## Why is this an issue?

According to the Java Language Specification, there is a contract between `equals(Object)` and `hashCode()`:

> If two objects are equal according to the `equals(Object)` method, then calling the `hashCode` method on each of the two objects must produce the same integer result.

> It is not required that if two objects are unequal according to the `equals(java.lang.Object)` method, then calling the `hashCode` method on each of the two objects must produce distinct integer results.

> However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

In order to comply with this contract, those methods should be either both inherited, or both overridden.

## How to fix it

### Code examples

#### Noncompliant code example

    class MyClass {    // Noncompliant - should also override "hashCode()"

      @Override
      public boolean equals(Object obj) {
        /* ... */
      }

    }

#### Compliant solution

    class MyClass {    // Compliant

      @Override
      public boolean equals(Object obj) {
        /* ... */
      }

      @Override
      public int hashCode() {
        /* ... */
      }

    }

## Resources

- CWE - CWE-581 - Object Model Violation: Just One of Equals and Hashcode Defined
- CERT, MET09-J. - Classes that define an equals() method must also define a hashCode() method