing desktop, mobile and web applications. The popularity of the language is intact for several years because the trends of the technology in the language keeps evolving from time to time. The programmers tend to the most recent or the upgraded version of this programming language to make use of its evolved enhancements and features. This object-oriented programming language helps the developers in scripting the best applications for various range of devices and gadgets. It is important for the software developers to stay updated regarding the emerging trends in Java to formulate top-rated programs.

Map in Java:

The Map is one of the most significant Data Structures in Java. The java.util.Map, a Java Map Interface, denotes the mapping between a value and a key. A map in Java can accumulate pairs of values and keys where a specific value links to each key.  Once the value corresponding to a particular key is saved in the Map, it can be accessed anytime just by using the key designated to that particular value. Only one value can be mapped to a key in Java. Maps can not be iterated directly using the iterators since map is not a collection in Java. However, there are several ways in which we can iterate maps in Java. 

More about Java Maps:

Maps are mostly used for mapping the key-value association, such as dictionaries. We also use maps to accomplish lookups or to update and retrieve elements with the help of keys. Here are a few examples of maps in Java.

  • A Java map of error codes along with their descriptions.
  • A map of cities and their respective zip codes. 
  • A map of employees and their managers. In this case, each key corresponding to a manager is linked to a list of employees managed by him that corresponds to the value assigned to the key.
  • A map of students and their respective classes. Each class represents a key that corresponds to a list of students that corresponds to a value assigned to the key. 

To implement a Map in the programs, the Java compiler offers two interfaces, namely Map and SortedMap. There are three map classes viz., HashMap, TreeMap and LinkedHashMap. The Java maps do not allow duplicate keys. However, you can assign duplicate values. LinkedHashMap and HashMap allow null values and keys. On the other hand, TreeMap doesn’t permit any null value or key. A map cannot be traversed. However, you can ensure the iteration of maps by converting them into sets using the entrySet() and keyset() methods.

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How to iterate Map in Java?

There are various methods in which the Map in Java can be iterated. In simpler words, the contents of a Map are extracted using keyset(), entrySet() or valueSet(). Since the return values of all these methods are sets, the same working principle applies to all of them. A collection-view of a Map in Java is returned by the Map.entrySet API. The only way in which a single map entry reference can be obtained through the iterator of the collection view Java Map. The key and the value are returned by the entry.getKey() and the entry.getValue() methods, respectively.

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Why Iterate through A Map?

We must traverse or iterate across the map to access, alter, or remove data from a map.

Commonly Used Map Methods In Java

Let’s clarify what each of the three methods does before we iterate map in Java using them:

  • entrySet() returns a map’s collection-view, whose elements are taken from the Map.entry level. Entry.getValue() returns the matching value after the entry.getKey() function returns the key.
  • keySet() returns a Set of all the keys in this map.
  • values() returns a Set of all the values in this map.

Methods To Iterate Over  A Map In Java

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There are typically five different ways to Java iterate over map. 

  • The method Map.entrySet() returns a collection view (SetMap.entries (EntryK, V>>) of the mappings. The getKey() and getValue() methods of the Map.Entry<K, V> can therefore be used to traverse across the key-value pairs. Use this technique if you require both map keys and values in the loop. 
  • Map.values() returns a collection view of the values in this map, while Map.keySet() returns a Set view of the keys in this map. Therefore, you can traverse through keySet or values using the quick method of the for-each loop.
  • In the first way, a for-each loop is used with a Map.Entry<K, V>, although in this case, iterators are used. Instead of a map.Entry<K,V>, using iterators, has its benefit in utilizing the iterator.remove() function to remove entries from the map while iterating.
  • If you want to Java iterate map, you can use the Map.forEach(action) method and lambda expressions. This method is fast and clean.
  • Here, we loop over the keys using the Map.keySet() function before looking for the value for each key using the Map.get(key) method. This method is not used in practice. After all, it is quite inefficient because retrieving values using a key may take some time.

Differences between Map and Set in Java:

MapSet
Map in Java is an interface that enables the mapping of specific keys to specified values.Set is an interface that does not support duplicate elements. It is a component of the collection hierarchy. 
It provides an independent interface.This interface is an extension of the collection interface.
It represents the mapping between a key and a value.It aids in storing unique values.

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Pavan Vadapalli

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Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.