Also For-Each loop will throw NullPointerException if you try to iterate over a map that is null, so before iterating you should always check for null references. In this tutorial we will learn about the four different ways of looping or iterating over Map in java. Simply put, we can extract the contents of a Map using keySet , valueSet or entrySet. This is the most common method and is preferable in most cases. HashMap int capacity — Used to create the HashMap with the given capacity.
Small tips and possible you may get in the future. Entries and stores internally into a set. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to contribute geeksforgeeks. First of all it is the only way to iterate over a map in older versions of Java. In this article, we will discuss all of them and also look at their advantages and disadvantages. Below are other related articles for your references:.
From performance point of view this method is equal to For-Each iteration. It is very basic concept to crack java interview for freshers. You will feel it every time, when you will have to process 100 messages per second. How to traverse or loop Map, HashMap or TreeMap in Java In next section of this Java tutorial, we will see four different ways of looping or iterating over Map in Java and will display each key and value from HashMap. An iterator should also be nondestructive in the sense that the act of iteration should not, by itself, change the collection. Certain principles hold for the use of iterators: In general, you should be able to have multiple traversals in progress at the same time; that is, an iterator should allow for the concept of nested looping. These basic ideas are easily extended to collections of more complicated objects such as employees , and where the processing for each object is a little more involved like giving each highly rated employee a 4.
In some cases, you will also need to have iterators with different traversal methods; for instance, preorder and postorder traversal of a tree, or depth-first and breadth-first traversal of a graph. Indeed Knut, Java 8 makes iterating over Map or List really a one liner, no boiler plate due to imperative nature of Java language any more. The will invoke the iterator method internally. This method might look redundant but it has its own advantages. Iterator to go through the HashMap. Below is the java program to demonstrate it.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Further Learning Anonymous In my opinion best way to iterate a Map is by using it's keySet instead of entrySet because you often need keys to perform filtering or any kind of processing logic, and only require value on need basis. In retrospect this was a bad name for the class. This is not a problem based on technical disagreements. Use of Generics and foreach loop result in very concise and elegant code, as shown below. Iterate HashMap using For-each Loop The For-Each loop is available for all the classes which implement the Iterable interface.
Anyway, that gives me an idea to write this Java tutorial and here are multiple ways to traverse, iterate or loop on a Map in Java, so remember this because you may also ask this question J. Since we are only using methods from java. HashMap , Hashtable , LinkedHashMap or even relatively new ConcurrentHashMap. Of course the operation being performed on the elements in a collection could possibly change some of the elements. It is used to iterate through the hashmap and also reduces the number of lines of code. Using for each to iterate through a HashMap This is the advised approach. It will iterate on each of the map entry and call the callback provided by us.
Iterating or looping Map Using keySet and iterator In this example of looping HashMap in java we have used java iterator instead of for each loop. If you are stuck with older version of Java less than 5 or planning to remove entries during iteration you have to use method 3. For each name in the collection, I will simply print its value to standard output. There are several ways to do that — 1. From performance point of view this method is equal to For-Each iteration. But in this tutorial we will go over all possible ways you could iterate for Map and List both. To avoid this, we need to use.
Try to process your emails for some strange service. Your Iterator may also throw ConcurrentModificationException , depending upon it's. I think entryset with iterator is the better way to iterate any kind of abj with respect to key as well as value said. Though foreach loop internally uses Iterator for traversing elements, It doesn't expose handle to that Iterator, which means you only have remove method of Map to remove entries. So we can iterate over key-value pair using getKey and getValue methods of. As a Java developer, everyone should know how to Iterate through HashMap, as it will be part of his routine programming. So If you need only keys or values from the map, you can iterate over keySet or values using for-each loops.
That is not too much in fact. Sometime back I have written an article which covers few methods. We have seen couple of ways to iterate over each entry, but as I said, best way is to use foreach loop and entry set, if you just need to traverse, without modifying actual Map. In the below code we will get EntrySet from the entrySet method, from the entrySet we will get the Stream through the stream method which we will be iterating using forEach. The major new features in Java 8 center on , along with related features such as streams, method references, and functional interfaces. Method 4: Iterating over keys and searching for values inefficient.
Using an Iterator to iterate over a List of strings. I think using Entry Set along with For loop for traversing Map is the fastest way to iterate. This method should be avoided. Related Post: We know that method returns a Set view of the keys contained in the map. Iterating or looping Map Using keySet and foreach loop In this we will use foreach loop to iterate over any map in java and using KeySet of the map to get keys.