BEST SITE FOR WEB DEVELOPERS
Java programming language. Lessons for beginners

Ua

Java HashMap


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);
  }
}
Try it Yourself »

Access an Item

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

Example

capitalCities.get("England");
Try it Yourself »

Remove an Item

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

Example

capitalCities.remove("England");
Try it Yourself »

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

Example

capitalCities.clear();
Try it Yourself »

HashMap Size

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

Example

capitalCities.size();
Try it Yourself »

Loop Through a HashMap

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

Note: Use the keySet() method if you only want the keys, and use the values() method if you only want the values:

Example

// Print keys
for (String i : capitalCities.keySet()) {
  System.out.println(i);
}
Try it Yourself »

Example

// Print values
for (String i : capitalCities.values()) {
  System.out.println(i);
}
Try it Yourself »

Example

// Print keys and values
for (String i : capitalCities.keySet()) {
  System.out.println("key: " + i + " value: " + capitalCities.get(i));
}
Try it Yourself »

Other Types

Keys and values in a HashMap are actually objects. In the examples above, we used objects of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:

Example

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

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

public class Main {
  public static void main(String[] args) {

    // Create a HashMap object called people
    HashMap<String, Integer> people = new HashMap<String, Integer>();

    // Add keys and values (Name, Age)
    people.put("John", 32);
    people.put("Steve", 30);
    people.put("Angie", 33);

    for (String i : people.keySet()) {
      System.out.println("key: " + i + " value: " + people.get(i));
    }
  }
}
Try it Yourself »