The name reflection is used to describe code which is able to inspect other code in the same system (or itself).
For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.
So, to give you a code example of this in Java (imagine the object in question is foo) :
Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);
One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.
There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html
And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.
Update from a comment:
The ability to inspect the code in the system and see object types is
not reflection, but rather Type Introspection. Reflection is then the
ability to make modifications at runtime by making use of
introspection. The distinction is necessary here as some languages
support introspection, but do not support reflection. One such example
is C++
There are several differences between HashMap
and Hashtable
in Java:
Hashtable
is synchronized, whereas HashMap
is not. This makes HashMap
better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable
does not allow null
keys or values. HashMap
allows one null
key and any number of null
values.
One of HashMap's subclasses is LinkedHashMap
, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap
for a LinkedHashMap
. This wouldn't be as easy if you were using Hashtable
.
Since synchronization is not an issue for you, I'd recommend HashMap
. If synchronization becomes an issue, you may also look at ConcurrentHashMap
.
Best Answer
JavaFX is part of OpenJDK
The JavaFX project itself is open source and is part of the OpenJDK project.
However, the OpenJDK project includes many projects, including incubating projects and other projects, such as OpenJFX, whose source and implementation are not shipped as part of some JDK/JRE distributions (e.g. Oracle JDK 11+ implementations and many open-source JDK Linux package installations do not include JavaFX runtimes).
Modern JavaFX distributions are modules
JavaFX is not part of most JDK 11+ downloads, it is usually downloaded by developers as modules or a separate SDK.
Specifically for JavaFX 11+, as detailed on the https://openjfx.io site:
This means that JavaFX can be used from any modern JDK running on a supported platform, by including the appropriate JavaFX modules (available from openjfx or Maven Central) on the runtime module path.
All JavaFX modules available from openjfx are open source implementations that contain no closed source code.
For distribution of JavaFX-based applications to end-users, it is advised to package a run-time image distribution based on the JDK and JavaFX modules + required dependent library code and application code. A full discussion of this packaging is out of scope for this answer, but one can create a distribution based purely on open-source JDK+JavaFX+library+application code if desired. See the openjfx.io documentation on run-time images for some more (though not comprehensive) information on this.
Update Sep 2021
Also, see the related question:
Which covers information regarding Java 8 and also some later versions.
Update Dec 2019
For current information on how to use Open Source JavaFX, visit https://openjfx.io. This includes instructions on using JavaFX as a modular library accessed from an existing JDK (such as an Open JDK installation).
The open source code repository for JavaFX is at https://github.com/openjdk/jfx.
At the source location linked, you can find license files for open JavaFX (currently this license matches the license for OpenJDK: GPL+classpath exception).
The wiki for the project is located at: https://wiki.openjdk.java.net/display/OpenJFX/Main
If you want a quick start to using open JavaFX, the Bellsoft's Liberica JDK distributions provide pre-built OpenJDK binaries that include OpenJFX for a variety of platforms.
For distribution as self-contained applications, Java 14 is scheduled to implement JEP 343: Packaging Tool, which "Supports native packaging formats to give end users a natural installation experience. These formats include msi and exe on Windows, pkg and dmg on macOS, and deb and rpm on Linux.", for deployment of OpenJFX based applications with native installers and no additional platform dependencies (such as a pre-installed JDK).
Older information that may become outdated over time
Building JavaFX from the OpenJDK repository
You can build an open version of OpenJDK (including JavaFX) completely from source which has no dependencies on the Oracle JDK or closed source code.
Update: Using a JavaFX distribution pre-built from OpenJDK sources
As noted in comments to this question and in another answer, the Debian Linux distributions offer a JavaFX binary distibution based upon OpenJDK:
https://packages.qa.debian.org/o/openjfx.html
Install via:
(currently this only works for Java 8 as far as I know).
Differences between Open JDK and Oracle JDK with respect to JavaFX
The following information was provided for Java 8. As of Java 9, VP6 encoding is deprecated for JavaFX and the Oracle WebStart/Browser embedded application deployment technology is also deprecated. So future versions of JavaFX, even if they are distributed by Oracle, will likely not include any technology which is not open source.
Oracle JDK includes some software that is not usable from the OpenJDK. The components that relate to JavaFX:
This means that an open version of JavaFX cannot play VP6 FLV files. This is not a big loss as it is difficult to find VP6 encoders or media encoded in VP6.
Other more common video formats, such as H.264 will playback fine with an open version of JavaFX (as long as you have the appropriate codecs pre-installed on the target machine).
The lack of WebStart/Browser Embedded deployment technology is really something to do with OpenJDK itself rather than JavaFX specifically. This technology can be used to deploy non-JavaFX applications.
It would be great if the Open Source community developed a deployment technology for Java (and other software) which completely replaced WebStart and Browser Embedded deployment methods, allowing a nice light-weight, low impact user experience for application distribution. I believe there have been some projects started to serve such a goal, but they have not yet reached a high maturity and adoption level.
Personally, I feel that WebStart/Browser Embedded deployments are legacy technology and there are currently better ways to deploy many JavaFX applications (such as self-contained applications).
Update Dec, 2019:
An open source version of WebStart for JDK 11+ has been developed and is available at https://openwebstart.com.
Who needs to create Linux OpenJDK Distributions which include JavaFX
It is up to the people which create packages for Linux distributions based upon OpenJDK (e.g. Redhat, Ubuntu etc) to create RPMs for the JDK and JRE that include JavaFX. Those software distributors, then need to place the generated packages in their standard distribution code repositories (e.g. fedora/red hat network yum repositories). Currently this is not being done, but I would be quite surprised if Java 8 Linux packages did not include JavaFX when Java 8 is released in March 2014.
Update, Dec 2019:
Now that JavaFX has been separated from most binary JDK and JRE distributions (including Oracle's distribution) and is, instead, available as either a stand-alone SDK, set of jmods or as a library dependencies available from the central Maven repository (as outlined as https://openjfx.io), there is less of a need for standard Linux OpenJDK distributions to include JavaFX.
If you want a pre-built JDK which includes JavaFX, consider the Liberica JDK distributions, which are provided for a variety of platforms.
Advice on Deployment for Substantial Applications
I advise using Java's self-contained application deployment mode.
A description of this deployment mode is:
You can build a self-contained application either from the Oracle JDK distribution or from an OpenJDK build which includes JavaFX. It currently easier to do so with an Oracle JDK.
As a version of Java is bundled with your application, you don't have to care about what version of Java may have been pre-installed on the machine, what capabilities it has and whether or not it is compatible with your program. Instead, you can test your application against an exact Java runtime version, and distribute that with your application. The user experience for deploying your application will be the same as installing a native application on their machine (e.g. a windows .exe or .msi installed, an OS X .dmg, a linux .rpm or .deb).
Note: The self-contained application feature was only available for Java 8 and 9, and not for Java 10-13. Java 14, via JEP 343: Packaging Tool, is scheduled to again provide support for this feature from OpenJDK distributions.
Update, April 2018: Information on Oracle's current policy towards future developments