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    <p>It's not a problem when 2 different objects of 2 different types have the same hash code.</p> <p>Hopefully, when you are going to build your <code>HashMap</code> you are not going to mix Students and Teachers as the keys to that map. And even in the case when you want to do <code>HashMap&lt;Object, Object&gt;</code> you will be OK, because</p> <pre><code>assertFalse( new Teacher( "John Smith" ).equals( new Student( "John Smith" ) ); </code></pre> <p>This is why it's important to override both <code>hashCode</code> and <code>equals</code>.</p> <p>The only drawback of delegating to <code>Arrays.hashCode(Object[])</code> may be that sometimes it may be too expensive from the performance point of view.</p> <p>For example, in your case, this would be a much better hash method for either Teacher or Student.</p> <pre><code>@Override public int hashCode() { return name.hashCode(); } </code></pre>
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    1. POguava-libraries: Is Objects.hashCode(Object[]) collision safe?
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    1. USAlexander Pogrebnyak
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    1. COThis conversation is a bit reversed. Usually, developers override `equals` and forget `hashcode`. I don't see how `HashMap<Object, Object>` would be ok in this case. Where did your `assertFalse` come from? I can't find it in `HashMap`. From what I see, `HashMap` only uses `hashCode`, not `equals`. So, two different type objects could collide, and that's why I'm leery of `Objects.hashCode(Object[])`. I agree that it's always important to override `hashCode` and `equals` as a pair, but from my understanding, if two objects have the same hash code, they should be equal and vise versa.
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      1. USJohn McCarthy
    2. CO@nondescript1 - I guess you misunderstood equals and hashcode contract - you must override hashcode when you override equals and two equal objects should produce same hashcode but it's not vice versa. the general contract of Object#hashcode says - It is **not required** that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.
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      1. USPremraj
    3. CO@nondescript1 - and about your statement "From what I see, HashMap only uses hashCode, not equals" > how you conclude this? can you point me the doc/spec link? As per my understanding the HashSet will store the object in hash buckets by using hashcode value but will call equals on it to compare the equality. So if your Teacher and Student returns the same hashcode they will be placed in the same bucket but if you want to get Teacher from Map it will check the equals and then will return you the valid Teacher or null, so there won't be any collision.
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