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What is a HashMap in Java?

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Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

Arrays store items in an ordered collection and are accessed using an index number (which is an integer). HashMap stores items as key/value pairs. Values can be accessed by indexes, known as keys, of a user-defined type. Java HashMap class implements the map interface by using a hash table1.

Declaration

A Java map is declared using the keyword Map. This is followed by angle brackets <> which containe data types for keys and values. The first parameter is the data type for the key and the second parameter is the data type for the mapped value. This is followed by the name of the map.

import java.util.HashMap; // import HashMap class
import java.util.Map; // import Map Interface
class MyClass {
public static void main( String args[] ) {
HashMap<Integer, String> shapes = new HashMap<Integer,String>(); // Create an ArrayList object with string data type
}
}

Useful methods in HashMap

Below are some useful methods in the HashMap class:

Add an item: The put() method is used to add a new key/value pair in a HashMap or it can replace the value of an existing key. First parameter is the key and second parameter is the mapped value.

shapes.put(6,"hexagon")

Access an item: The get() method takes a key as an input to access its associated value in the HashMap.

shapes.get(3)

Remove all items: The clear() method is used to remove all elements in a HashMap.

shapes.clear()

Remove an item: The remove() method takes a specific key as the input and removes its associated value in the HashMap.

shapes.remove(1)

Check if HashMap is empty: The isEmpty() method is to check whether the map is empty or not. It returns true if the map is empty and false if it is not.

shapes.isEmpty()

Size of HashMap: The size() method is used to find the number of key/value pairs in a HashMap.

shapes.size()

HashMap implementation

Below is a Java code implementation of a HashMap:

import java.util.HashMap; //importing the HashMap class
import java.util.Map; // importing the Map interface
class MyClass {
public static void main( String args[] ) {
HashMap<Integer,String> shapes = new HashMap<Integer,String>(); //create a HashMap with Integer keys and String Values
shapes.put(4,"square"); // add square at key 4
shapes.put(3,"triangle"); // add triangle at key 3
shapes.put(6,"hexagon"); // add hexagon at key 6
shapes.put(8,"octagon"); // add octagon at key 8
shapes.put(5,"pentagon"); // add pentagon at key 5
System.out.println(shapes); // print keys and mapped values
shapes.remove(3); // removing value at key 3
shapes.remove(5); // removing value at key 5
System.out.println("Updated map:");
System.out.println(shapes); // prints keys and mapped values
shapes.put(4,"rhombus"); // replaces value at key 4 with rhombus
System.out.println("Updated map:");
System.out.println(shapes); // prints keys and mapped values
System.out.println("Size of map is "+shapes.size());
shapes.clear(); // removes all key/value pairs in the HashMap
System.out.println("Updated map:");
System.out.println(shapes); // prints keys and mapped values
}
}

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hashmap
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Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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