Area of a circle

What’s inside a reference varible in Java?

We know that the object reference variable holds bits that represent a way to access an object.
It doesn't hold the object itself, but it holds something like a pointer or an address.
I was going through the Head-First Java (2nd edition) book and in the book it is written that

"In Java we don't really know what is inside a reference variable. We do know that whatever it is, it represents one and only one object. And the JVM knows how to use the reference to get to the object." - Chapter 3 (page 54)

I want to ask:

• Do an object reference variable holds Pointer, Address or what?
• How does JVM interpret that?

Answers

1. It's entirely up to the JVM to determine what goes inside a reference.

In the simplest case it would just be a pointer (i.e. an address). In more sophisticated cases, the VM may use different representations depending on the situation - for example, you may want to read the paper on "Compressed oops in HotSpot" to see how the HotSpot VM can avoid doubling the size of references in some (but not all) places when running as a 64-bit VM.
The important thing is that you should neither know nor care. As far as you're concerned as a programmer, it's just an opaque set of bits - its only purpose is to let you get to the object in question (or identify a null reference).    Jon Skeet

2. That's up to the JVM. A Java reference isn't guaranteed to have any semantics besides that you can access an object through it. Sunacle might do that differently to IBM as well.

In practice it may often be a pointer of some sort, though maybe not directly to the object since that can be moved by the GC.    Johannes Rössel

3. It's entirely JVM specific. It may be an address, a pointer, or something more complicated. You don't have, or need, any harder guarantees than the fact that you can get an Object using the reference.    GaryF

4. It's up to the JVM to decide exactly how the reference is implemented.

Having said that, any "sane" implementation would probably use either a direct pointer or some form of compressed pointer for performance reasons. I believe this applies to all current production JVMs.

5. Have a look at an open implementation such as OpenJDK if you're that curious :)    Samuel_xL

6. A legitimate question would be "how are references implemented in 's JVM" though.    Samuel_xL

7. @Samuel - it depends on how you define "legitimate". Unless you're looking to modify the JVM itself there's no practical use for this information. Curiousity is fine, but these kind of questions always make me think that someone is clinging to a C mindset.    Andrzej Doyle

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GuessGame Program

Summary: 

The guessing game involves a 'game' object and three 'player' objects. The game generates a random number between 0 and 9 and the three player objects try to guess it.

Classes:

GuessGame.class   Player.class   GameLauncher.class

Logic:

1) The GameLauncher class is where the application starts; it has the main() method.

2) In the main() method, a GuessGame object is created, and its startGame() method is called.

3) The GuessGame object's startGame() method is where the entire game plays out. It creates three players, then "thinks" of a random number (the target for the players to guess). It then asks each player to guess, checks the result, and either prints out information about the winning player(s) or asks them to guess again.

You can download the source code of the program from the link given below.

Source code

Finally got my first book of Java (Head First Java)

I ordered the Head First Java book (2nd edition) on 2 august and today I got a call from the Aramex courier company at 4 PM. One person told me that he came to deliver the book but he didn't find the home so he tried to call on my cellphone but my mobile was switched off. He asked me if I can come to their office and receive the package. I agreed and searched the company's address on internet just to see where the office is located I took help of the Google map. It was showing that the office is in "Chandpole" near "Janana hospital". It's about 6 km far from my home. I took my bike and went there. There people told me that there isn't any office of that company. Then I called back to that person and he told me that you went to a wrong place, He said "Our office is in Rambagh". It's about 4 km far from my home. I went back to Rambagh and then searched there. I saw a big board of the company from distance, I parked my bike and went there. I got my book there. I scolded the guys out there for wrong tagging there address in Google map. They said that are sorry and they will make it ok as soon as possible. Then I returned to home and unpacked the package. The book looks nice. I mean it's like GUI in printed pages. I checked about the authors "Kathy Sierra" and "Bert Bates" and had a look on the Table of Contents and How to use the book. I will start the first lesson "Breaking the Surface" from tomorrow. So before signing off I'll let you guys know the reviews of the book.

Here they are.. 

...The only way to decide the worth of a tutorial is to decide how well it teaches. Head First Java excels at teaching. ---slashdot.org

...It's definitely time to dive in--Head First. --Scott McNealy, Sun Microsystems, Chairman, President, and CEO

Head First Java transforms the printed page into the closest thing to a GUI you've ever seen. In a wry, hip manner, the authors make learning Java an engaging, 'what're they gonna do next?' experience. --Warren Keuffel, Software Development Magazine

It's fast, irreverent, fun, and engaging. Be careful you might actually learn something! --Ken Arnold, coauthor (with James Gosling, creator of Java), The Java Programming Language

Amazon Reviews

Eclipse and Java for total beginners

An introduction to using the Eclipse development platform to build Java applications. Includes installing Eclipse and the supporting downloads required to get up and running with Eclipse.

PART 1:

                        

PART 2: