The HashMap in Java is one of the most popular Collection classes among Java programmers. After my article on How HashMap works in Java, which describes the theory part of Java HashMap and becomes hugely popular among Java programmers, I thought to share how to use HashMap in Java with essential and fundamental HashMap examples, but couldn't do that earlier and it was slipped but today we are here with all the HashMap examples about getting values to checking if a key exists in Map, I am going to share everything a Java developer should know about HashMap. The HashMap is a data structure, based on hashing, which allows you to store an object as a key-value pair, an advantage of using HashMap is that you can retrieve objects on constant time i.e. O(1) if you know the key.
How to convert a Map to List in Java? HashMap to ArrayList Example
Before converting a Map to a List in Java, we should be very clear about these data structures which are widely used in Java. So let's begin with Map. What is Map? Map is an Interface in Java which store key and value object. It's a Java representation of a popular hash table data structure that allows you to search an existing element in O(1) time, at the same time also makes insertion and removal easier. We use a key object to retrieve the value object by using hashing functionality provided by Map. As we have seen in how the get method of HashMap works, In Java, the equals() and hashcode() methods are an integral part of storing and retrieving objects from it. The map allows duplicate values but no duplicate keys.
How HashSet works in Java [Explained with Example]
Not many Java programmers know that HashSet is internally implemented using HashMap in Java, so if you know How HashMap works internally in Java, more likely you can figure out how HashSet works in Java. But, now a curious Java developer can question that, how come HashSet uses HashMap because you need a key-value pair to use with Map, while in HashSet we only store one object. Good question, isn't it? If you remember some functionality of the earlier class, then you know that HashMap allows duplicate values, and this property is exploited while implementing HashSet in Java.
How get() and put() methods of HashMap works in Java? [Explained]
In this article, I am revisiting a couple of interesting questions related to the internal working of HashMap in Java, mostly asked senior Java developers, ranging from 4 to 6 and up to 8 years of experience. I did cover a lot of these questions from HashMap, ranging from thread-safety to race conditions, in my post about the internal working of Java HashMap, but I thought to revisit two of those questions, how does get and put method of HashMap or Hashtable works internally in Java and what happens if two different keys return the same hashCode, how do you return value from HashMap in that case.
How to check if a Key Object Exists in HashMap Java? containsKey() Example Tutorial
Hello Java programmers, if you have working in Java programming language or writing server side programs then you may know that one of the common programming tasks while using HashMap in Java is to check if a given key exists in the map or not. This is supposed to be easy, right? Yes, it is easy if you know JDK API well, all you need to is call the containsKey() method, it returns true if the given key exists in HashMap, otherwise false; but I have seen many programmers write code like below which is not correct, which inspired me to write this blog post.
if(map.get(key) != null){
System.out.println("key exists in Map");
}
This code is fragile, it will not work if you have added null values into HashMap because HashMap does allow null values.
if(map.get(key) != null){
System.out.println("key exists in Map");
}
This code is fragile, it will not work if you have added null values into HashMap because HashMap does allow null values.
How to sort HashMap by values in Java 8 [using Lambdas and Stream] - Example Tutorial
In the past, I have shown you how to sort a HashMap by values in Java, but that was using traditional techniques of the pre-Java 8 world. Now the time has changed and Java has evolved into a programming language that can also do functional programming. How can you, a Java Programmer take advantage of that fact to do your day-to-day task better like how do you sort a Map by values in Java using lambda expressions and Stream API. That's what you are going to learn in this article. It will serve two purposes, first, it will tell you a new way to sort a Map by values in Java, and, second and more important it will introduce you to essential Java 8 features like Lambda Expression and Streams, which every Java Programmer should learn.
