JAVA HASHMAP
Java Hashmap basic programs and important method.
INTRODUCTION:
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:
Syntax:
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:
Program 1:Add item in hashmap.
Here,you have need to add key-value pair.
CODE:
// 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(“Virat”, “India”);
capitalCities.put(“AB”, “Africa”);
capitalCities.put(“Kane”, “NZ”);
capitalCities.put(“SMITH”, “AUS”);
System.out.println(capitalCities);
}
}
OUTPUT:
{AB=Africa, SMITH=AUS, Kane=NZ, Virat=India}Note*:IT is stored in random order in map
Access an Item
To access a value in the HashMap, use the get() method and refer to its key:
Program 2:Access item using key
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
System.out.println(capitalCities.get(“England”));
}
}
OUTPUT:
London
Note*:Key we have passed England so it will printead london value.
Remove an Item
To remove an item, use the remove() method and refer to the key:
Program 3:Remove key values.
If we remove key then its value also automatically remove.
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
capitalCities.remove(“England”);
System.out.println(capitalCities);
}
}
OUTPUT:
{USA=Washington DC, Norway=Oslo, Germany=Berlin}
Clear Method.
To remove all items, use the clear() method:
Program 4: To remove all items, use the clear() method:
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
capitalCities.clear();
System.out.println(capitalCities);
}
}
OUTPUT:
{}
HashMap Size
To find out how many items there are, use the size() method:
Program 5:Calculating hashmap size.
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
System.out.println(capitalCities.size());
}
}
OUTPUT:
4
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:
Program 6:Printing hashmaplist using for loop
Note:Here we have used Keyset() so it will print all keys.
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
for (String i : capitalCities.keySet()) {
System.out.println(i);
}
}
}
OUTPUT:
USA
Norway
England
Germany
Program 7:Printing values of hashmap
Note:Here we have used values() so it will print all values
CODE:
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put(“England”, “London”);
capitalCities.put(“Germany”, “Berlin”);
capitalCities.put(“Norway”, “Oslo”);
capitalCities.put(“USA”, “Washington DC”);
for (String i : capitalCities.values()) {
System.out.println(i);
}
}
}
OUTPUT:
Washington DC
Oslo
London
Berlin
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:
Program 8:String integer key value pair
CODE:
// 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));
}
}
}
OUTPUT:
Name: Angie Age: 33
Name: Steve Age: 30
Name: John Age: 32
You can use any other mix data type such as string,float and so on.
Program 9:Change value using key and put method
CODE:
// Java program to change
// elements of HashMap
import java.io.*;
import java.util.*;
class ChangeElementsOfHashMap {
public static void main(String args[])
{
// Initialization of a HashMap
HashMap<Integer, String> hm
= new HashMap<Integer, String>();
// Change Value using put method
hm.put(1, “Somnath”);
hm.put(2, “Sachin”);
hm.put(3, “You”);
System.out.println(“Initial Map “ + hm);
hm.put(2, “good”);
System.out.println(“Updated Map “ + hm);
}
}
OUTPUT:
Initial Map {1=Somnath, 2=Sachin, 3=You}Updated Map {1=Somnath 2=good, 3=You}
Program 10:Checking its key is present or not using containsKey() method
CODE:
import java.util.*;
public class Hash_Map_Demo {
public static void main(String[] args)
{
// Creating an empty HashMap
HashMap<Integer, String> hash_map = new HashMap<Integer, String>();
// Mapping string values to int keys
hash_map.put(10, “Somnath”);
hash_map.put(15, “4”);
hash_map.put(20, “Sachin”);
hash_map.put(25, “Welcomes”);
hash_map.put(30, “You”);
// Displaying the HashMap
System.out.println(“Initial Mappings are: “ + hash_map);
// Checking for the key_element ‘20’
System.out.println(“Is the key ‘20’ present? “ +
hash_map.containsKey(20));
// Checking for the key_element ‘5’
System.out.println(“Is the key ‘5’ present? “ +
hash_map.containsKey(5));
}
}
OUTPUT:
Initial Mappings are: {20=Sachin, 25=Welcomes, 10=Somnath, 30=You, 15=4}Is the key '20' present? trueIs the key '5' present? false
Thank you.I have discussed all important method in hashmap .So I hope you like it.