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What is the difference between the ways to read an Image file in Java?

There are various ways of reading an image file in java such as BufferedImage and ImageIcon to name a few. I want to know what is the difference between these cases? Are they context dependent that in a particular case only one of them can be used?
What would be the best way of reading a image selected by JFileChooser by the user and separating the color channels of an image?

A good way is to use the different ImageIO.read methods, which return BufferedImage objects.
Image is an abstract class, so I think the real question is which subclass is more efficient for your program. Use VolatileImage if you need hardware acceleration. More on that here.

ImageIcon (and Toolkit#createImage/Toolkit#getImage) use a background loading process. That is, after you call these methods, they will return immediately, having created a background thread to actually load the image data.
These were/are used when loading large images across slow connections, like ye old 28k modems (ah, how I remember the days). This means that your application can continue running while the images are been downloaded.
You'll find in the Graphics class the drawImage methods accept an ImageObserver interface and that java.awt.Component implements this interface, this allows components the ability to automatically update themselves once the image has actually finished loading.
ImageIO on the other hand will not return until the image is fully loaded. It also makes it easier to introduce new readers/writers, making the API far more flexible then the original API. ImageIO also supports a wider range of images out of the box.
BufferedImage is also a far more flexible image class, especially when it comes to apply effects to the image.
Now, I, personally, prefer ImageIO. If I know I'm loading large images or images over a potentially slow connection, I will create my own background thread to load them. While a little more complicated, the trade offs greatly out weight the small amount of extra work -IMHO
What would be the best way of reading a image selected by JFileChooser by the user and separating the color channels of an image?
ImageIO without a doubt. In order to do any serious manipulation of an image loaded using something ImageIcon, you'd have to convert that image to a BufferedImage anyway

Image vs. BufferedImage

Whenever dealing with the loading and rendering of images in Java, I have previously always used BufferedImages to store and manipulate the images in memory.
However, I have recently come across a few different sites that use the Image class instead of BufferedImage and this got me wondering - what are the differences?
I'm aware that a BufferedImage has a larger/optimised toolset, but does come at any cost? If so, when does this cost become noticeable? In which situations would you use an Image over a BufferedImage, or vice-versa?

BufferedImage extends Image. Image is just a base abstract class and you can't instantiate it. Under the hood you are using BufferedImage or another implementation for sure.

There shouldn't be any real performance difference between directly creating a BufferedImage and a Toolkit image (java.awt.Toolkit or Image#getScaledInstance). You'll never have an actual instance of Image because it's an abstract class; you'll only be dealing with its subclasses (e.g. BufferedImage).

Java image with double value pixel

Actually I'm working with BufferedImages, that provide me pixel values in int type.
Do you know a Java object to represent an image that the pixel value in double or float type?

I'm not sure exactly what your end goal is, but you could look into java.awt.image.ColorModel
. Either that or you could do your processing on a float[][]/double[][] and then round if you need to do something with your BufferedImage.

I do know of an implementation off the top of my head, but you can use the SampleModel/WritableRaster to access bands independently and create your own backing however you like. It likely won't be much fun though. Although, I'd question the "access component (??) as a double/float" requirement (unless you're doing something fun like OpenGL 8-bit floats ... which don't map to Java floats anyway :-)

You can use Java Advanced Imaging API (JAI).
JAI introduces two new data classes, which extend the Java 2D DataBuffer image data class: DataBufferFloat and DataBufferDouble.
http://java.sun.com/products/java-media/jai/forDevelopers/jai1_0_1guide-unc/J2D-concepts.doc.html#52401

output a jFrame to jpeg or bitmap

I've been working on an assignment, and have all the requirements completed. The project is to compare the differences in runtime between a linear search algorithm and a binary search. I have a graph class that puts out the results of those searches in a xy graph.
The graph object is a Turtle class that extends JFrame. Is there any way I can convert that graph object to a bitmap and save it for future printing?
The professor requires printouts of the results. Since I don't want a printout of every time the program is run, I would prefer to save the graphics results in a designated folder, rather than using screen-grab.
Unfortunately, I haven't come up with any answers on Google or here. Is something like this even possible?

Another approach is to tell your Component to paint() itself into a BufferedImage, as seen in this complete example. The Container returned by a JFrame's getContentPane() method is suitable. In summary:
Component component = f.getContentPane();
BufferedImage image = new BufferedImage(…);
component.paint(image.getGraphics());
ImageIO.write(image,…);

You can pass the bounds of the area into the Robot.createScreenCapture(Rectangle) method to create a BufferedImage of the area. The easiest way to save the screenshot as an image file is to use the ImageIO class.

Java type that represents a PNG image?

What is a Java type which can hold a PNG implement and provide access to it's pixel buffer?

BufferedImage img = ImageIO.read(new File("my.png"));
int color = img.getRGB(23,12);

I would take a look at Java Advanced Imaging, it handle multiple types of image files.

Take a look ImageIO and its numerous static helpers for reading and writing bytes/streams containing an image.

If you want to do pixel based operations on the entire image, I've found calling the getRGB() method every time to be fairly slow. In that case, you might want to try and get access to the actual pixel array holding the image data using something like:
byte[] pixel_array = ((DataBufferByte)img.getRaster().getDataBuffer()).getData()
There may be a more flexible way that doesn't make any presumptions on the array data type.