HashMap in Java is an unordered collection that stores items (objects) as key value pairs (called elements).

It is expressed as HashMap<key, value> or HashMap<K, V>, where K is the key and V is the value. The key and the value are both objects. HashMap uses one object to get another.

If the key is specified, the corresponding value can be easily extracted from HashMap. Keys on the card must be unique, which means we can’t use duplicate data for keys in HashMap.

If we try to enter an entry with a double key, the card will replace the old entry with the new one.

The Java HashMap class is one of the four specific implementations of the mapping interface. It has been added to version 1.2 of Java. It’s in the java package. HashMap<K, V>. It is effective for finding values, adding a record and deleting a record.

HashMap Class Hierarchy in Java

The HashMap class in Java extends the AbstractMap class and implements the map interface. The diagram of the HashMap hierarchy is shown in the following figure. Hierarchy diagram

HashMap declaration Class

HashMap is a generic class that can be explained as follows:

public class HashMap<K,V>
extended AbstractMap<K,V>
implemented Folder<K,V>, cloned, serialized

Java HashMap offers class

There are several features of the HashMap class that you should take into account. They’re identical:

1. The basic structure of the HashMap data is HashTable. Simply put: HashMap uses the hashtable internally to store records. This means that accessing and adding a record is almost as fast as accessing a table.

2. The insertion sequence is not saved in HashMap (i.e. not saved). This means that we cannot get the keys and values in the same order as in HashMap.

3. It is based on the hash code of the key, not the hash code of the value.

4. Java HashMap contains only single keys, i.e. double keys are not allowed, but values can be duplicated. We derive values from the key.

5. Heterogeneous objects are allowed for both keys and values.

6. Java HashMap can only have a zero key, because double keys are not allowed.

7. Multiple zero values are allowed in HashMap.

8. HashMap in Java is not synchronized, which means we get unreliable results if we use multiple threads on the HashMap object.

9. Java HashMap implements cloned and serial ports, but does not implement random access.

10. HashMap is the best choice if our frequent operation is a search.

11. If there is no element in HashMap, an exception called NoSuchElementException is thrown.

12. Java HashMap only stores links to objects. That’s why we can’t use primitive data types like double or int. Instead, we can use a packaging class such as Whole or Double.

Hash Card Designers

The HashMap class in Java offers four manufacturers that look like this:

1. HashMap() : It is used to create an empty HashMap object with a standard initial capacity of 16 and a standard load factor of 0.75. The syntax for creating the hash map object is as follows:

HashMap hmap = new HashMap();
HashMap<K,V> hmap = new HashMap<K,V>(); // General form.

2. HashMap (Initial Power) : It is used to create a blank hash map object with a specified initial capacity at a standard load factor of 0.75. The general syntax is as follows:

HashMap<K,V> hmap = new HashMap<K,V>(int initialCapacity) ;

3. HashMap(m card) : This constructor is used to create a cardash object by initializing the elements of this m-card object.

4. HashMap (int initialCapacity, floating point load factor) : This constructor is used to create a hash card object with a certain initial speed and load factor.

The general syntax for creating a hash map object with an initial bandwidth and load factor is the same:

HashMap<K,V> hmap = new HashMap<>(int initialCapacity, floatloadFactor) ;

For example:
HashMap<String, integer> hmap = new HashMap<>(30, 0.7f) ;

In the syntax above, the capacity is the number of bins in which the hash card values can be stored. The occupancy factor is the number of knives used before the automatic capacity increase.

The value is a floating point number that varies from 0 (empty) to 1.0 (full). 0.7 means that the capacity increases when the buckets are 70% full. The standard power is 16 and the load factor 0.75, which is often sufficient.

HashMap methods in Java

In this section we have listed a number of important methods available in the HashMap class, which look as follows

1. invalid clear(() : Delete records from the specified card.

2. boolean isEmpty() : This method is used to check whether a card is blank or not. If the hash memory card does not display the value of the key, the value will be returned true, otherwise false.

