>> HashMap in Java

HashMap in Java

HashMap is a class  which implements Map (an interface).

Hashmap has set of Key and Value.

Each key - value set must have unique keys but values can be duplicate.

Hashmap accepts only one null Key but there can be multiple null values.

There is no specific order of the elements in the hashmap.

Syntax:

HashMap

To enter a key - value set we use "put" and to retrieve the VALUE  using the KEY , we use "get".

Example:

//To use HashMap

import java.util.HashMap; //You need to import this class

public class HashMapTest {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("Name", "Smith"); // This is a key value pair being stored in Hashmap

            

             mapObject.put("Salary","2000");

            

             mapObject.put("Department","Technical");

            

             //Now to retrieve the value using respective KEY.

             String name = mapObject.get("Name");

             System.out.println("Name is : "+name);

            

             String salary = mapObject.get("Salary");

             System.out.println("Salary is : "+salary);

            

             String dept = mapObject.get("Department");

             System.out.println("Department is : "+dept);

            

       }

}

Output:

Name is : Smith

Salary is : 2000

Department is : Technical

You can put different data types for Key-Value:

Example 1:

import java.util.HashMap; //You need to import this class

public class HashMapTest {

       public static void main(String[] args) {

            

             HashMapInteger

> mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("SNO", 10); // This is a key value pair being stored in Hashmap

            

             mapObject.put("Salary",2000);

            

             mapObject.put("Phone", 1234567890);

            

             //Now to retrieve the value using respective KEY.

             int sno = mapObject.get("SNO");

             System.out.println("SNO is : "+sno);

            

             int salary = mapObject.get("Salary");

             System.out.println("Salary is : "+salary);

            

             int ph = mapObject.get("Phone");

             System.out.println("Phone is : "+ph);

            

       }

}

OutPut:

SNO is : 10

Salary is : 2000

Phone is : 1234567890

Example 2:

//To use HashMap

import java.util.HashMap; //You need to import this class

public class HashMapTest {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put(1, 10.11f); // This is a key value pair being stored in Hashmap

            

             mapObject.put(2,20.25f);

            

            

             //Now to retrieve the value using respective KEY.

             float value1 = mapObject.get(1);

             System.out.println("Value1 is : "+value1);

            

             float value2 = mapObject.get(2);

             System.out.println("Value2 is : "+value2);

            

       }

}

OutPut:

Value1 is : 10.11

Value2 is : 20.25

Remove the key-value and other features in HashMap:

From above example, remove the key-value set with Key=2:

mapObject.remove(2);

After running above code, the key-value set 2-20.25 is removed from HashMap.

mapObject.size(); --- this will show the number of key value pairs in the HashMap, size of the hashmap.

mapObject.clear(); --- this will removes all the Key-Value sets from Hashmap. After using this , you run mapObject.size(); , this will return zero. And if you try to "get" the value , you will get "null"

mapObject.isEmpty(); --- To check if the hashmap is empty.

Examples:

import java.util.HashMap; //You need to import this class

public class HashMapTest {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("Name", "Smith"); // This is a key value pair being stored in Hashmap

            

             mapObject.put("Salary","2000");

            

             mapObject.put("Department","Technical");

            

             //Now to retrieve the value using respective KEY.

             String name = mapObject.get("Name");

             System.out.println("Name is : "+name);

            

             String salary = mapObject.get("Salary");

             System.out.println("Salary is : "+salary);

            

             String dept = mapObject.get("Department");

             System.out.println("Department is : "+dept);

            

            

            

             int size = mapObject.size();

             System.out.println("HashMap size is : "+size);

            

             mapObject.clear();

            

             int size1 = mapObject.size();

             System.out.println("HashMap size is : "+size1);

            

             String name1 = mapObject.get("Name");

             System.out.println("Name is : "+name1);

            

             String salary1 = mapObject.get("Salary");

             System.out.println("Salary is : "+salary1);

            

             String dept1 = mapObject.get("Department");

             System.out.println("Department is : "+dept1);

             Boolean Empty = mapObject.isEmpty() ;

             System.out.println("is the HashMap Empty ? "+ Empty);

            

            

       }

}

OutPut:

Name is : Smith

Salary is : 2000

Department is : Technical

HashMap size is : 3

HashMap size is : 0

Name is : null

Salary is : null

Department is : null

is the HashMap Empty ? false

To know all key-value pairs in HashMap:

import java.util.HashMap;

public class HelloWorld {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("Name", "Smith");

            

             mapObject.put("Salary","2000");

            

             mapObject.put("Department","Technical");

            

             System.out.println(mapObject);

            

                            

}

OutPut:

{Salary=2000, Department=Technical, Name=Smith}

To know/print all Keys and/or their value present in the HashMap(To iterate through HashMap):

import java.util.HashMap;

import java.util.Set; // you need to import this additional class

public class HelloWorld {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("Name", "Smith");

            

             mapObject.put("Salary","2000");

            

             mapObject.put("Department","Technical");

            

             System.out.println(mapObject);

            

             //To know all the keys

        Set allKeys = mapObject.keySet();

        for(String eachKEY: allKeys){

            System.out.println("Key = "+eachKEY);

           

            //To know value of each key

            System.out.println("Value for "+eachKEY+" = " + mapObject.get(eachKEY));

