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Class files for a project in Python are contained with in a source folder. With java, classes for a project are typically stored within a package for purposes of visibilty and accessability.

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Explains how to apply Cramer's Rule and demonstrates the technique.

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I'm extremely new to Java and just wanted to confirm what Double is? Is it similar to Float or Int? Any help would be appreciated. I also sometimes see the uppercase Double and other times the lower case double. If someone could clarify what this means that'd be great!

Double is a wrapper class,

The Double class wraps a value of the primitive type double in an object. An object of type Double contains a single field whose type is double.

In addition, this class provides several methods for converting a double to a String and a String to a double, as well as other constants and methods useful when dealing with a double.

The double data type,

The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is 4.94065645841246544e-324d to 1.79769313486231570e+308d (positive or negative). For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.

Check each datatype with their ranges : Java's Primitive Data Types.

Important Note : If you'r thinking to use double for precise values, you need to re-think before using it. Java Traps: double

answered Jun 12 '11 at 2:27

In a comment on @paxdiablo's answer, you asked:

"So basically, is it better to use Double than Float?"

That is a complicated question. I will deal with it in two parts

Deciding between double versus float

On the one hand, a double occupies 8 bytes versus 4 bytes for a float. If you have many of them, this may be significant, though it may also have no impact. (Consider the case where the values are in fields or local variables on a 64bit machine, and the JVM aligns them on 64 bit boundaries.) Additionally, floating point arithmetic with double values is typically slower than with float values ... though once again this is hardware dependent.

On the other hand, a double can represent larger (and smaller) numbers than a float and can represent them with more than twice the precision. For the details, refer to Wikipedia.

The tricky question is knowing whether you actually need the extra range and precision of a double. In some cases it is obvious that you need it. In others it is not so obvious. For instance if you are doing calculations such as inverting a matrix or calculating a standard deviation, the extra precision may be critical. On the other hand, in some cases not even double is going to give you enough precision. (And beware of the trap of expecting float and double to give you an exact representation. They won't and they can't!)

There is a branch of mathematics called Numerical Analysis that deals with the effects of rounding error, etc in practical numerical calculations. It used to be a standard part of computer science courses ... back in the 1970's.

Deciding between Double versus Float

For the Double versus Float case, the issues of precision and range are the same as for double versus float, but the relative performance measures will be slightly different.

A Double (on a 32 bit machine) typically takes 16 bytes + 4 bytes for the reference, compared with 12 + 4 bytes for a Float. Compare this to 8 bytes versus 4 bytes for the double versus float case. So the ratio is 5 to 4 versus 2 to 1.

Arithmetic involving Double and Float typically involves dereferencing the pointer and creating a new object to hold the result (depending on the circumstances). These extra overheads also affect the ratios in favor of the Double case.

Correctness

Having said all that, the most important thing is correctness, and this typically means getting the most accurate answer. And even if accuracy is not critical, it is usually not wrong to be "too accurate". So, the simple "rule of thumb" is to use double in preference to float, UNLESS there is an overriding performance requirement, AND you have solid evidence that using float will make a difference with respect to that requirement.

answered Jun 12 '11 at 3:25

A double is an IEEE754 double-precision floating point number, similar to a float but with a larger range and precision.

IEEE754 single precision numbers have 32 bits (1 sign, 8 exponent and 23 mantissa bits) while double precision numbers have 64 bits (1 sign, 11 exponent and 52 mantissa bits).

A Double in Java is the class version of the double basic type - you can use doubles but, if you want to do something with them that requires them to be an object (such as put them in a collection), you'll need to box them up in a Double object.

answered Jun 12 '11 at 2:27

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Stack and heap are two important concepts you should understand in relation to Java memory allocation. Let's take a look at the two concepts, why they matter, and when you should use each.

What Is Java Stack?

A Java stack is part of your computer's memory where temporary variables, which are created by all functions you do, are stored. It is used to execute a thread and may have certain short-lived values as well as references to other objects. It uses LIFO data structure, or last in first out.

What does this mean? When a method is invoked, it creates a new block in the stack for that particular method. The new block will have all the local values, as well as references to other objects that are being used by the method. When the method ends, the new block will be erased and will be available for use by the next method. The objects you find here are only accessible to that particular function and will not live beyond it.

This makes it very easy to keep track of the stack, where the latest reserved block is also the first to be freed. The variables created for the method are directly stored in the memory, allowing for fast access.

The memory size of a Java stack is generally much less than in a Java heap space because when a method ends, all the variables created on the stack are erased forever.

