// this is bad
public class Bar {
  public boolean equals(Object o) {
      // do some comparison
  }
}

// and so is this
public class Baz {
  public int hashCode() {
      // return some hash value
  }
}

// this is OK
public class Foo {
  public boolean equals(Object other) {
      // do some comparison
  }
  public int hashCode() {
      // return some hash value
  }
}

Justification:

“You must override hashCode() in every class that overrides equals(). Failure to do so will result in a violation of the general contract for Object.hashCode(), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.” from Effective Java, by Joshua Bloch.

If two objects are equal according to the {@code equals(Object)} method, then calling the {@code hashCode} method on each of the two objects must produce the same integer result.​ So, implementing an equals() method comparing 2 members, and then implementing a hashCode() which only calls super.hashCode() is just as bad.