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Postsdbo
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    <ol> <li><p>Immutable keys cannot change. Consequently, the hashcode which is computed at the time of insertion cannot change. So when you try to <em>get</em> an element from the map, the hashcode of the object to get is computed against known hashcodes. Had your key changed from outside(it was mutable), the new key's hashcode would be different from the one which you inserted.</p></li> <li><p>Let's see an example. for(<code>2</code> and <code>4</code>)</p> <pre><code>public class RandomPair { int p; int q; public RandomPair(int p, int q) { this.p = p; this.q = q; } @Override public int hashCode() { return 31 * p + q; } @Override public boolean equals(Object obj) { if (!(obj instanceof RandomPair)) { return false; } if (obj == this) { return true; } RandomPair other = (RandomPair) obj; if (p != other.p) return false; if (q != other.q) return false; return true; } public static void main(String[] args) { RandomPair pair = new RandomPair(10, 10); Map&lt;RandomPair, Integer&gt; map = new HashMap&lt;RandomPair, Integer&gt;(); map.put(pair, 1); System.out.println(map.get(pair)); //returns 1 //someone somewhere just changed the value of pair pair.p = 20; //the object was the same, someone somewhere just changed value of pair and now you can't //find it in the map System.out.println(map.get(pair)); //had you made p and q final, this sort of modification wouldn't be possible //Strings are immutable and thus prevent this modification } } </code></pre></li> <li><p>Since strings are immutable, the hashcode value once computed can be reused again. The hashcode is lazily computed. ie on the first call to hashcode and then the value of hashcode is cached.</p></li> </ol>
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    1. COThanks bsd. Can you also add a few lines about what my manager is talking about? We are receiving our data from a customer as a JSON string. So they will be changing the address for sure. I want to understand how that can affect the logic I have?
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    2. CODo not keep address a part of computing `hashcode` and `equals`. Fields which are constants, may be their unique 10 digit identification number(may be SSN) should be part of `hashcode`. Names may change, addresses may change. If you can can find out what changed in the `Person` object, you can remove that item from the map and re-insert with the new fresh values.
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