How big is an object reference in Java and precisely what information does it contain?

by

While on many VMs the size of a reference is the native pointer size (i.e. 32 bits for a 32 bit JVM and 64 bits for a 64 bit JVM) this isn’t guaranteed – and in particular HotSpot either does now or soon will support “Compressed Oops” which are 32 bit references in a 64 bit JVM. (That doesn’t mean that every reference is compressed – read the linked article for more information, and there are plenty of blog posts about it too.)

In response to another comment, note that the reference itself is typically just a way of addressing the object itself. Whether it’s a direct memory pointer or not, its goal is to get to the data for the object. That’s basically all that really matters. If there’s some “spare” bits (e.g. it’s a 64-bit reference and you don’t need all of that width just to represent the object’s location) then the VM can use that data for other information such as its type, which may allow some optimisations. (See Tom’s comment for more details.)

The object itself contains type information (probably in the form of a reference to the instance of Class, or something similar – I don’t know in enough detail) as well as other necessary “stuff” in the header, before you get to the user data for the object.

More Related Contents:

  1. How do I get a platform-dependent new line character?
  2. Why is Java’s SimpleDateFormat not thread-safe? [duplicate]
  3. How to set a background picture in JPanel
  4. Java time-based map/cache with expiring keys [closed]
  5. How to initialize List object in Java?
  6. Why are Java Streams once-off?
  7. JFormattedTextField : input time duration value
  8. Generate random date of birth
  9. Can a Collector’s combiner function ever be used on sequential streams?
  10. Keystore type: which one to use?
  11. Best Practice to Use HttpClient in Multithreaded Environment
  12. JavaFX Nested Controllers (FXML )
  13. How do I get the max and min values from a set of numbers entered?
  14. Constantly checking if a bullet has touched a node
  15. Re-sizing an image without losing quality
  16. Password Verification with PBKDF2 in Java
  17. Why null cast a parameter? [duplicate]
  18. Generic JSF entity converter [duplicate]
  19. Internet listener Android example
  20. How can I crop a bitmap for ImageView?
  21. Simple way to sort strings in the (case sensitive) alphabetical order
  22. Does Java recognize infinite loops?
  23. Spring properties that depend on other properties
  24. Font Awesome with Swing
  25. Why does an IllegalThreadStateException occur when Thread.start is called again
  26. Getting difference between two dates Android
  27. is it is possible to disable the windows keys using java
  28. Packaging Java apps for the Windows/Linux desktop
  29. JSP: EL expression is not evaluated [duplicate]
  30. Regex to remove spaces between numbers only

Leave a Comment