3. Clone the object() : It is used to make a flat copy of this HashMap. Only the hashp and all the plates are cloned, not the elements. Both this card and the new card have common links to the same keys and values.

4. Install EntSet() It is used to return a type of record containing all key/value pairs of this card.

5. install keySet() : This method is used to obtain the type of key defined on this card.

6. V put(object key, object value) : This method is used to insert a set of data into the map.

7. Cancel the setup: This method is used to enter all records of the specified card into another card.

8. V putIfAbsent(key K, value V) : This method only adds a specified value to the specified key on the card if it is not already specified.

9. V (Delete object key) : This method is used to delete a record for the specified key.

10. Boolean deletion (object key, object value) : This method removes the specified value associated with a specific key from the card.

11. int Size() : This method indicates the number of records on the card.

12. Collective values() : This method gives a collective view containing all the values of the card.

13. V get(object key) : This method is used to obtain a value linked to the key. The way he comes back is the point.

14. Replace V (K-button, V-value) : This method is used to replace the specified value with the specified key.

15. Boolean replacement (key K, V old value, V new value) : This method is used to replace the old value of the given key with the new one.

16. Boolean containsValue(object v) : This method is used to determine whether the card contains a specific value. Displays the truth as a value v.

17. Boolean contains keys (object k) : This method is used to determine whether the card contains a specific key. Shows the truth if a key on this map is k.

18. Boolean equations (object o) : This method is used to compare the specified object with the chart.

Examples of Java HashMap for programs

1. Let’s create a program in which we simply add entries to the HashMap and display them on the console.

Program source code 1 :

import java.util.HashMap ;
public class HashMapEx1 [
public static void main(String[] args)
[
// Create HashMap.
HashMap<String,Integer> hmap = new HashMap<>() ;

// The HashMap control is empty or not
Empty Boolean = hmap.isEmpty();
System.out.println(Is HashMap empty: +empty) ;

// Adding records to the hash card.
hmap.put(John, 24); // hmap.size() is 1.
hmap.put(Deep, 22); // hmap.size() is 2.
hmap.put(Shubh, 15); // hmap.size() is 3.
hmap.put(Riky, 22); // hmap.size() is 4. // Add a double value.
hmap.put(Mark, 30); // hmap.size() is 5.

System.out.println(records in HashMap: +hmap);
int Size = hmap.size();
System.out.println(HashMap: +size) ;

// Added a zero key and a value.
hmap.put(null, zero); // hmap.size() is 6.
System.out.println(Updated data in HashMap: +hmap);
}
}

A way out:
HashMap is empty: real HashMap
entries: {Riky=22, Shubh=15, John=24, Mark=30, Deep=22}HashMap size
: 5
Updated HashMap data: {zero=zero, Riky=22, Shubh=15, John=24, Mark=30, Low=22}.

As you can see from the result above, the records in HashMap do not have a special order in which they are inserted into the map. We saved String as key and Integer as values, so we use HashMap<String, Integer> as type. The well() method adds entries to the map.

2. Let’s take the example of a program in which we try to add double keys and values to HashMap.

Program source code 2 :

import java.util.HashMap ;
public class HashMapEx2 {
public static void main(String[] args)
{
HashMap<Integer, String> hmap = new HashMap<>() ;

hmap.put(5, banana);
hmap.put(10, mango);
hmap.put(15, apple);
System.out.println(records in HashMap: +hmap);
System.out.println(HashMap size: +hmap.size() ;

// Adding a secondary key to the hash card.
hmap.put(10, guava); // Always hmap.size 3.
hmap.put(20, banana); // Added double value.
System.out.println(Updated data in HashMap: +hmap);
System.out.println(Size after adding a double value: +hmap.size());
}
}

A way out:
HashMap Registration: {5=banana, 10=mango, 15=apple}
Size HashMap : 3
Updated HashMap data: {20= banana, 5= banana, 10= guava, 15= apple}
Size after adding a double value : 4

As you can see in this program, we cannot store duplicate keys in HashMap. However, we tried to save a duplicate of the key with a different value, but it only replaced the value.

