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POHow do maps check keys?
 

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  1. POHow do maps check keys?
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    <p>This has been buzzing me for quite some time now. It's basically that whenever I want to store my own set of objects (I called it MyObject here) as keys in the map, I can't get the keyvalue unless I have the same exact object somewhere saved in my class. Even though I tried to override the equals method in MyObject, where it usually returned true when comparing 2 objects with the same values, nothing changed.</p> <p>Just to give you a demonstration of what I mean:</p> <hr> <pre><code>Map&lt;Integer, Integer&gt; map = new HashMap&lt;Integer, Integer&gt;(); map.put(2, 3); System.out.println(map.get(2))); </code></pre> <p>Now, as you'd probably expect, it searches the map for the Integer-object 2 and then prints out 3. If the integer doesn't exist, it prints null. So far so good.</p> <hr> <pre><code>Map&lt;String, Integer&gt; map = new HashMap&lt;String, Integer&gt;(); map.put(new String("hi"), 3); System.out.println(map.get(new String("hi"))); </code></pre> <p>This one works also as expected. We're just getting the value for the key "hi".</p> <hr> <pre><code>Map&lt;MyObject, Integer&gt; map = new HashMap&lt;MyObject, Integer&gt;(); map.put(new MyObject(), 3); System.out.println(map.get(new MyObject())); </code></pre> <p>Even though, there isn't technically a difference between "new MyObject()" and "new MyObject()", it returns null anyway, unless I saved the new MyObject as an instance in my class and used that instance as the parameter for the get-method.</p> <p>Contrary to my MyObject, the map easily grabbed the key values, if the keys were Strings or Integers. Are those types just privileged or is there a way to tell the map: "Hey, the newly created object is similar with that one in that list"? How does a map compare objects?</p>
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    1. COMap will check keys depending on the implementation. HashMap uses the hashcode to do it.
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    2. CO@porfiriopartida Is there another Map that I could use to compare two similar objects?
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    3. COYes, there are few in the Map documentation. Just google for java.util.Map and see All Known Implementing Classes section.
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