        }

            

       }

}

OutPut:

{Salary=2000, Department=Technical, Name=Smith}

Key = Salary

Value for Salary = 2000

Key = Department

Value for Department = Technical

Key = Name

Value for Name = Smith

To know/print if any Key and/or Value is Present in the HashMap:

import java.util.HashMap;

import java.util.Set; // you need to import this additional class

public class HelloWorld {

       public static void main(String[] args) {

            

             HashMap mapObject = new HashMap<>();

            

             //Entering values(Key-Value set) in HashMap

            

             mapObject.put("Name", "Smith");

            

             mapObject.put("Salary","2000");

            

             mapObject.put("Department","Technical");

            

             System.out.println(mapObject);

            

             //Check if the key is present

             if(mapObject.containsKey("Department")) {

                    System.out.println("The key is present.");

        } else {

             System.out.println("The key is not present.");

        }

            

             //Check if the value is present

             if(mapObject.containsValue("Smith")) {

                    System.out.println("The value is present.");

        } else {

             System.out.println("The value is not present.");

        }

            

       }

}

OutPut:

{Salary=2000, Department=Technical, Name=Smith}

The key is present.

The value is present.

Copy one hashmap to another hashmap:

import java.util.HashMap;

public class HelloWorld {

       public static void main(String[] args) {

            

             //First hashmap

             HashMap map1 = new HashMap<>();            

             map1.put("Name", "Smith");            

             map1.put("Salary","2000");

             map1.put("Department","Technical");

            

             System.out.println("map1 before copy " +map1);

            

             //Second hashmap

               HashMap map2 = new HashMap<>();

               map2.put("Education", "Engineering");

              

               System.out.println("map2 before copy "+map2);

           //Copy map1 to map2   

               map2.putAll(map1);

               System.out.println("map2 after copy "+map2);

               System.out.println("map1 after copy "+map1);

        }

}

OutPut:

map1 before copy {Salary=2000, Department=Technical, Name=Smith}

map2 before copy {Education=Engineering}

map2 after copy {Salary=2000, Department=Technical, Education=Engineering, Name=Smith}

map1 after copy {Salary=2000, Department=Technical, Name=Smith}

Additional Information

package test;

 

import java.util.HashMap;

import java.util.Map;

 

public class HashmapTest {

 

       public static void main(String[] args) {

             //using map interface

             Map<String, String> map = new HashMap<String, String>();

            OR Map map = new HashMap<String, String>();

            OR Map map = new HashMap<>(); //for 

             Map<Integer, Integer> map1 = new HashMap<Integer, Integer>();

             Map<String, Integer> map2 = new HashMap<String, Integer>();

             map.put("Name", "Ram");

             map1.put(2001, 100);

             map2.put("No", 15);

             System.out.println(map.get("Name"));

             System.out.println(map1.get(2001));

             System.out.println(map2.get("No"));

             //directly using hashmap class

             HashMap<String, String> hmap = new HashMap<String, String>();

            OR HashMap hmap = new HashMap<String, String>();

            OR HashMap hmap = new HashMap<>(); 

             HashMap<Integer, Integer> hmap1 = new HashMap<Integer, Integer>();

             HashMap<String, Integer> hmap2 = new HashMap<String, Integer>();

             hmap.put("State", "TX");

             hmap1.put(200, 100);

             hmap2.put("EmpNo", 105);

             System.out.println(hmap.get("State"));

             System.out.println(hmap1.get(200));

             System.out.println(hmap2.get("EmpNo"));

            

             //no parameters on left side

             HashMap hmap3 = new HashMap<String, Integer>();

             hmap3.put("EmpNo", 1000);

             System.out.println(hmap3.get("EmpNo"));

            

             Map map3 = new HashMap<String, Integer>();

             map3.put("EmpNo", 1001);

             System.out.println(map3.get("EmpNo"));

            

             //hashmap with float/decimal value, NOT using "f", Integer is working for float

             HashMap hmap4 = new HashMap<String, Integer>();

             hmap4.put("Pay", 123.45);

             System.out.println(hmap4.get("Pay"));

             //map with float/decimal value, NOT using "f", Integer is working for float

             Map map4 = new HashMap<String, Integer>();

             map4.put("Pay", 1001.123456789);

             System.out.println(map4.get("Pay"));

            

             //hashmap WITH OUT parameters

             HashMap hmap5 = new HashMap<>();

             hmap5.put("Pay", 100.0001);

             System.out.println(hmap5.get("Pay"));

            

             //map WITH OUT parameters

             //HashMap<String, float> hmap5 = new HashMap<String, float>(); Error

             //Map map5 = new HashMap<String, long>(); Error

             Map map5 = new HashMap<>();

             map5.put("float value", 1001.123456789f);//float, but no need to use the char f

             System.out.println(map5.get("float value"));

            

             Map map6 = new HashMap<>();

             //map6.put("long value", 10.1234789l); Error

             map6.put("double value", 10.1234789d);//double, but no need to use the char d

             System.out.println(map6.get("double value"));

            

            

       }

 

}

 Output:

Ram
100
15
TX
100
105
1000
1001
123.45
1001.123456789
100.0001
1001.1235
10.1234789