How to design good custom key object for HashMap

By Lokesh Gupta | Filed Under: Java HashMap

Can we use an object as a key for a hashmap in java? This is a very popular interview question indeed. It is asked immediately after “How HashMap works?”. Lets make a reasoning around user defined object as key in hashmap in java.

1. The contract between hashCode() and equals()

The very basic need for designing a good key is that “we should be able to retrieve the value object back from the map without failure“, otherwise no matter how fancy data structure you build, it will be of no use. To decide that we have created a good key, we MUST know that “how HashMap works?”. I will leave, how hashmap works, part on you to read from linked post, but in summary it works on principle of Hashing.

Learn how HashMap works in java

Key’s hash code is used primarily in conjunction to its equals() method, for putting a key in map and then getting it back from map. So, our only focus point is these two methods. So if hash code of key object changes after we have put a key value pair in map, then its almost impossible to fetch the value object back from map. It is a case of memory leak.

On runtime, JVM computes hash code for each object and provide it on demand. When we modify an object’s state, JVM set a flag that object is modified and hash code must be AGAIN computed. So, next time you call object’s hashCode() method, JVM recalculate the hash code for that object.

2. Make HashMap key object immutable

For above basic reasoning, key objects are suggested to be IMMUTABLE. Immutability allows you to get same hash code every time, for a key object. So it actually solves most of the problems in one go. Also, this class must honor the hashCode() and equals() methods contract.

This is the main reason why immutable classes like String, Integer or other wrapper classes are a good key object candidate. and it is the answer to question why string is popular hashmap key in java?

How to make a class immutable

But remember that immutability is recommended and not mandatory. If you want to make a mutable object as key in hashmap, then you have to make sure that state change for key object does not change the hash code of object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also.

3. HashMap custom key object example

An example is always better for demonstration, right? Then lets have one.

In this example, I have created an Account class with only two fields for simplicity. I have overridden the hashcode and equals method such that it uses only account number to verify the uniqueness of Account object. All other possible attributes of Account class can be changed on runtime.

package com.howtodoinjava.demo.map;

public class Account 
{
	private int accountNumber;
	private String holderName;

	public Account(int accountNumber) {
		this.accountNumber = accountNumber;
	}

	public String getHolderName() {
		return holderName;
	}

	public void setHolderName(String holderName) {
		this.holderName = holderName;
	}

	public int getAccountNumber() {
		return accountNumber;
	}

	//Depends only on account number
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + accountNumber;  
		return result;
	}

	//Compare only account numbers
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Account other = (Account) obj;
		if (accountNumber != other.accountNumber)
			return false;
		return true;
	}

}

Will this cause any undesired behavior???

NO, it will not. The reason is that Account class’s implementation honor the contract that “Equal objects must produce the same hash code as long as they are equal, however unequal objects need not produce distinct hash codes.” i.e.

Whenever a.equals(b) is true, then a.hashCode() must be same as b.hashCode().
Whenever a.equals(b) is false, then a.hashCode() may/may not be same as b.hashCode().

4. Test the HashMap custom key object

Lets test our Account class for above analysis.

package com.howtodoinjava.demo.map;

import java.util.HashMap;

public class TestMutableKey
{
    public static void main(String[] args)
    {
        //Create a HashMap with mutable key
        HashMap<Account, String> map = new HashMap<Account, String>();
         
        //Create key 1
        Account a1 = new Account(1);
        a1.setHolderName("A_ONE");
        //Create key 2
        Account a2 = new Account(2);
        a2.setHolderName("A_TWO");
         
        //Put mutable key and value in map
        map.put(a1, a1.getHolderName());
        map.put(a2, a2.getHolderName());
         
        //Change the keys state so hash map should be calculated again
        a1.setHolderName("Defaulter");
        a2.setHolderName("Bankrupt");
         
        //Success !! We are able to get back the values
        System.out.println(map.get(a1)); //Prints A_ONE
        System.out.println(map.get(a2)); //Prints A_TWO
         
        //Try with newly created key with same account number
        Account a3 = new Account(1);
        a3.setHolderName("A_THREE");
         
        //Success !! We are still able to get back the value for account number 1
        System.out.println(map.get(a3)); //Prints A_ONE
    }
}

Program Output.

A_ONE
A_TWO
A_ONE

This is my understanding on designing custom key object for HashMap. If you disagree or feel otherwise, please drop me a comment.

Happy Learning !!

References:

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About Lokesh Gupta

A family guy with fun loving nature. Love computers, programming and solving everyday problems. Find me on Facebook and Twitter.

