Implementations of Object#hashCode should not incorporate fields which the implementation of Object#equals does not. This violates the contract of hashCode: specifically, equal objects must have equal hashCodes.

class Foo {
  private final int a;
  private final int b;

  Foo(int a, int b) {
    this.a = a;
    this.b = b;
  }

  @Override
  public boolean equals(@Nullable Object o) {
    return o instanceof Foo && ((Foo) o).a == a;
  }

  @Override
  public int hashCode() {
    return a + 31 * b;
  }
}

Foo first = new Foo(10, 20);
Foo second = new Foo(10, 40);

first.equals(second) // true
first.hashCode() == second.hashCode() // false

The fix for this class is either to include a comparison of b in the #equals method, or remove b from #hashCode. The former is more likely to be correct.