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Java HashMap
Java HashMap
In the ArrayList chapter, you learned that Arrays stores items as a ordered order, and you should access them by reference number (int type). HashMap however, save items with "key / value" pairs, and you can access them with another type indicator (eg a String).
One item is used as a key (index) for another item (value). It can store different types: String keys with Integer values, or similar type, such as: Cord keys and cable 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 features. 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 value in 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 delete 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: Use the keySet() method if you only want keys, and use the values() method if you only want values:
Example
// Print keys
for (String i : capitalCities.keySet()) {
System.out.println(i);
}
Example
// Print values
for (String i : capitalCities.values()) {
System.out.println(i);
}
Example
// Print keys and values
for (String i : capitalCities.keySet()) {
System.out.println("key: " + i + " value: " + capitalCities.get(i));
}
Other Types
Keys and values in HashMap items actually. In the examples above, we used "String" items. Remember that String in Java is an object (not an old version). To use other types, such as int, you must specify an equal wrapper category: Integer. For other classic types, use: Boolean boolean, Character for Character, 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));
}
}
}