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POHow to ensure finalize() is always called (Thinking in Java exercise)
 

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    <p>I'm slowly working through Bruce Eckel's <em>Thinking in Java 4th edition</em>, and the following problem has me stumped:</p> <blockquote> <p>Create a class with a finalize( ) method that prints a message. In main( ), create an object of your class. Modify the previous exercise so that your finalize( ) will always be called.</p> </blockquote> <p>This is what I have coded:</p> <pre><code>public class Horse { boolean inStable; Horse(boolean in){ inStable = in; } public void finalize(){ if (!inStable) System.out.print("Error: A horse is out of its stable!"); } } public class MainWindow { public static void main(String[] args) { Horse h = new Horse(false); h = new Horse(true); System.gc(); } } </code></pre> <p>It creates a new <code>Horse</code> object with the boolean <code>inStable</code> set to <code>false</code>. Now, in the <code>finalize()</code> method, it checks to see if <code>inStable</code> is <code>false</code>. If it is, it prints a message. </p> <p>Unfortunately, no message is printed. Since the condition evaluates to <code>true</code>, my guess is that <code>finalize()</code> is not being called in the first place. I have run the program numerous times, and have seen the error message print only a couple of times. I was under the impression that when <code>System.gc()</code> is called, the garbage collector will collect any objects that aren't referenced.</p> <p>Googling a correct answer gave me <a href="http://greggordon.org/java/tij4/initialization/BankTest.java" rel="nofollow">this link</a>, which gives much more detailed, complicated code. It uses methods I haven't seen before, such as <code>System.runFinalization()</code>, <code>Runtime.getRuntime()</code>, and <code>System.runFinalizersOnExit()</code>.</p> <p>Is anybody able to give me a better understanding of how <code>finalize()</code> works and how to force it to run, or walk me through what is being done in the solution code?</p>
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    1. CO"I was under the impression that when System.gc() is called, the garbage collector will collect any objects that aren't referenced" In your case, there *is* in fact a reference to your object, so why should it be destroyed? Also, I don't think it's guaranteed that `System.gc()` actually recycles anything.
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    2. COOn the line `h = new Horse(true);` I set **h** to a new object, making the previous one (which had **inStable** set to false) unreferenced.
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    3. COYeah, in that case consult the second half of my comment. There are no guarantees. The solution from that external page is actually not a real solution
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