visual basic barcode printing A simplified hashcode example in Java

Generator PDF-417 2d barcode in Java A simplified hashcode example

A simplified hashcode example
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Overriding hashCode() and equals() (Exam Objective 9.2)
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letters, so the hashcode will be identical for both names. That s acceptable, but it does mean that when someone asks you (the bucket-clerk) for the Amy piece of paper, you ll still have to search through the target bucket reading each name until we find Amy rather than May. The code tells you only which bucket to go into, but not how to locate the name once we re in that bucket.
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In real-life hashing, it s not uncommon to have more than one entry in a bucket. Good hashing retrieval is typically a two-step process: 1. Find the right bucket. 2. Search the bucket for the right element.
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So for efficiency, your goal is to have the papers distributed as evenly as possible across all buckets. Ideally, you might have just one name per bucket so that when someone asked for a paper you could simply calculate the hashcode and just grab the one paper from the correct bucket (without having to go flipping through different papers in that bucket until you locate the exact one you re looking for). The least efficient (but still functional) hashcode generator would return the same hashcode (say, 42) regardless of the name, so that all the papers landed in the same bucket while the others stood empty. The bucket-clerk would have to keep going to that one bucket and flipping painfully through each one of the names in the bucket until the right one was found. And if that s how it works, they might as well not use the hashcodes at all but just go to the one big bucket and start from one end and look through each paper until they find the one they want. This distributed-across-the-buckets example is similar to the way hashcodes are used in collections. When you put an object in a collection that uses hashcodes, the collection uses the hashcode of the object to decide in which bucket/slot the object should land. Then when you want to fetch that object (or, for a hashtable, retrieve the associated value for that object), you have to give the collection a reference to an object which the collection compares to the objects it holds in the collection. As long as the object (stored in the collection, like a paper in the bucket) you re trying to search for has the same hashcode as the object you re using for the search (the name you show to the person working the buckets), then the object will be found. But and this is a Big One, imagine what would happen if, going back to our name example, you showed the bucket-worker a name and they calculated the code based on only half the letters in the name instead of all of them. They d never find the name in the bucket because they wouldn t be looking in the correct bucket!
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7: Objects and Collections
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Now can you see why if two objects are considered equal, their hashcodes must also be equal Otherwise, you d never be able to find the object since the default hashcode method in class Object virtually always comes up with a unique number for each object, even if the equals method is overridden in such a way that two or more objects are considered equal. It doesn t matter how equal the objects are if their hashcodes don t reflect that. So one more time: If two objects are equal, their hashcodes must be equal as well.
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Implementing hashCode()
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What the heck does a real hashcode algorithm look like People get their PhDs on hashing algorithms, so from a computer science viewpoint, it s beyond the scope of the exam. The part we care about here is the issue of whether you follow the contract. And to follow the contract, think about what you do in the equals() method. You compare attributes. Because that comparison almost always involves instance variable values (remember when we looked at two Moof objects and considered them equal if their int moofValues were the same ). Your hashCode() implementation should use the same instance variables. Here s an example:
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class HasHash { public int x; HasHash(int xVal) { x = xVal; } public boolean equals(Object o) { HasHash h = (HasHash) o; // Don't try at home without // instanceof test if (h.x == this.x) { return true; } else { return false; } } public int hashCode() { return (x * 17); } }
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Because the equals() method considers two objects equal if they have the same x value, we have to be sure that objects with the same x value will return identical hashcodes.
Overriding hashCode() and equals() (Exam Objective 9.2)
A hashCode() that returns the same value for all instances whether they re equal or not is still a legal even appropriate hashCode() method! For example,
public int hashCode() { return 1492; }
would not violate the contract. Two objects with an x value of 8 will have the same hashcode. But then again, so will two unequal objects, one with an x value of 12 and the other a value of -920. This hashCode() method is horribly inefficient, remember, because it makes all objects land in the same bucket, but even so, the object can still be found as the collection cranks through the one and only bucket using equals() trying desperately to finally, painstakingly, locate the correct object. In other words, the hashcode was really no help at all in speeding up the search, even though search speed is hashcode s intended purpose! Nonetheless, this one-hash-fits-all method would be considered appropriate and even correct because it doesn t violate the contract. Once more, correct does not necessarily mean good.
Typically, you ll see hashCode() methods that do some combination of ^-ing (XOR-ing) the instance variables, along with perhaps multiplying them by a prime number. In any case, while the goal is to get a wide and random distribution of objects across buckets, the contract (and whether or not an object can be found) requires only that two equal objects have equal hashcodes. The exam does not expect you to rate the efficiency of a hashCode() method, but you must be able to recognize which ones will and will not work (work meaning will cause the object to be found in the collection ). Now that we know that two equal objects must have identical hashcodes, is the reverse true Do two objects with identical hashcodes have to be considered equal Think about it you might have lots of objects land in the same bucket because their hashcodes are identical, but unless they also pass the equals() test, they won t come up as a match in a search through the collection. This is exactly what you d get with our very inefficient everybody-gets-the-same-hashcode method. It s legal and correct, just slooooow. So in order for an object to be located, the search object and the object in the collection must have both identical hashcode values and return true for the equals() method. So there s just no way out of overriding both methods to be absolutely certain that your objects can be used in Collections that use hashing.
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