Java equals() and hashCode() Contracts

1. Overview

In this tutorial, we’ll introduce two methods that closely belong together: .equals() and .hashCode(). We’ll focus on their relationship with each other, how to correctly override them, and why we should override both or neither.

2. The .equals() Method

By default, the Object class defines both the .equals() and .hashCode() methods. As a result, every Java class implicitly has these two methods.:

class Money {
    int amount;
    String currencyCode;
}

Money income = new Money(55, "USD");
Money expenses = new Money(55, "USD");
boolean balanced = income.equals(expenses)

We would expect income.equals(expenses) to return true, but with the current implementation of the Money class, it won’t.

The default implementation of equals() in the Object class compares the identity of the object. In our example, the income and expenses instances of the Money class have two different identities. Therefore, comparing them with the .equals() method returns false.

To change this behavior we must override this method.

2.1. Overriding equals()

Let’s override the .equals() method so that it doesn’t consider only object identity but also the value of the two relevant properties:

@Override
public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof Money))
        return false;
    Money other = (Money)o;
    boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
      || (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
    return this.amount == other.amount && currencyCodeEquals;
}

Above, we have three conditions to check whether the Money instance is the same as any other object. First, if the object is equal to itself it will return true. Second, if it is not an instance of Money it will return false. Third, we compare it with the attributes of another Money class instance. In detail, we ensure that all attributes of the compared class match those of the comparing class.

2.2. The .equals() Contract

Java SE defines the contract that our implementation of the equals() method must fulfill. In short, most criteria follow common sense but we can define the formal rules that the equals() method must follow. It must be:

• reflexive: an object must equal itself
• symmetric: x.equals(y) must return the same result as y.equals(x)
• transitive: if x.equals(y) and y.equals(z), then also x.equals(z)
• consistent: the value of .equals() should change only if a property that is contained in .equals() changes (no randomness allowed)

We can look up the exact criteria in the Java SE Docs for the Object class.

2.3. Violating equals() Symmetry With Inheritance

If the criteria for .equals() is such common sense, then how can we violate it at all? Well, violations of the .equals() contract are more likely to occur when we extend a class that has overridden the .equals() method as well. Let’s consider a Voucher class that extends our Money class:

class WrongVoucher extends Money {

    private String store;

    @Override
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof WrongVoucher))
            return false;
        WrongVoucher other = (WrongVoucher)o;
        boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
          || (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
        boolean storeEquals = (this.store == null && other.store == null)
          || (this.store != null && this.store.equals(other.store));
        return this.amount == other.amount && currencyCodeEquals && storeEquals;
    }

    // other methods
}

At first glance, the Voucher class and its override for .equals() seem to be correct. Both .equals() methods behave correctly as long as we compare Money to Money or Voucher to Voucher. But what happens if we compare these two objects:

Money cash = new Money(42, "USD");
WrongVoucher voucher = new WrongVoucher(42, "USD", "Amazon");

voucher.equals(cash) => false // As expected.
cash.equals(voucher) => true // That's wrong.

Thus, we have a violation of the symmetry criteria.

2.4. Fixing equals() Symmetry With Composition

To avoid making mistakes, we should favor composition over inheritance.

Instead of subclassing Money, let’s create a Voucher class with a Money property:

class Voucher {

    private Money value;
    private String store;

    Voucher(int amount, String currencyCode, String store) {
        this.value = new Money(amount, currencyCode);
        this.store = store;
    }

    @Override
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof Voucher))
            return false;
        Voucher other = (Voucher) o;
        boolean valueEquals = (this.value == null && other.value == null)
          || (this.value != null && this.value.equals(other.value));
        boolean storeEquals = (this.store == null && other.store == null)
          || (this.store != null && this.store.equals(other.store));
        return valueEquals && storeEquals;
    }

    // other methods
}

Now .equals() will work symmetrically as the contract requires.

3. The .hashCode() Method

The .hashCode() method returns an integer representing the current instance of the class. We should calculate this value consistently with the class definition of equality.

For more details, check out our guide to .hashCode().

3.1. The .hashCode() Contract

Java SE also defines a contract for the .hashCode() method. A thorough look at this contract reveals how closely related .hashCode() and .equals() are.