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kjp

how to do hashmap value object (is my approach correct ), I have written this code, how to get the city details,

My csv file contains these data,

city,city_ascii,lat,lng,country,iso2,iso3,admin_name,capital,population,id
Malishevë,Malisheve,42.4822,20.7458,Kosovo,XK,XKS,Malishevë,admin,,1901597212
Prizren,Prizren,42.2139,20.7397,Kosovo,XK,XKS,Prizren,admin,,1901360309
Zubin Potok,Zubin Potok,42.9144,20.6897,Kosovo,XK,XKS,Zubin Potok,admin,,1901608808
Kamenicë,Kamenice,42.5781,21.5803,Kosovo,XK,XKS,Kamenicë,admin,,1901851592
Viti,Viti,42.3214,21.3583,Kosovo,XK,XKS,Viti,admin,,1901328795
Shtërpcë,Shterpce,42.2394,21.0272,Kosovo,XK,XKS,Shtërpcë,admin,,1901828239
________________________

import java.io.File;
import java.io.FileNotFoundException;
import java.util.HashMap;
import java.util.Scanner;

public class HashTable {

    String csvFile;
    HashMap<String, CM> cityMap;
    Scanner input = new Scanner(System.in);
    
    public static void main(String[] args) {   	
    	HashTable csvFileReaders = new HashTable();
    	csvFileReaders.createHashTable();
   		for(int i=0;i<5;i++){
   				csvFileReaders.searchFromHashTable();
   		}
    }
    
    public HashTable() {
    	while(true){
    		System.out.println("Enther the File Path");
    		String csvFileInput = input.nextLine();
    		File csv = new File(csvFileInput);
    		
       		if(csv.exists()){
       			this.csvFile= csvFileInput;
       			break;
       		}
       		else{
       			System.out.println(" The filename you entered doesnt exist please re-enter the correct filepath\n");
       		}
    	}	
     }
    
    public void createHashTable() {
        cityMap = new HashMap<>(); 
        CM CmHashMap = new CM();       
        try (Scanner scanFile= new Scanner(new File(csvFile), "UTF-8")) {        	
            while (scanFile.hasNextLine()) {
            	String name = scanFile.nextLine();
            	String[] line = name.split(",");
            	for (int i = 0; i < line.length; i++) {
            		CmHashMap.setCity(line[0]);
            		CmHashMap.setCity_ascii(line[1]);
            		CmHashMap.setLat(line[2]);
            		CmHashMap.setLng(line[3]);
            		CmHashMap.setCountry(line[4]);
            		CmHashMap.setIso2(line[5]);
            		CmHashMap.setIso3(line[6]);
            		CmHashMap.setAdmin_name(line[7]);
            		CmHashMap.setPopulation(line[8]);
            		CmHashMap.setId(line[9]);
            		cityMap.put(line[1], CmHashMap);
            	}
            }
            scanFile.close();
        }        
        catch (FileNotFoundException e) {
        	e.printStackTrace();
        }
    }

    public void searchFromHashTable() {
   		System.out.println(" \n Enter the City name to be Searched from HashTable :   \n -> ");      
		String searchTerm = input.nextLine().toLowerCase();        
        //Search the city        
        long start = System.currentTimeMillis();
        	if (cityMap.containsKey(searchTerm)) {
        		
        		System.out.println("City name is : " + searchTerm + "\nCity details are accordingly in the order :"
                                       + "\n[city , city_ascii , lat , lng , country , iso2 , iso3 , admin_name , capital"
                                       + " , population , id] \n"+ cityMap.get(searchTerm) + "");
        	} 
        	else {
            System.out.println("Enter the correct City name");
        	}
        	
        long end = System.currentTimeMillis();
        System.out.println(" \n It took " + (end - start) + " Milli Seconds to search the result \n");    	
    }
}

___________________________

public class CM {
	
    private  String city;
    private  String city_ascii;
    private  String lat;
    private  String lng;
    private  String country;
    private  String iso2;
    private  String iso3;
    private  String admin_name;
    private  String capital;
    private  String population;
    private  String id;
    
   public CM( String city, String city_ascii, String lat, String lng, String country, String iso2, String iso3, String admin_name, String capital, String population, String id){
	   this.city = city;
	   this.city_ascii = city_ascii;
	   this.lat = lat;
	   this.lng = lng;
	   this.country = country;
	   this.iso2 = iso2;
	   this.iso3 = iso3;
	   this.admin_name = admin_name;
	   this.capital = capital;
	   this.population = population;
	   this.id = id;	   
   }
	   public CM() {
	// TODO Auto-generated constructor stub
	   }
	public String getCity() {
			return city;
		}

