7: Objects and Collections in Java

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7: Objects and Collections
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15. 16.
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Now, let s run the program and check the results:
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Total JVM memory: 1048568 Before Memory = 703008 After Memory = 458048 After GC Memory = 818272
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As we can see, the VM actually did decide to garbage collect (i.e. delete) the eligible objects. In the preceding example, we suggested to the JVM to perform garbage collection with 458,048 bytes of memory remaining, and it honored our request. This program has only one user thread running, so there was nothing else going on when we called rt.gc(). Keep in mind that the behavior when gc() is called may be different for different JVMs, so there is no guarantee that the unused objects will be removed from memory. About the only thing you can guarantee is that if you are running very low on memory, the garbage collector will run before it throws an OutOfMemoryException.
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Cleaning Up Before Garbage Collection the Finalize() Method
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Java provides you a mechanism to run some code just before your object is deleted by the garbage collector. This code is located in a method named finalize() that all classes inherit from class Object. On the surface this sounds like a great idea; maybe your object opened up some resources, and you d like to close them before your object is deleted. The problem is that, as you may have gathered by now, you can t count on the garbage collector to ever delete an object. So, any code that you put into your class s overridden finalize() method is not guaranteed to run. The finalize() method for any given object might run, but you can t count on it, so don t put any essential code into your finalize() method. In fact, we recommend that in general you don t override finalize() at all.
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Tricky Little Finalize() Gotcha s
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There are a couple of concepts concerning finalize() that you need to remember.
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For any given object, finalize() will be called only once by the
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garbage collector.
Calling finalize() can actually result in saving an object from deletion.
Garbage Collection (Exam Objectives 3.1, 3.2, 3.3)
Let s look into these statements a little further. First of all, remember that finalize() is a method, and any code that you can put into a normal method you can put into finalize(). For example, in the finalize() method you could write code that passes a reference to the object in question back to another object, effectively uneligiblizing the object for garbage collection. If at some point later on this same object becomes eligible for garbage collection again, the garbage collector can still process this object and delete it. The garbage collector, however, will remember that, for this object, finalize() already ran, and it will not run finalize() again. Now that we ve gotten down and dirty with garbage collection, verify that the following scenarios and solutions make sense to you. If they don t, reread the last part of this chapter. While awake.
SCENARIO & SOLUTION
I want to allocate an object and make sure that it never is deallocated. Can I tell the garbage collector to ignore an object My program is not performing as well as I would expect. I think the garbage collector is taking too much time. What can I do No. There isn t a mechanism for marking an object as undeletable. You can instead create a static member of a class, and store a reference to the object in that. Static members are considered live objects. First, if it really is the garbage collector (and it probably isn t), then the code is creating and dropping many references to many temporary objects. Try to redesign the program to reuse objects or require fewer temporary objects. The object won t be deleted until the last reference to the object is dropped. If you return the object as a method return value, the method that called it will contain a reference to the object. Set the member to null. Alternatively, if you set a reference to a new object, the old object loses one reference. If that is the last reference, the object becomes eligible for deletion. Prior to Java 1.2, you would have to check the amount of free memory yourself and guess. Java 1.2 introduced soft references for just this situation. This is not part of the Java 2 exam, however.
I am creating an object in a method and passing it out as the method result. How do I make sure the object isn t deleted before the method returns How do I drop a reference to an object if that object is referred to in a member of my class
I want to keep objects around as long as they don t interfere with memory allocation. Is there any way I can ask Java to warn me if memory is getting low
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