Java Map

//Create a map
Map<String, Integer> tel_dir_map = new HashMap<String, Integer>();

//Add elements by ‘put‘ method
tel_dir_map.put("Mike", 4567891);

//Retrieved by ‘get‘ method
int value = tel_dir_map.get("Mike");

//Number of key-value pairs by ‘size’
int map_size = tel_dir_map.size();

I will show you examples of declaring, adding key-value pairs, and accessing the values by keys in Maps by using implementation classes in the following part, so keep reading.

I will also show you examples of a few useful methods of Map Java interface.

An example of creating a Java Map with HashMap

In this example, a map object is created with the HashMap class.

The HashMap provides all map operations while it also permits null key and values.

In the example, the type of keys is set as String and values as integers. For demonstration, the phone directory map contains the names as keys and numbers as values. Have a look:

import java.util.*;

public class map_demo {

  public static void main(String[] args) {

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

       tel_dir_map.put("Mike", 4567891);
       tel_dir_map.put("Nina", 1424541);
       tel_dir_map.put("Danial", 45678977);
       tel_dir_map.put("Usman", 14712314);

          System.out.println("Telephone Numbers:");

           System.out.print("\t" + tel_dir_map);

   }

}

Output:

You see, the keys and values are separated by a comma as using the put method. In the output, you can see the order of insertion is different from the order displayed by using the System.out.print.

A demo of creating a Map with LinkedHashMap

If you require the same order of iteration as map entries are in the Map object, then use the Java LinkedHashMap implementation class.

See this example where a Map is created with LinkedHashMap class. The employee salary map contains four entries with employee Name and the respective salary.

For iterating through the LinkedHashMap, a for loop is used and key/value pairs are displayed as follows:

The code of creating the map with LinkedHashMap:

import java.util.*;

public class map_demo {

  public static void main(String[] args) {

    Map<String, String> salary_map = new LinkedHashMap<String, String>();

     salary_map.put("Mike", "$5000");
     salary_map.put("Nina", "$4500");
     salary_map.put("Danial", "$400");
     salary_map.put("Usman", "$6000");


         System.out.println("Displaying Map entries with LinkedHashMap");

         System.out.println("\n");

           for (Map.Entry<String, String> entry : salary_map.entrySet())

    {

             System.out.println("Employee Name: " + entry.getKey() + " and Salary = " + entry.getValue());

   }                             
 }

}

Output:

You may also notice the order of inserting and iteration is the same in above example.

To make it clearer, I am using the same example as above with the exception of using the HashMap instead of LinkedHashMap and see the output:

The same code with HashMap class:

import java.util.*;

public class map_demo {

  public static void main(String[] args) {

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

     salary_map.put("Mike", "$5000");
     salary_map.put("Nina", "$4500");
     salary_map.put("Danial", "$400");
     salary_map.put("Usman", "$6000");

        System.out.println("Displaying Map entries with HashMap");

         System.out.println("\n");

         for (Map.Entry<String, String> entry : salary_map.entrySet())

        {

         System.out.println("Employee Name: " + entry.getKey() + " and Salary = " + entry.getValue());

   }                             
 }

}

Output:

You can see the difference; the iteration order is different as compared to insertion order.

An example of using the iterator object

Rather than using the for loop, you may use iterator object with the while loop to iterate through the Map object.

See this example where I have created a Map object with name/salary pairs. After that, an iterator object is created and while loop is used to display the Map pairs:

The code with iterator object:

import java.util.*;

public class map_demo {

public static void main(String[] args) {

          Map<String, String> salary_map = new LinkedHashMap<String, String>();

          salary_map.put("Mike", "$5000");
          salary_map.put("Nina", "$4500");
          salary_map.put("Danial", "$400");
          salary_map.put("Usman", "$6000");

          System.out.println("Displaying Map key/value pairs:");

          Iterator<Map.Entry<String, String>> map_iterator = salary_map.entrySet().iterator();

          String str="";

          while (map_iterator.hasNext()) {

                        Map.Entry<String, String> pair = map_iterator.next();

                        str += pair.getKey() +"\t" +pair.getValue() + "\n";

                    }

System.out.println(str);

       }
}

Output:

A demo of Map clear() method

The clear() method of Map interface can be used for removing all key/value pairs from the specified Map.