		public void setCity(String city) {
			this.city = city;
		}

		public String getCity_ascii() {
			return city_ascii;
		}

		public void setCity_ascii(String city_ascii) {
			this.city_ascii = city_ascii;
		}

		public String getLat() {
			return lat;
		}

		public void setLat(String lat) {
			this.lat = lat;
		}

		public String getLng() {
			return lng;
		}

		public void setLng(String lng) {
			this.lng = lng;
		}

		public String getCountry() {
			return country;
		}

		public void setCountry(String country) {
			this.country = country;
		}

		public String getIso2() {
			return iso2;
		}

		public void setIso2(String iso2) {
			this.iso2 = iso2;
		}

		public String getIso3() {
			return iso3;
		}

		public void setIso3(String iso3) {
			this.iso3 = iso3;
		}

		public String getAdmin_name() {
			return admin_name;
		}

		public void setAdmin_name(String admin_name) {
			this.admin_name = admin_name;
		}

		public String getCapital() {
			return capital;
		}

		public void setCapital(String capital) {
			this.capital = capital;
		}

		public String getPopulation() {
			return population;
		}

		public void setPopulation(String population) {
			this.population = population;
		}

		public String getId() {
			return id;
		}

		public void setId(String id) {
			this.id = id;
		}
		public String toString() {
	        return "\n The following details are of city : " + city +"\n The Ascii string would be : "
					+ city_ascii +"\n Its having the lattitude around : "
					+ lat + "\n and Longitude of : "+ lng +"\n It is situated in : "
					+ country +"\n These have iso code like  : "+ iso2 +" and : "+ iso3 +"\n It comes under  : "
					+ admin_name +" State \n Capital of this city is : "+ capital +"\n The population is around : "
					+ population +"\n ZIP code is : "+id+"\n ";
			}
}

arun singh

pretty deep and difficult to understand in one go.
but great article, thanks for the information.

bala

Save my day! Thanks Lokesh!

Divya

How do you access values of a hashmap in a class from a different class in Java? please help!

Awesome..felling that I m learning hashmap now..

Subodh

Hi Lokesh,
One more point to add to the answer is not using the transient variables in hashCode()
If the object is serailized, it would get default value for transient variable instead of instance value , making it impossible to locate

jyotsna

Hi Lokesh,
could you plz explain me below doubts ?
if HashMap key is a String object/user defined class obj then how the hashcode generates ?
HashMap hm= new HashMap();
hm.put(21, 121);
normally hashcode calulates : key %capacity = 21%4= 1 // suppose my Initial capacity is 4 and it will tore @ index position 1
same way want to know how it calculates for a String and userdefined class as a key ..

Sandeep

Interview questions:

public class Employee {
    private int id;
    private String name;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {/*
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Employee employee = (Employee) o;

        if (id != employee.id) return false;
        return name != null ? name.equals(employee.name) : employee.name == null;*/
        return false;

    }

    @Override
    public int hashCode() {
        /*int result = id;
        result = 31 * result + (name != null ? name.hashCode() : 0);
        return result;*/
        return 1;
    }
}

what is the output

System.out.println(employeeObjectMap.size());
System.out.println(employeeObjectMap.get(employee3));

RAJIV SARKAR

MapSize :1 as every time it generates same hashcode so same bucket is use for out opeation.
As equals always returns false it wont be able to get the object from map.

RAJIV SARKAR

MapSize will be number of put operations you do as every time it generates same hashcode so same bucket is use for out opeation but since equals method returns false every time it stores in different entry of map.
As equals always returns false it wont be able to get the object from map.

RAJIV SARKAR

please ignore my first answer

GURDEEP SINGH SAINI

is equals methods compare each key on the same bucket and if keys are equal then replace the values to the recent keys value?
please explain how equal method works in hashmap?

siddharhta

In Hash map equals method works on keys. Generally Hash code is may same or may not same for two different keys. if hash code is same and and the index is same but keys are different In this case hash map create a linked list and stores the values at the next node of present key value pair

Arun Vadivel

This article is very informative.. Thank you very much..

Sangram

above hashcode method :

final int prime = 31;
int result = 1;
result = prime * result + accountNumber;
return result;

is same as :
return 31 * accountNumber;

Any reason why you wrote those 4 lines?

1. The contract between hashCode() and equals()

2. Make HashMap key object immutable

3. HashMap custom key object example

4. Test the HashMap custom key object

About Lokesh Gupta

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