How to use ConcurrentHashSet from ConcurrentHashMap in Java 8 - Example Tutorial
There is no ConcurrentHashSet in JDK 8, but you can still create one for yourself using the ConcurrentHashMap class of java.util. Concurrent package. A new method is added into ConcurrentHashMap in JDK 8, newKeySet(), which allows you to create a concurrent hash set backed by a concurrent hash map. If you remember, whenever you get keys from the map, they are returned in a Set e.g. for the old keySet() method, because the map only has unique keys. Since the map doesn't allow duplicate keys, it can be used as a set, as long as you only care for keys or just one element.
How to initialize HashMap with values in Java? Example
There is often a situation where you would like to create a HashMap with some pre-defined mapping, but unfortunately, Java doesn't provide map literals like Groovy or Scala to create the map with values in the same line. If you remember, we have faced the same issue with other Collection classes as well e.g. ArrayList, Vector, or LinkedList. But ArrayList is lucky because you can still use the Arrays.asList() method to create and initialize the ArrayList in just one line as shown here. But, how do you initialize the HashMap with values in Java?
How to Sort HashMap in Java based on Keys and Values
HashMap is not meant to keep entries in sorted order, but if you have to sort HashMap based upon keys or values, you can do that in Java. Sorting HashMap on keys is quite easy, all you need to do is to create a TreeMap by copying entries from HashMap. TreeMap is an implementation of SortedMap and keeps keys in their natural order or a custom order specified by Comparator provided while creating TreeMap. This means you can process entries of HashMap in sorted order but you cannot pass a HashMap containing mappings in a specific order, this is just not possible because HashMap doesn't guarantee any order.
Top 21 Java HashMap Interview Questions and Answers
The java.util.HashMap is one of the workhorses of JDK. Along with ArrayList, it is one of the most used classes from Java's collection framework. There is hardly a real-world Java project, where I haven't seen the use of HashMap. It is an implementation of hash table data structure, and it's not a surprise that HashMap is so useful, as someone has rightly said, "if you could have just one data structure, make it a hash table". The hash table data structure allows you to search for a value in O(1) time if you have a key. In Java, several implementations of hash table data structures exist like Hashtable, ConcurrentHashMap, LinkedHashMap, etc. but HashMap is your general-purpose map.
Difference between HashMap vs TreeMap vs LinkedHashMap in Java
Hello guys, if you are wondering what is difference between HashMap, TreeMap and LinkedHashMap in Java and when to use HashMap, TreeMap and LinkedHashMap in Java then you are at the right place. Earlier, I have shared difference between HashSet, TreeSet, and LinkedHashSet and in this article, I will explain the difference between these three common Map implementation HashMsp, TreeMap, and LinkedHashMap. Though all three classes like HashMap, LinkedHashMap and TreeMap are
implementation of java.util.Map interface, there is some functionality difference
between them. Perhaps most notable difference between them comes from their
iteration order. HashMap makes absolute no guarantee about in which order you
can iterate their keys, any application depending upon iteration order of
HashMap is fragile, because it can change anytime. In fact, in Java 7,
iteration order of HashMap is different than Java 6.
3 Examples to Loop Map in Java - Foreach vs Iterator
There are multiple ways to loop through Map in Java, you can either use a foreach loop or Iterator to traverse Map in Java, but always use either Set of keys or values for iteration. Since Map by default doesn't guarantee any order, any code which assumes a particular order during iteration will fail. You only want to traverse or loop through a Map, if you want to transform each mapping one by one. Now Java 8 release provides a new way to loop through Map in Java using Stream API and forEach method. For now, we will see 3 ways to loop through each element of Map.
How to convert List Of of Object to Map of Object (key, value) In Java? Example Tutorial
Hello guys, if you have a list of object and you want to conver into Map of object or key value pair then you can use different ways like looping over list and then adding values to map, or you can use Collectors.toMap() by using Stream API, You can even use flatMap() function if you want to convert list of one type of object to Map of another type of object. All this is possible and I will show you code example of how to do that in this article, But, before we get into the process of how you can convert a list of one object to another in Java, let me tell you a bit more about what Java really is.