Here's an example of how to create an object in the stack:

What Is Java Heap?

Java objects are in an area, which is called the heap. It is created when the program is run, and its size may decrease or increase as your program runs. It can easily become full, and when it does, garbage collection is initiated. This is when objects that are no longer used are deleted to make way for new objects.

Unlike in a Java stack where memory allocation is done when your program is compiled, in a heap it is allocated as your program is run. Accessing variables placed here is a bit slower compared to a stack's direct and fast access.

Heap is likened to a global memory pool. A method or function will use the heap for memory allocation if you need the data or variables to live longer than the method or function in question. The objects you find here are accessible to all the functions.

Also, there is no specific order in reserving blocks in a heap. You can allocate blocks at any time, and then you can free it when you wish. As you can imagine, it is much more complex to keep track of the parts that are free and can be allocated, but it can also be divided into two generations or sub-areas.

These sub-areas are called the young space (or nursery) and the old space. The young space is typically earmarked for the memory allocation for new objects. When the young space becomes full, garbage collection happens. Short-lived or temporary objects typically use the young space. This help makes garbage collection faster when compared to a heap without any divisions.

Here's an example of how to create an object in the heap:

Similarities and Differences Between Stack and Heap

Both are ways that Java allocates memory and both are stored in the RAM. However, to make things easier to remember, heap is used for dynamic memory allocation, while stack is for static allocations.

Where is it stored? Variables that are allocated on the stack are accessible directly from memory, and as such, these can run very fast. Accessing objects on the heap, on the other hand, takes more time.

When does the allocation happen? On the stack, memory allocation happens when the program is compiled. Meanwhile, on the heap, it begins when the program is run.

And since this is the case, you would need to know just how much data and memory you are going to need before compiling if you want to use the stack. Another limitation that the stack has is that it cannot handle big chunks of variables that need a lot of memory. If you do not know how much data you are going to need at run time or if you need memory for a lot of data, then you need to use heap.

In a Nutshell...

Stack

The size of the stack will vary as methods and functions create and delete local variables as needed.

Memory is allocated and then subsequently freed without you needing to manage the memory allocation.

Stack has size limits, which can vary according to the operating system you use.

Variables that are stored on the stack exist for as long as the function that created them are running.

Heap

Memory is not managed automatically nor is it as tightly managed by the central processing unit the way stack is managed. You would need to free allocated memory yourself when these blocks are no longer needed.

The heap is prone to memory leaks, where memory is allocated to unused objects and will not be available to processes other than that.

There is no size limit in the heap.

Compared to stack, objects in the heap are much slower to access. It is also slower to write to the memory on the heap.

Stack is easier and faster to use, but it comes with a lot of limitations that you can ignore if you use heap.

When do you use stack? Stack can only be used for local variables that use up small amounts of memory. The good news is that memory allocation and management is not going to be your problem and access to these objects is very fast. It does suffer from size limitations and the fact that you cannot resize variables on the stack.

When do you use heap? You use the heap to allocate memory if there are variables that you need to be accessed globally, as opposed to just being available only to the methods and functions that created it. Heap is also good when you have a need for a lot of memory since it has no limit on memory size. You can also resize the variables on the heap.

Additional Resources and Tutorials

To learn more about the differences between stack and heap, and the best use cases for each, visit the following resources and tutorials:

Stack and heap are two ways Java allocates memory. Heap is better in instances in which you have variables requiring global access, while stack is your go-to for local variables requiring only small amounts of memory. Understanding when and how to use a stack and a heap is critical for developing better Java programs.

It's also helpful to understand how memory allocation works when dealing with memory leaks. For a comprehensive guide to all the tools, websites, blogs, and other resources you need to level-up your Java game, download our Comprehensive Java Developer's Guide.

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Stack and heap are two important concepts you should understand in relation to Java memory allocation. Let's take a look at the two concepts, why they matter, and when you should use each. A Java stack is part of your computer's memory where temporary variables, which are created by all functions you do, are stored.

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Background on Instantiation

Java is an object-oriented programming language. In object-oriented programming, an object is an instance of a class. Think of the common example that is the Employee class; any new employee object used in the program is considered an instance of that class. Therefore, the fancy term for creating a new instance of that class is instantiation.

So what does this really mean?

When we say Employee class, don't think of a specific employee. Rather, think generally about what an employee would be or have; an ID, name, job title, pay rate, and so on.

Then, if we hire an employee named Jane, she will also possess all of those attributes (pay rate, name, and so on). Jane is now considered to be an instance of the Employee class. And to put that concept into Java-speak, she has inherited the values from the generic Employee class.

