How can I see the impression of the Garbage Collector?

[Zamioculcas]

Hello Experts,
I want to get acquainted with the Garbage Collector.
I'm pretty new to Java, so far I have written very simple programs. I 'm using the System.gc() statement in my codes, but I can't see the Garbage Collector's influence.

Can someone give me a little code or explaination where I can see clearly the impact of the garbage collector? Is there any function to display the unreferenced objects or memory usage?

At the moment I try to test the functionality of the gc with the following code:

import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Iterator;
public class Garbage{
	

public static void main(String[] args)
	 {
	    Queue qu = new PriorityQueue();
	    for (int i=0;i< 100;i++)
              {
		  String str=new String(Integer.toString(i)+"pear");	
		  qu.add(str);
		  [B]System.gc();	[/B]
	      }

	    Iterator it = qu.iterator();	
	    while(it.hasNext())
		{
		   System.out.println(it.next());	
		}
         }
}

How can I tranform this code to see the impression of gc?

I would be very grateful for any help! :)

[FlyNn]

GC is a very mysterious thing in JAVA. :D Calling a function System.gc() only advices the GC to run. Java Virtual Machine decides when to run it by it self. You really should not worry about gc.

[Zamioculcas]

Dear FlyNn,

Thank you very much for your quickly answer. GC has got a truly mysterious behaviour:D

At the moment I can find a small utility which gives information about our heap. (But heap /stack is not really clear to me yet:) )
If you run java program from command line, you will specify the -verbose:gc specifier.
for example: java -verbose:gc ForumGC

When you call the System.gc function in your code, the program prints information about your heap.

I test it with the following small code:

import java.util.Queue;
import java.util.PriorityQueue;
public class ForumGC{
public static void main(String[] args)
    {
        Queue chain = new PriorityQueue();        
        chain.offer("apple");
        chain.offer("walnut");
        chain.offer("pear");
        System.out.println("list size: " + chain.size());    
        System.out.println("call gc");
        System.gc();            
        chain.poll();        
        chain.poll();        
        System.out.println("list size: " + chain.size());
        System.out.println("call gc");    
        System.gc();
        chain.offer("plum");
        chain.offer("cherry");
        chain.offer("raspberry");
        chain.offer("strawberry");
        System.out.println("list size: " + chain.size());
        System.out.println("call gc");    
        System.gc();
       }}

the output:

list size: 3
call gc
[Full GC 277K->119K(15744K), 0.0147829 secs]
list size: 1
call gc
[Full GC 208K->119K(15872K), 0.0113771 secs]
list size: 5
call gc
[Full GC 208K->119K(15872K), 0.0112291 secs]

My code is too small to show the effects of gc. But I guess, this utility might be useful in large programs.

Best regards

[Tolls]

There are profilers and heap dump tools (and heap analysers) that are far more useful than sticking that sort of code into your system.