Hashmap in java working

Hierarchy of Java HashMap in Java

Characteristics of HashMap:

A HashMap is a data structure that is used to store and retrieve values based on keys. Some of the key characteristics of a hashmap include:

• Fast access time: HashMaps provide constant time access to elements, which means that retrieval and insertion of elements are very fast, usually O(1) time complexity.
• Uses hashing function: HashMaps uses a hash function to map keys to indices in an array. This allows for a quick lookup of values based on keys.
• Stores key-value pairs: Each element in a HashMap consists of a key-value pair. The key is used to look up the associated value.
• Supports null keys and values: HashMaps allow for null values and keys. This means that a null key can be used to store a value, and a null value can be associated with a key.
• Not ordered: HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved. However, LinkedHashMap is a variation of HashMap that preserves the insertion order.
• Allows duplicates: HashMaps allow for duplicate values, but not duplicate keys. If a duplicate key is added, the previous value associated with the key is overwritten.
• Thread-unsafe: HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies. If thread safety is required, ConcurrentHashMap can be used.
• Capacity and load factor: HashMaps have a capacity, which is the number of elements that it can hold, and a load factor, which is the measure of how full the hashmap can be before it is resized.

Creating HashMap in Java:

Java

Constructors in HashMap

HashMap provides 4 constructors and the access modifier of each is public which are listed as follows:

1. HashMap()
2. HashMap(int initialCapacity)
3. HashMap(int initialCapacity, float loadFactor)
4. HashMap(Map map)

Now discussing the above constructors one by one alongside implementing the same with help of clean Java programs.

1. HashMap()

It is the default constructor which creates an instance of HashMap with an initial capacity of 16 and a load factor of 0.75.

Java

Mappings of HashMap hm1 are : Mapping of HashMap hm2 are :

2. HashMap(int initialCapacity)

It creates a HashMap instance with a specified initial capacity and load factor of 0.75.

HashMap hm = new HashMap(int initialCapacity);

Java

Mappings of HashMap hm1 are : Mapping of HashMap hm2 are :

3. HashMap(int initialCapacity, float loadFactor)

It creates a HashMap instance with a specified initial capacity and specified load factor.

HashMap hm = new HashMap(int initialCapacity, float loadFactor);

Java

Mappings of HashMap hm1 are : Mapping of HashMap hm2 are :

4. HashMap(Map map)

It creates an instance of HashMap with the same mappings as the specified map.

Java

Mappings of HashMap hm1 are : Mapping of HashMap hm2 are :

Performing Various Operations on HashMap

1. Adding Elements

In order to add an element to the map, we can use the put() method. However, the insertion order is not retained in the Hashmap. Internally, for every element, a separate hash is generated and the elements are indexed based on this hash to make it more efficient.

Java

Mappings of HashMap hm1 are : Mapping of HashMap hm2 are :

2. Changing Elements

After adding the elements if we wish to change the element, it can be done by again adding the element with the put() method. Since the elements in the map are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we wish to change.

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3. Removing Element

In order to remove an element from the Map, we can use the remove() method. This method takes the key value and removes the mapping for a key from this map if it is present in the map.

Java

Mappings of HashMap are : Mappings after removal are :

4. Traversal of HashMap

We can use the Iterator interface to traverse over any structure of the Collection Framework. Since Iterators work with one type of data we use Entry < ? , ? >to resolve the two separate types into a compatible format. Then using the next() method we print the entries of HashMap.

Java

Key: vaibhav Value: 20 Key: vishal Value: 10 Key: sachin Value: 30

Important Features of HashMap

To access a value one must know its key. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to a small String that represents the same String. A shorter value helps in indexing and faster searches. HashSet also uses HashMap internally.
A few important features of HashMap are:

• HashMap is a part of java.util package.
• HashMap extends an abstract class AbstractMap which also provides an incomplete implementation of the Map interface.
• It also implements a Cloneable and Serializable interfaces. K and V in the above definition represent Key and Value respectively.
• HashMap doesn’t allow duplicate keys but allows duplicate values. That means A single key can’t contain more than 1 value but more than 1 key can contain a single value.
• HashMap allows a null key also but only once and multiple null values.
• This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. It is roughly similar to HashTable but is unsynchronized.

Internal Structure of HashMap

Internally HashMap contains an array of Node and a node is represented as a class that contains 4 fields:

It can be seen that the node is containing a reference to its own object. So it’s a linked list.