In this example, a Map is created with a few pairs. The Map entries are displayed before and after using the clear() method so you may see the difference:

The code before and after using the clear Map method:

import java.util.*;

public class map_demo {

public static void main(String[] args) {

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

      tel_dir_map.put("Mike", 4567891);
      tel_dir_map.put("Nina", 1424541);
      tel_dir_map.put("Danial", 45678977);
      tel_dir_map.put("Usman", 14712314);

      System.out.println("The Map before clear method:");

      System.out.print("\t" + tel_dir_map);

      tel_dir_map.clear();

      System.out.print("\n");

      System.out.print("\n");

      System.out.println("The Map before after clear() method:");

      System.out.print("\t" + tel_dir_map);



   }

}

Output:

You see, an empty Map is displayed after using the clear() method.

Accessing the value of a key by using Map get method

By using the get() method of Map, the value of a key can be accessed.

import java.util.*;

import java.util.Scanner;

public class map_demo {

public static void main(String[] args) {

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

          tel_dir_map.put("Mike", 4567891);
          tel_dir_map.put("Nina", 1424541);
          tel_dir_map.put("Danial", 45678977);
          tel_dir_map.put("Usman", 14712314);


                    System.out.println("Enter Name (Mike, Nina, Danial, Usman): ");

                    Scanner scanner = new Scanner(System.in);

                    String Str_name = scanner.nextLine();

          System.out.println("The Number of " + Str_name + ": " + tel_dir_map.get(Str_name));

       }

}

Sample Output:

An example of using the remove method

The remove() method can be used for deleting a specified entry from the given Map. The remove method takes the key as a parameter.

In the following demo, the Map is displayed before using the remove() method. Again, the user is asked to enter a key to be removed from the Map.

If you enter a key from the displayed options, the map will display again after removing that key.

If you enter some other key that does not exist, the map will still display all four key/value pairs:

The code:

import java.util.*;

import java.util.Scanner;

public class map_demo {

public static void main(String[] args) {

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

          tel_dir_map.put("Mike", 4567891);
          tel_dir_map.put("Nina", 1424541);
          tel_dir_map.put("Danial", 45678977);
          tel_dir_map.put("Usman", 14712314);


          System.out.println("The Map before remove method:");
          System.out.print("\t" + tel_dir_map);
          System.out.print("\n");

          //User inout for the key to be removed from Map

          System.out.println("Enter a Key from: (Mike, Nina, Danial, Usman) ");

                    Scanner scanner = new Scanner(System.in);

                    String Str_name = scanner.nextLine();

                    tel_dir_map.remove(Str_name);

                          System.out.println("The Map after remove method:");

                          System.out.print("\t" + tel_dir_map);

       }

}

Sample output:

Know the size of the specified map by size() method

The size() method can be used to return the number of key/value pairs in the given map. The following example shows that:

import java.util.*;

import java.util.Scanner;

public class map_demo {

public static void main(String[] args) {

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

          fruit_map.put(1, "Apple");
          fruit_map.put(2, "Mango");
          fruit_map.put(3, "Banana");
          fruit_map.put(4, "Pine");
          fruit_map.put(5, "Apple");
          fruit_map.put(6, "Mango");
          fruit_map.put(7, "Orange");

          System.out.print("The size of map is: " + fruit_map.size());
       }

}

Output:

The put and putAll methods

You may be surprised to see the put method in the last part of this tutorial. This is because; the put method is used in each example of this tutorial.

As the name of the method suggests, the put method adds a key/value pair in the specified map.