How can I do unit test for hashCode()?

Question

How can I test the hashCode() function in unit testing?

public int hashCode(){     int result = 17 + hashDouble(re);     result = 31 * result + hashDouble(im);     return result; } 

Answer 1

Whenever I override equals and hash code, I write unit tests that follow Joshua Bloch's recommendations in "Effective Java" Chapter 3. I make sure that equals and hash code are reflexive, symmetric, and transitive. I also make sure that "not equals" works properly for all the data members.

When I check the call to equals, I also make sure that the hashCode behaves as it should. Like this:

@Test public void testEquals_Symmetric() {     Person x = new Person("Foo Bar");  // equals and hashCode check name field value     Person y = new Person("Foo Bar");     Assert.assertTrue(x.equals(y) && y.equals(x));     Assert.assertTrue(x.hashCode() == y.hashCode()); }