Читать книгу OCP Oracle Certified Professional Java SE 17 Developer Study Guide - Jeanne Boyarsky - Страница 122

Garbage collection refers to the process of automatically freeing memory on the heap by deleting objects that are no longer reachable in your program. There are many different algorithms for garbage collection, but you don't need to know any of them for the exam.

As a developer, the most interesting part of garbage collection is determining when the memory belonging to an object can be reclaimed. In Java and other languages, eligible for garbage collection refers to an object's state of no longer being accessible in a program and therefore able to be garbage collected.

Does this mean an object that's eligible for garbage collection will be immediately garbage collected? Definitely not. When the object actually is discarded is not under your control, but for the exam, you will need to know at any given moment which objects are eligible for garbage collection.

Think of garbage-collection eligibility like shipping a package. You can take an item, seal it in a labeled box, and put it in your mailbox. This is analogous to making an item eligible for garbage collection. When the mail carrier comes by to pick it up, though, is not in your control. For example, it may be a postal holiday, or there could be a severe weather event. You can even call the post office and ask them to come pick it up right away, but there's no way to guarantee when and if this will actually happen. Hopefully, they will come by before your mailbox fills with packages!

Java includes a built-in method to help support garbage collection where you can suggest that garbage collection run.

System.gc();

Just like the post office, Java is free to ignore you. This method is not guaranteed to do anything.