Performance of HashMap

The performance of HashMap depends on 2 parameters which are named as follows:

1. Initial Capacity – It is the capacity of HashMap at the time of its creation (It is the number of buckets a HashMap can hold when the HashMap is instantiated). In java, it is 2^4=16 initially, meaning it can hold 16 key-value pairs.

2. Load Factor – It is the percent value of the capacity after which the capacity of Hashmap is to be increased (It is the percentage fill of buckets after which Rehashing takes place). In java, it is 0.75f by default, meaning the rehashing takes place after filling 75% of the capacity.

3. Threshold – It is the product of Load Factor and Initial Capacity. In java, by default, it is (16 * 0.75 = 12). That is, Rehashing takes place after inserting 12 key-value pairs into the HashMap.

4. Rehashing – It is the process of doubling the capacity of the HashMap after it reaches its Threshold. In java, HashMap continues to rehash(by default) in the following sequence – 2^4, 2^5, 2^6, 2^7, …. so on.

If the initial capacity is kept higher then rehashing will never be done. But by keeping it higher increases the time complexity of iteration. So it should be chosen very cleverly to increase performance. The expected number of values should be taken into account to set the initial capacity. The most generally preferred load factor value is 0.75 which provides a good deal between time and space costs. The load factor’s value varies between 0 and 1.

Note: From Java 8 onward, Java has started using Self Balancing BST instead of a linked list for chaining. The advantage of self-balancing bst is, we get the worst case (when every key maps to the same slot) search time is O(Log n).

Synchronized HashMap

As it is told that HashMap is unsynchronized i.e. multiple threads can access it simultaneously. If multiple threads access this class simultaneously and at least one thread manipulates it structurally then it is necessary to make it synchronized externally. It is done by synchronizing some object which encapsulates the map. If No such object exists then it can be wrapped around Collections.synchronizedMap() to make HashMap synchronized and avoid accidental unsynchronized access. As in the following example:

Map m = Collections.synchronizedMap(new HashMap(. ));

Now the Map m is synchronized. Iterators of this class are fail-fast if any structure modification is done after the creation of the iterator, in any way except through the iterator’s remove method. In a failure of an iterator, it will throw ConcurrentModificationException.

The time complexity of HashMap:

HashMap provides constant time complexity for basic operations, get and put if the hash function is properly written and it disperses the elements properly among the buckets. Iteration over HashMap depends on the capacity of HashMap and the number of key-value pairs. Basically, it is directly proportional to the capacity + size. Capacity is the number of buckets in HashMap. So it is not a good idea to keep a high number of buckets in HashMap initially.

Applications of HashMap:

HashMap is mainly the implementation of hashing. It is useful when we need efficient implementation of search, insert and delete operations. Please refer to the applications of hashing for details.

Methods in HashMapassociate

• K – The type of the keys in the map.
• V – The type of values mapped in the map.

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Hashmap in java working

“Анна Ивановна Решетникова, 4211 717171” И как они собирались хранить такой номер паспорта в типе Integer? Тем самым показав, что номер паспорта хранить в базе данных как число — это не лучшая идея. Так как номера паспорта могут содержать дефисы, пробелы, буквы и т.д. Поправьте меня, если я не прав. Иначе у меня вопросы к господину Милану.

Скажите, почему хронология вывода на экран поменялась?

Получение списка всех ключей и значений Еще одна удобная особенность HashMap — можно по-отдельности получить список всех ключей и всех значений. Для этого используются методы keySet() и values() Для чего в примере создаются Set и ArrayList? Если в sout можно прямо вызвать методы keySet() и values()?

Возможно ли в HashMap переопределить метод toString() и если да, то каким образом? P/S: Спросил нынче модный ChatGPT, так он какую-то чушь городит: то пишет можно и даёт нерабочие коды, каждый раз разные, то пишет что нельзя потому что HashMap происходит от абстрактного класса. Хотя я уже понял что верить ему нельзя во всем, на вопрос можно ли реализовать в Java множественное наследование (не через интерфейсы) он бодро рапортовал, что конечно можно и вывалил код типа public сlass A extends B, C, который естественно не работает. Извините за возможный оффтоп. Так что роботы пока не завоюют мир, поэтому, вопрос по toString() все еще актуален. )

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