![]() Note, that even if only one of the methods hashCode() or equals() return different results(when a property is modified), the key becomes useless from the lookup perspective. To do away with this issue altogether, recommendation is to use immutable classes as keys. If the property being modified is not being used by hashCode() and equals(), then we will still be able to find the key in hashMap. If hashCode() and equals() method make use of this property, then we may not find this key in hashMap. Later I modify one of the properties of this key. Let's say I use an object as key, and put this key and associated value into hashMap. ![]() If the object which is used as key in hashMap is modified, then we may have problem retrieving values from hashMap. Note the the value object is not used in any of the calculations within hashMap.Ī class that we plan to use as a “key” in hashMap needs to follow certain restrictions. Step# 2: Go over the mini-list and return the element that matches the key (using key.equals()). Step# 1: Quickly determine the bucket number in which this element may reside (using key.hashCode()). Lookup process is at the heart of HashMap and almost all the complexity of hashMap lies here. So with basics out of the way, here are 10 things every java programmer should know about HashMap. That is what hashMap does, it builds an array of these mini-lists. However, if you split this list into multiple mini-lists AND IF you can quickly tell in which mini-list this element may reside, then the time-complexity for look-up can be reduced greatly. If you are looking for an element in a "list", the order of lookup would be proportional to the length of the list, This writeup assumes that you have a basic familiarity with Java’s HashMap and dives into some of its lesser known but very important characteristics.Let me start with a very brief primer before we take a deep dive.īasic syntax: HashMap hashMap = new HashMap()
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