visual basic barcode printing 7: Objects and Collections in Java

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7: Objects and Collections
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Reassigning a Reference Variable
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We can also decouple a reference variable from an object by setting the reference variable to refer to another object. Examine the following code:
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class GarbageTruck { public static void main(String [] args) { StringBuffer s1 = new StringBuffer("hello"); StringBuffer s2 = new StringBuffer("goodbye"); System.out.println(s1); // At this point the StringBuffer "hello" is not eligible s1 = s2; // Redirects s1 to refer to the "goodbye" object // Now the StringBuffer "hello" is eligible for collection } }
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Objects that are created in a method also need to be considered. When a method is invoked, any local variables created exist only for the duration of the method. Once the method has returned, the objects created in the method are eligible for garbage collection. There is an obvious exception, however. If an object is returned from the method, its reference might be assigned to a reference variable in the method that called it; hence, it will not be eligible for collection. Examine the following code:
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import java.util.Date; public class GarbageFactory { public static void main(String [] args) { Date d = getDate() doComplicatedStuff(); System.out.println("d = " + d); } public static Date getDate() { Date d2 = new Date(); String now = d2.toString(); System.out.println(now); return d2; } }
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In the preceding example, we created a method called getDate() that returns a Date object. This method creates two objects: a Date and a String containing the date information. Since the method returns the Date object, it will not be eligible for collection even after the method has completed. The String object, though, will be eligible, even though we did not explicitly set the now variable to null.
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Garbage Collection (Exam Objectives 3.1, 3.2, 3.3)
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Isolating a Reference
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There is another way in which objects can become eligible for garbage collection, even if they still have valid references! We think of this scenario as islands of isolation. A simple example is a class that has an instance variable that is a reference variable to another instance of the same class. Now imagine that two such instances exist and that they refer to each other. If all other references to these two objects are removed, then even though each object still has a valid reference, there will be no way for any live thread to access either object. When the garbage collector runs, it will discover any such islands of objects and will remove them. As you can imagine, such islands can become quite large, theoretically containing hundreds of objects. Examine the following code:
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public class Island { Island i; public static void main(String [] args) { Island i2 = new Island(); Island i3 = new Island(); Island i4 = new Island(); i2.i = i3; i3.i = i4; i4.i = i2; i2 = null; i3 = null; i4 = null; // do complicated, memory intensive stuff } } // i2 refers to i3 // i3 refers to i4 // i4 refers to i2
When the code reaches // do complicated, the three Island objects (previously known as i2, i3, and i4) have instance variables so that they refer to each other, but their links to the outside world (i2, i3, and i4) have been nulled. These three objects are eligible for garbage collection. This covers everything you will need to know about making objects eligible for garbage collection. Study Figure 7-5 to reinforce the concepts of objects without references and islands of isolation.
7: Objects and Collections
FIGURE 7-5
Objects eligible for garbage collection
Forcing Garbage Collection
The first thing that should be mentioned here is, contrary to this section s title, garbage collection cannot be forced. However, Java provides some methods that allow you to request that the JVM perform garbage collection. For example, if you are about to perform some time-sensitive operations, you probably want to minimize the chances of a delay caused by garbage collection. But you must remember that the methods that Java provides are requests, and not demands; the virtual machine will do its best to do what you ask, but there is no guarantee that it will comply.
In reality, it is possible only to suggest to the JVM that it perform garbage collection. However, there are no guarantees the JVM will actually remove all of the unused objects from memory. It is essential that you understand this concept for the exam.
Garbage Collection (Exam Objectives 3.1, 3.2, 3.3)
The garbage collection routines that Java provides are members of the Runtime class. The Runtime class is a special class that has a single object (a Singleton) for each main program. The Runtime object provides a mechanism for communicating directly with the virtual machine. In order to get the Runtime instance, you can use the method Runtime.getRuntime(), which returns the Singleton. Alternatively, for the method we are going to discuss, you can call the same method on the System class, which has static methods that can do the work of obtaining the Singleton for you. The simplest way to ask for garbage collection (remember just a request) is
System.gc();
Theoretically, after calling System.gc(), you will have as much free memory as possible. We say theoretically because this routine does not always work that way. First, the JVM you are using may not have implemented this routine; the language specification allows this routine to do nothing at all. Second, another thread (again, see 9) may perform a substantial memory allocation right after you run the garbage collection. This is not to say that System.gc() is a useless method it s much better than nothing. You just can t rely on System.gc() to free up enough memory so that you don t have to worry about the garbage collector being run. The certification exam is interested in guaranteed behavior, not probable behavior. Now that we are somewhat familiar with how this works, let s do a little experiment to see if we can see the effects of garbage collection. The following program lets us know how much total memory the JVM has available to it and how much free memory it has. It then creates 10,000 Date objects. After this, it tells us how much memory is left and then calls the garbage collector (which, if it decides to run, should halt the program until all unused objects are removed). The final free memory result should indicate whether it has run. Let s look at the program:
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. import java.util.Date; public class CheckGC { public static void main(String [] args) { Runtime rt = Runtime.getRuntime(); System.out.println("Total JVM memory: " + rt.totalMemory()); System.out.println("Before Memory = " + rt.freeMemory()); Date d = null; for(int i = 0;i<10000;i++) { d = new Date(); d = null; } System.out.println("After Memory = " + rt.freeMemory()); rt.gc(); // an alternate to System.gc() System.out.println("After GC Memory = " + rt.freeMemory());
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