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Java HashMap

In the ArrayList chapter, you learned that Arrays store items as an ordered collection, and you have to access them with an index number (int type). A HashMap however, store items in "key/value" pairs, and you can access them by an index of another type (e.g. a String).

One object is used as a key (index) to another object (value). It can store different types: String keys and Integer values, or the same type, like: String keys and String values:

Example

Create a HashMap object called capitalCities that will store String keys and String values:

import java.util.HashMap; // import the HashMap class

 

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

Add Items

The HashMap class has many useful methods. For example, to add items to it, use the put() method:

Example

// Import the HashMap class

import java.util.HashMap;

 

public class Main {

  public static void main(String[] args) {

    // Create a HashMap object called capitalCities

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

 

    // Add keys and values (Country, City)

    capitalCities.put("England", "London");

    capitalCities.put("Germany", "Berlin");

    capitalCities.put("Norway", "Oslo");

    capitalCities.put("USA", "Washington DC");

    System.out.println(capitalCities);

  }

}

 

Access an Item

To access a value in the HashMap, use the get() method and refer to its key:

Example

capitalCities.get("England");

 

Remove an Item

To remove an item, use the remove() method and refer to the key:

Example

capitalCities.remove("England");

 

To remove all items, use the clear() method:

Example

capitalCities.clear();

 

HashMap Size

To find out how many items there are, use the size() method:

Example

capitalCities.size();

 

Loop Through a HashMap

Loop through the items of a HashMap with a for-each loop.

Note:

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