How to override hashcode in Java example - Tutorial

Equals and hashcode methods are two primaries but yet one of the most important methods for java developers to be aware of. Java intends to provide equals and hashcode for every class to test equality and to provide a hash or digest based on the content of the class. The importance of hashcode increases when we use the object in different collection classes which works on hashing principle e.g. hashtable and hashmap. A well-written hashcode method can improve performance drastically by distributing objects uniformly and avoiding a collision.

In this article, we will see how to correctly override the hashcode() method in java with a simple example.


We will also examine the important aspects of hashcode contracts in java. This is in continuation of my earlier post on overriding the equals method in Java, if you haven’t read it already I would suggest going through it.

General Contracts for hashCode() in Java

1) If two objects are equal by the equals() method then their hashcode returned by the hashCode() method must be the same.

2) Whenever the hashCode() method is invoked on the same object more than once within a single execution of the application, hashCode() must return the same integer provided no information or fields used in equals and hashcode is modified. This integer is not required to be the same during multiple executions of application though.

3) If two objects are not equaled by the equals() method it is not required that their hashcode must be different. Though it’s always good practice to return different hashCode for unequal object. Different hashCode for a distinct objects can improve the performance of hashmap or hashtable by reducing collision.

To better understand concept of equals and hashcode and what happens if you don’t override them properly I would recommend understanding of How HashMap works in Java

Overriding hashCode method in Java

We will follow step by step approach for overriding hashCode method. This will enable us to understand the concept and process better.

1) Take a prime hash e.g. 5, 7, 17 or 31 (prime number as hash, results in distinct hashcode for distinct object)

2) Take another prime as multiplier different than hash is good.

3) Compute hashcode for each member and add them into final hash. Repeat this for all members which participated in equals.

4) Return hash

  Here is an example of hashCode() method

   @Override

    public int hashCode() {

        int hash = 5;

        hash = 89  hash + (this.name != null ? this.name.hashCode() : 0);

        hash = 89  hash + (int) (this.id ^ (this.id >>> 32));

        hash = 89  hash + this.age;

        return hash;

    }

It’s always good to check null before calling hashCode() method on members or fields to avoid NullPointerException, if member is null than return zero. Different data types has different way to compute hashCode.Integer members are simplest we just add there value into hash, for other numeric data-type are converted into int and then added into hash. Joshua bloach has full tables on this. I mostly relied on IDE for this.

Better way to override equals and hashCode

In my opinion, a better way to override both equals and hashcode methods should be left to IDE. I have seen Netbeans and Eclipse and found that both have excellent support of generating code for equals and hashcode and their implementations seem to follow all best practices and requirement e.g. null check, instanceof check, etc and also frees you to remember how to compute hashcode for different data-types.

Let’s see how we can override the hashcode method in Netbeans and Eclipse.

In Netbeans

1) Write your Class.

2) Right click + insert code + Generate equals() and hashCode().

In Eclipse

1) Write to your Class.

2) Go to Source Menu + Generate hashCode() and equals()

Things to remember while overriding hashcode in Java

1. Whenever you override the equals method, hashcode should be overridden to be in compliance with equals hashcode contract.

2. hashCode() is declared in Object class and return type of hashcode method is int and not long.

3. For an immutable object, you can cache the hashcode once generated for improved performance.

4. Test your hashcode method for equals hashcode compliance.

5. If you don't override hashCode() method properly your Object may not function correctly on hash-based collection e.g. HashMap, Hashtable or HashSet.

A complete example of equals and hashCode

public class Stock {

       private String symbol;

       private String exchange;

       private long lotSize;

       private int tickSize;

       private boolean isRestricted;

       private Date settlementDate;

       private BigDecimal price;

      

      

       @Override

       public int hashCode() {

              final int prime = 31;

              int result = 1;

              result = prime * result

                           + ((exchange == null) ? 0 : exchange.hashCode());

              result = prime * result + (isRestricted ? 1231 : 1237);

              result = prime * result + (int) (lotSize ^ (lotSize >>> 32));

              result = prime * result + ((price == null) ? 0 : price.hashCode());

              result = prime * result

                           + ((settlementDate == null) ? 0 : settlementDate.hashCode());

              result = prime * result + ((symbol == null) ? 0 : symbol.hashCode());

              result = prime * result + tickSize;

              return result;

       }

       @Override

       public boolean equals(Object obj) {

              if (this == obj) return true;

              if (obj == null || this.getClass() != obj.getClass()){

                     return false;

              }

              Stock other = (Stock) obj;

             

return  

this.tickSize == other.tickSize && this.lotSize == other.lotSize && 

this.isRestricted == other.isRestricted &&

(this.symbol == other.symbol|| (this.symbol != null && this.symbol.equals(other.symbol)))&& 

(this.exchange == other.exchange|| (this.exchange != null && this.exchange.equals(other.exchange))) &&

(this.settlementDate == other.settlementDate|| (this.settlementDate != null && this.settlementDate.equals(other.settlementDate))) &&

(this.price == other.price|| (this.price != null && this.price.equals(other.price)));

                       

        

 }

}

Writing equals and hashcode using Apache Commons EqualsBuilder and HashCodeBuilder

EqualsBuilder and HashCodeBuilder from Apache commons are a much better way to override equals and hashcode method, at least much better than ugly equals, hashcode generated by Eclipse. I have written the same example by using HashCodebuilder and EqualsBuilder and now you can see how clear and concise they are.

    @Override

    public boolean equals(Object obj){

        if (obj instanceof Stock) {

            Stock other = (Stock) obj;

            EqualsBuilder builder = new EqualsBuilder();

            builder.append(this.symbol, other.symbol);

            builder.append(this.exchange, other.exchange);

            builder.append(this.lotSize, other.lotSize);

            builder.append(this.tickSize, other.tickSize);

            builder.append(this.isRestricted, other.isRestricted);

            builder.append(this.settlementDate, other.settlementDate);

            builder.append(this.price, other.price);

            return builder.isEquals();

        }

        return false;

    }

  

    @Override

    public int hashCode(){

        HashCodeBuilder builder = new HashCodeBuilder();

        builder.append(symbol);

        builder.append(exchange);

        builder.append(lotSize);

        builder.append(tickSize);

        builder.append(isRestricted);

        builder.append(settlementDate);

        builder.append(price);

        return builder.toHashCode();

    }

  

    public static void main(String args[]){

        Stock sony = new Stock("6758.T", "Tkyo Stock Exchange", 1000, 10, false, new Date(), BigDecimal.valueOf(2200));

        Stock sony2 = new Stock("6758.T", "Tokyo Stock Exchange", 1000, 10, false, new Date(), BigDecimal.valueOf(2200));

        System.out.println("Equals result: " + sony.equals(sony2));

        System.out.println("HashCode result: " + (sony.hashCode()== sony2.hashCode()));

    }

The only thing to concern is that it adds a dependency on the apache-commons jar, most people use it but if you are not using then you need to include it for the writing equals and hashcode method.

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