What is WeakHashMap in Java? HashMap vs WeakHashMap Example Tutorial
Hello friends, we are here today again on our journey to Java. I hope everyone is fine and ready to board our train of knowledge. Today we are gonna learn something very interesting and very exciting. Today's topic will definitely be very useful in coding and programming. This topic would surely decrease your time complexity, and space requirements for any task very significantly :p So what's the wait? Let's start!
2 Ways to sort a Map in Java by Keys? TreeMap and HashMap Example
So you have a Map in your Java program and you want to process its data in sorted order. Since Map doesn't guarantee any order for its keys and values, you always end up with unsorted keys and Map. If you really need a sorted Map then think about using the TreeMap, which keeps all keys in sorted order. This could be either natural order of keys (defined by Comparable) or custom order (defined by Comparator), which you can provide while creating an instance of TreeMap. If you don't have your data in a sorted Map then the only option that remain for you is to get the keys and values, sort them and then process your Map in that order.
How to Remove Entry (key/value) from HashMap in Java while Iterating? Example Tutorial
Hello guys, Can you remove a key while iterating over HashMap in Java? This is one of the interesting interview questions as well as a common problem Java developers face while writing code using HashMap. Some programmers will say No, you cannot remove elements from HashMap while iterating over it. This will fail fast and throw concurrent modification exceptions. They are right but not completely. It's true that the iterator of HashMap is a fail-fast iterator but you can still remove elements from HashMap while iterating by using Iterator's remove() method.
How to use Map.compute(), computeIfPresent() and ComputeIfAbsent in Java? HashMap and ConcurrentHashMap Example
The JDK 8 has added several useful methods in existing interfaces e.g. java.util.Map, java.util.Collection, and java.util.concurrent.ConcurrentMap. Thanks to default methods, the much-needed evolution of existing interfaces becomes possible. Out of many useful methods, one method which stands out to me is the compute() method, which allows you to update a value in ConcurrentHashMap atomically. As per Java documentation, The compute() function tries to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). The entire function is performed atomically.
Difference between HashMap vs IdentityHashMap in Java? Example
The IdentityHashMap is one of the lesser known Map implementations from JDK. Unlike general purposes Map implementations like HashMap and LinkedHashMap, it is very special and its internal working is quite different than HashMap. The main difference between IdentityHashMap and HashMap in Java is that the former uses the equality operator (==) instead of the equals() method to compare keys. This means you need the same key object to retrieve the value from IdentityHashMap, you cannot retrieve values by using another key that is logically equal to the previous key.
How to Synchronize HashMap in Java? Collections.synchronizedMap() Example Tutorial
Hello guys, if you have used HashMap class in Java before than you will know that HashMap is not synchronized, which means you cannot use it on multi-threaded Java programs without external synchronization. In other words, if you share an instance of HashMap between multiple threads, where each of them is either adding, removing or updating entries then it's possible that HashMap may lose its structure and not behave as expected. In simple words, exposing HashMap to multiple threads can corrupt the Map and you may not able to retrieve the data you want. If you have read my earlier article about HashMap, you know that during re-sizing its possible that HashMap exposed to multiple threads, may end up in an infinite loop.
Difference between WeakHashMap , IdentityHashMap, and EnumMap in Java?
Hello guys, if you are wondering what is the difference between WeakHashMap, IdentityHashMap, and EnumMap in Java then you are at the right place. In last article, we have seen difference between HashMap, TreeMap, and LinkedHashMap in Java and in this article we will difference between WeakHashMap, EnumMap, and IdnetityHashMap in Java. Apart from popular implementation like HashMap and LinkedHashMap, java.util.Map also has some specialized implementation classes e.g. IdentifyHashMap which uses == instead of equals() method for comparing keys, WeakHashMap which uses WeakReference to wrap the key object and a key/value mapping is removed when the key is no longer referenced from elsewhere, and EnumMap where keys are Enum constants.
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