3. Let’s create a program in which we perform the removal operation. We will delete the HashMap record using the remove() method. Look at the source code.

Program source code 3 :

import java.util.HashMap ;
public class HashMapEx3 {
public static void main(String[] args)
{
HashMap<Character,String> hmap = new HashMap<>() ;

hmap.put(R, red);
hmap.put(O, orange);
hmap.put(G, green);
hmap.put(B, brown);
hmap.put(W, white) ;

// Display of HashMap items.
System.out.println(records in HashMap: +hmap) ;

// Deletion of key value pairs for key B.
The object removeEntry = hmap.remove(‘B’) ;
System.out.println(deleted entry: +removeEntry) ;
System.out.println(HashMap entries after deletion: +hmap) ;

// The record check has been deleted or not.
Object removeEntry2 = hmap.remove(‘W’, White);
System.out.println(Removed records: +removeEntry2);
System.out.println(HashMap updated records: +hmap);
}
}

A way out:
Registration on HashMap : {R = red, B = brown, G = green, W = white, O = orange}
Remote shooting: Brown HashMap records
after removal : {R=Red, G=Green, W=White, O=Orange}
record deleted: real
HashMap updated records : {R=Red, G=Green, O=Orange}

4. Let’s take an example of a program in which we replace the specified value with the specified key. Look at the source code.

Program source code 4 :

import java.util.HashMap ;
public class HashMapEx1 {
public static void main(String[] args)
{
HashMap<Character,String> hmap = new HashMap<>() ;

hmap.put(R, red);
hmap.put(O, orange);
hmap.put(G, green);
hmap.put(B, brown);
hmap.put(W, white) ;

// Display of HashMap items.
System.out.println(records in HashMap: +hmap) ;

// Replace the specified value with the specified key.
The replaceValue object = hmap.replace(‘B’, black);
System.out.println (replaced value: +replaceValue);
System.out.println(HashMap: +hmap updated data) ;

boolean replaceValue2 = hmap.replace(‘G’, green, greenish);
System.out.println(If the value has changed: +replaceValue2);
System.out.println(hmap);
}
}

A way out:
Registration on HashMap : {R=Red, B=Brown, G=Green, W=White, O=Orange}
Replace value : Brown
Updated HashMap Items : {R=Red, B=Black, G=Green, W=White, O=Orange}
The value is replaced by: true
{R=Red, B=Black, G=Green, W=White, O=Orange}.

Using HashMap in Java

Java HashMap may be the best choice if we want to perform a search. It’s designed to find things quickly. The best example of this type is the phonebook. The person’s name (line) can be used to find the person’s phone number.

Let’s look at an example of a simple program. Take a look at the following source code.

Program source code 5 :

import java.util.HashMap ;
public class HashMapEx1 {
public static void main(String[] args)
{
HashMap<String, Long> hmap = new HashMap<>() ;

hmap.put(Deep, 8292736478L);
hmap.put(Shubh, 8123543268L);
hmap.put(Mark, 9876789142L);
hmap.put(Ricky, 8768976872L) ;

// You can get a number with your key by calling the get() method.
Long number = hmap.get(Deep) ;
System.out.println(Deep phone number: + number) ;

Long number2 = hmap.getOrDefault(Steave, -1L);
System.out.println(Alex’s phone: + number2);
}
}

A way out:
Deep phone number: 8292736478
Alex : -1

In this example we created a HashMap, called a phonebook, with keys (name of the person), which are strings, and values (phone number of the person), which are long objects. Objects are saved in the hash card by calling the well method (object key, object value).

This method saves the key element of the map as name and the value as phone number. hmap.put(John, 9431676282L);. If the specified key is not found, it returns -1 by default.

We hope this tutorial has covered the most important points about HashMap in Java using sample programs. I hope you understand the subject.
Thanks for reading!

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