visual basic barcode printing 7: Objects and Collections in Java

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7: Objects and Collections
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The hashCode() Contract
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Now coming to you straight from the fabulous Java API documentation for class Object, may we present (drum roll) the hashCode() contract:
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Whenever it is invoked on the same object more than once during an execution
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of a Java application, the hashCode() method must consistently return the same integer, provided no information used in equals() comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
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If two objects are equal according to the equals(Object) method, then
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calling the hashCode() method on each of the two objects must produce the same integer result.
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It is not required that if two objects are unequal according to the
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equals(java.lang.Object) method, then calling the hashCode() method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. And what this means to you is
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Condition x.equals(y) == true x.hashCode() == y.hashCode() x.equals(y) == false x.hashCode() != y.hashCode() x.equals(y)== false Required x.hashCode() == y.hashCode() x.equals(y) == true No hashCode() requirements Not Required (But Allowed)
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So let s look at what else might cause a hashCode() method to fail. What happens if you include a transient variable in your hashCode() method While that s legal (compiler won t complain), under some circumstances an object you put in a collection won t be found. The exam doesn t cover object serialization, so we won t go into any details here. Just keep in mind that serialization saves an object so that it can be reanimated later by deserializing it back to full objectness.
Overriding hashCode() and equals() (Exam Objective 9.2)
But danger Will Robinson remember that transient variables are not saved when an object is serialized. A bad scenario might look like this:
class SaveMe implements Serializable{ transient int x; int y; SaveMe(int xVal, int yVal) { x = xVal; y = yVal; } public int hashCode() { return (x ^ y); //Legal, but not correct to // use a transient variable } public boolean equals(Object o) { SaveMe test = (SaveMe)o; if (test.y == y && test.x == x) { // Legal, not correct return true; } else { return false; } } }
Here s what could happen using code like the preceding example:
Give an object some state (assign values to its instance variables). Put the object in a HashMap, using the object as a key. Save the object to a file using object serialization without altering any of its state. Retrieve the object from the file through deserialization. Use the deserialized (brought back to life on the heap) object to get the object
out of the HashMap. Oops. The object in the collection and the supposedly same object brought back to life are no longer identical. The object s transient variable will come back with a default value rather than the value the variable had at the time it was saved (or put into the HashMap). So using the preceding SaveMe code, if the value of x is 9 when the instance is put in the HashMap, then since x is used in the calculation of the hashcode, when the value of x changes the hashcode changes too. And when that same instance of SaveMe is brought back from deserialization, x == 0, regardless
7: Objects and Collections
of the value of x at the time the object was serialized. So the new hashcode calculation will give a different hashcode, and the equals() method fails as well since x is used as one of the indicators of object equality. Bottom line: transient variables can really mess with your equals and hashcode implementations. Either keep the variable nontransient or, if it must be marked transient, then don t use it in determining an object s hashcode or equality.
CERTIFICATION OBJECTIVE
Collections (Exam Objective 9.1)
Make appropriate selection of collection classes/interfaces to suit specific behavior requirements. Can you imagine trying to write object-oriented applications without using data structures like hashtables or linked lists What would you do when you needed to maintain a sorted list of, say, all the members in your Simpsons fan club Obviously you can do it yourself; Amazon.com must have thousands of algorithm books you can buy. But with the kind of schedules programmers are under today ( Here s a spec. Can you have it all built by tomorrow morning ), it s almost too painful to consider. The Collections Framework in Java, which took shape with the release of JDk1.2 (the first Java 2 version) and expanded in 1.4, gives you lists, sets, and maps to satisfy most of your coding needs. They ve been tried, tested, and tweaked. Pick the best one for your job and you ll get at the least reasonably good performance. And when you need something a little more custom, the Collections Framework in the java.util package is loaded with interfaces and utilities.
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