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Editorial Team
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Asked: 2026-05-20T09:52:39+00:00

The SCJP 6 Study Guide from Bert Bates and Kathy Sierra states on page

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The SCJP 6 Study Guide from Bert Bates and Kathy Sierra states on page 554 (among other requirements) that x.hashCode() != y.hashCode() requires that x.equals(y) == false.

But the Javadoc for Object doesn’t mention such requirement explicitly. Quote:
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.

Should I take what Javadoc says as a material implication, such as eq -> hc? Then there would be no conflict between these two sources.

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1 Answer

  1. Editorial Team

    As z5h says, the statements are equivalent.

    For logical conditions x and y, “x implies y” is the same as “!y implies !x”.

    “If something is a bus, it’s red” is logically equivalent to “if something isn’t red, it’s not a bus.”

    This is contraposition.

    Should I take what Javadoc says as a material implication, such as eq -> hc.

    Yes, that’s exactly what it’s saying: two objects being equal under equals implies their hashcodes must be equal.

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