How can I do unit test for hashCode()?

java unit-testing junit hashcode
Tomasz Gutkowski picture Tomasz Gutkowski · Dec 15, 2010 · Viewed 34.2k times · Source

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

duffymo picture duffymo · Dec 15, 2010

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());
}

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