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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.
Noncompliant Code Example

class MyClass {    // Non-Compliant - 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() {
    /* ... */
  }

}

See


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