151: COMPARING OBJECTS in Java

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151: COMPARING OBJECTS
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Example 155 Implications of Overriding the equals() Method public class TestUsableVNO { public static void main(String[] args) { // Three individual version numbers UsableVNO latest = new UsableVNO(9,1,1); UsableVNO inShops = new UsableVNO(9,1,1); UsableVNO older = new UsableVNO(6,6,6); // An array of version numbers UsableVNO[] versions = new UsableVNO[] { new UsableVNO( 3,49, 1), new UsableVNO( 8,19,81), new UsableVNO( 2,48,28), new UsableVNO(10,23,78), new UsableVNO( 9, 1, 1)}; // An array with number of downloads Integer[] downloads = {245, 786, 54,1010, 123}; TestCaseVNOtest(latest, inShops, older, versions, downloads); } }
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Output from the program:
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class UsableVNO Test object reference and value equality: latest: (911), inShops: (911), older: (666) latest == inShops: false latestequals(inShops): true latest == older: false latestequals(older): false Array: [(3491), (81981), (24828), (102378), (911)] Search key (911) found in array: true List: [(3491), (81981), (24828), (102378), (911)] Search key (911) contained in list: true Map: {(911)=123, (24828)=54, (81981)=786, (3491)=245, (102378)=1010} Hash code for keys in the map: (3491): 8451275 (81981): 4669910 (24828): 3374351 (102378): 5737707 (911): 31771588 Search key (911) has hash code: 31393597 Map contains search key (911): false Exception in thread "main" javalangClassCastException: UsableVNO cannot be cast to javalangComparable at TestCaseVNOtest(TestCaseVNOjava:59) at TestUsableVNOmain(TestUsableVNOjava:18)
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CHAPTER 15: COLLECTIONS AND MAPS
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The output from the program shows that object value equality is compared correctly Object value equality is now based on identical states, as defined by the equals() method The search for a UsableVNO object in an array or a list of UsableVNO objects is now successful, since the equals comparison is based on the states of the objects and not on their reference values However, searching in a map or creating sorted collections is still not feasible For searching in a HashMap, we have to look at the relationship between the equals() and the hashCode() methods For creating sorted collections or sorted maps, we will provide an implementation of the compareTo() method
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The hashCode() Method
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Hashing is an efficient technique for storing and retrieving data A common hashing scheme uses an array where each element is a list of items The array elements are called buckets Operations in a hashing scheme involve computing an array index from an item Converting an item to its array index is done by a hash function The array index returned by the hash function is called the hash value of the item The hash value identifies a particular bucket Storing an item involves the following steps: 1 2 Hashing the item to determine the bucket If the item does not match one already in the bucket, it is stored in the bucket
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Note that no duplicate items are stored Retrieving an item is based on using a key The key represents the identity of the item Item retrieval is also a two-step process: 1 2 Hashing the key to determine the bucket If the key matches an item in the bucket, this item is retrieved from the bucket
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Different items can hash to the same bucket, meaning that the hash function returns the same hash value for these items This condition is called a collision The list maintained by a bucket contains the items that hash to the bucket The hash value only identifies the bucket Finding an item in the bucket entails a search and requires an equality function to compare items The items maintained in a hash-based storage scheme must, therefore, provide two essential functions: a hash function and an equality function The performance of a hashing scheme is largely affected by how well the hash function distributes a collection of items over the available buckets A hash function should not be biased toward any particular hash values An ideal hash function produces a uniform distribution of hash values for a collection of items across all possible hash values Such a hash function is not an easy task to design Fortunately, heuristics exist for constructing adequate hash functions
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