All three criteria in the .hashCode() contract mention the .equals() method in some way:

• internal consistency: the value of hashCode() may only change if a property that is in equals() changes
• equals consistency: objects that are equal to each other must return the same hashCode
• collisions: unequal objects may have the same hashCode

3.2. Violating the Consistency of hashCode() and equals()

The second criterion of the .hashCode() contract has an important consequence: If we override equals(), we must also override hashCode(). This is by far the most widespread violation regarding the equals() and hashCode() methods contracts.

Let’s see such an example:

class Team {

    String city;
    String department;

    @Override
    public final boolean equals(Object o) {
        // implementation
    }
}

The Team class overrides only equals(), but it still implicitly uses the default implementation of hashCode() as defined in the Object class. Consequently, it will return a different hashCode() for every instance of the class and violate the second rule.

Now, if we create two Team objects, both with city “New York” and department “marketing,” they will be equal, but they’ll return different hashCodes.

3.3. HashMap Key With an Inconsistent hashCode()

But why is the contract violation in our Team class a problem? Well, the trouble starts when some hash-based collections are involved. Let’s try to use our Team class as a key of a HashMap:

Map<Team,String> leaders = new HashMap<>();
leaders.put(new Team("New York", "development"), "Anne");
leaders.put(new Team("Boston", "development"), "Brian");
leaders.put(new Team("Boston", "marketing"), "Charlie");

Team myTeam = new Team("New York", "development");
String myTeamLeader = leaders.get(myTeam);

We would expect myTeamLeader to return “Anne,” but with the current code, it doesn’t.

If we want to use instances of the Team class as HashMap keys, we have to override the hashCode() method so that it adheres to the contract; equal objects return the same hashCode.

Let’s see an example implementation:

@Override
public final int hashCode() {
    int result = 17;
    if (city != null) {
        result = 31 * result + city.hashCode();
    }
    if (department != null) {
        result = 31 * result + department.hashCode();
    }
    return result;
}

After this change, leaders.get(myTeam) returns “Anne” as expected.

4. When Do We Override .equals() and .hashCode()?

Generally, we want to override either both .equals() and .hashCode() or neither of them. We just saw in Section 3 the undesired consequences if we ignore this rule.

Domain-driven design can help us decide circumstances when we should leave them be. For entity classes, for objects having an intrinsic identity, the default implementation often makes sense.

However, for value objects, we usually prefer equality based on their properties. Thus, we want to override .equals() and .hashCode(). Remember our Money class from Section 2: 55 USD equals 55 USD, even if they’re two separate instances.

5. Implementation Helpers

We typically don’t write the implementation of these methods by hand. As we’ve seen, there are quite a few pitfalls.

One common option is to let our IDE generate the .equals() and .hashCode() methods.

Apache Commons Lang and Google Guava have helper classes to simplify writing using both methods.

Project Lombok also provides an @EqualsAndHashCode annotation. Note again how .equals() and .hashCode() “go together” and even have a common annotation.

6. Verifying the Contracts

If we want to check whether our implementations adhere to the Java SE contracts and best practices, we can use the EqualsVerifier library.

Let’s add the EqualsVerifier Maven test dependency:

<dependency>
    <groupId>nl.jqno.equalsverifier</groupId>
    <artifactId>equalsverifier</artifactId>
    <version>3.15.3</version>
    <scope>test</scope>
</dependency>

Now let’s verify that our Team class follows the equals() and hashCode() contracts:

@Test
public void equalsHashCodeContracts() {
    EqualsVerifier.forClass(Team.class).verify();
}

It’s worth noting that EqualsVerifier tests both the equals() and hashCode() methods.

EqualsVerifier is much stricter than the Java SE contract. For example, it makes sure that our methods can’t throw a NullPointerException. Also, it enforces that both methods, or the class itself, are final.

It’s important to realize that the EqualsVerifier‘s default configuration allows only immutable fields. This is a stricter check than what the Java SE contract allows. It adheres to a recommendation of Domain-Driven Design to make value objects immutable.

If we find some of the built-in constraints unnecessary, we can add a suppress(Warning.SPECIFIC_WARNING) to our EqualsVerifier call.

7. Conclusion 

In this article, we discussed the equals() and hashCode() contracts. We should remember to:

• Always override hashCode() if we override equals()
• Override equals() and hashCode() for value objects
• Be aware of the traps of extending classes that have overridden equals() and hashCode()
• Consider using an IDE or a third-party library for generating the equals() and hashCode() methods
• Consider using EqualsVerifier to test our implementation