Garbage Collection and Unused Objects

Java Tutorials - Herong's Tutorial Examples

∟Garbage Collection and Unused Objects

This section describes what is garbage collection and what is an unused object.

Garbage Collector: An execution thread that automatically frees up the memory occupied by unused objects. The garbage collector is managed by the JVM and executed separately from the application program threads.

Garbage Collection: The process during which the garbage collector actively identifying unused objects, and remove them to frees up the memory.

There are two key questions involved in the garbage collection:

How garbage collector identifies objects as unused objects?
When garbage collector performs the collection process?

Unused Object: An object that has zero live reference pointing to it. Java does not provide us any facilities to check how many references are there for a given object. But we can read the program source code to figure this out.

For example, after executing the following section of code:

   String a = new String("Apple");
   a = new String("Banana);
   new String("Orange");
   String g = sub("Grape");
   String x = "Kiwi";
   String y = x;
   x = null;
   ...
   static String sub(String b) {
      String l = new String{"Peach");
      return new String("Pear");
   }

objects created by the code will have the following status:

String object of "Apple": Unused, because the only reference from "a" is gone now. "a" is pointing to another object now.
String object of "Banana": Used, because it is still referred by "a".
String object of "Orange": Unused, because it was created, but never referenced by any variables.
String object of "Grape": Unused, because the reference from "b" is gone now. "a" is out of scope now after the execution of the method sub().
String object of "Peach": Unused, because the reference from "l" is gone now. "l" is out of scope now after the execution of the method sub().
String object of "Pear": Used, because the reference returned from the method sub() is in "g" now.
String object of "Kiwi": Used, because it is still referred by "y", even the reference from "x" is gone now.