One of the key principles of object-oriented programming is the concept of inheritance, that is, objects can inherit traits and properties from other objects. In the Employee example, every instance of an Employee that is created in the code will have all of the characteristics of the original Employee class. If we create the ideal Employee, Java lets us clone a hundred more.

In the real world, instances will not be clones. As we run through our code to process a payroll, each instance of the Employee class may have methods to calculate wages and hours. These are static, but the values of the variables within those methods will vary. Still, this is all transparent to Java.

Java is only interested in creating new instances of the Employee, completing the processing, and repeating the process until done. It's a very efficient way of processing, since we only need to create a single Employee object, and any instances of that class contain all the methods and variables created in it. No need for repetitive and redundant code.

Java Syntax

The syntax requires the new keyword to create a new object. Let's say we have a class called Employee, and it might have some methods or variables. We want to create a new instance of the employee class that is instantiate Employee. What you're looking at below is the basic syntax:

And the output would look like this:

Constructors

The previous syntax looks like a subroutine is being called (the parentheses give it away). Every class will have what Java calls the constructor; much like a subroutine, but this is the key piece of code that builds the instance. The constructor tells the system to go out and grab some memory for the object and initialize variables.

In our previous Employee example, the default constructor is used, since we didn't create one. There's nothing to worry about here, since Java takes care of this step. However, we can create our own constructor that accepts some values. Actually, we could create a dozen, or a hundred constructors. Each time you create a new Employee, you can use one of them. We'll cover that in the examples.

What you're looking at below is a basic example of a constructor. Note that the code to create the constructor needs to be within the class definition.

Before we get into some examples, let's quickly recap the concepts: Instantiation is the creation of a new instance of a class. In order for this to work, a constructor is needed. If you don't specify one, that's okay; Java uses a default constructor. However, you can create multiple constructors if the need arises.

Some Examples

Let's put this into practice and expand upon the sample Employee class, which includes a more detailed constructor for Employee.

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what is instantiation in java? - definition & example - video & lesson transcript | study.com

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What is Instantiation in Java? - Definition & Example - Video & Lesson Transcript | Study.com

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Java is an object-oriented programming language. In object-oriented programming, an object is an instance of a class. Think of the common example that is the Employee class; any new employee object used in the program is considered an instance of that class. Therefore, the fancy term for creating a new instance of that class is instantiation.

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Java 3D proves a natural choice for any Java programmer wanting to write an interactive 3D graphics program

The Java 3D application programmer's interface (API) provides a very flexible platform for building a broad range of graphics applications. Developers have already used Java 3D to build applications in a variety of domains including mechanical CAD, molecular visualisation, scientific visualisation, animation previews, geographic information systems, business graphics, 3D logos and educational offerings. Virtual reality applications have included immersive workbench applications, headtracked shutter-glass-based desktop applications and portals (a cave-like room with multiple back-projected walls).

Writing a VR program requires substantial programming effort. A developer must either write code to handle the various input and display devices that the application might encounter or, alternatively, the developer will need to rely on a programming API designed to support VR applications. A typical VR application must track a user's head position and orientation (so that it can generate images of the virtual world as if the user's head were in that position and orientation). Additionally, the application might need to track other parts of the user's body, such as an arm, a hand or a leg, so that it can make user interaction with that environment easier by including a virtual view of that body part. The application must use these tracker inputs in placing objects within a view and in specifying the appropriate position and orientation in generating 3D images. An API that enables VR applications must support generating 3D graphics, handling input trackers and continuously integrating that tracker information into the rendering loop. The Java 3D API includes specific features for automatically incorporating head tracker inputs into the image generation process and for accessing other tracker information to control other features. It does this through a new view model that separates the user's environment from the application code. The API also explicitly defines the values returned by six-degrees-of-freedom detectors as primitive Java 3D objects, called Sensors. Together, the new view and input models make it quite easy to turn interactive 3D graphics applications into VR-enabled applications. A detailed description of the API appears in the Java 3D specification. A Java 3D program Creating a Java 3D application, or applet, involves constructing a virtual universe and inserting one or more scene graphs into that universe. A virtual universe consists of superstructure objects including one Universe object, one but possibly more Locale objects, and one or more scene graphs containing node objects arranged in tree structures. The scene graphs, which are called branch graphs because their root node must always be a BranchGroup node, contain the objects to render, the lights to apply, the behaviours to execute, the sounds to play and so on. Branch graphs containing content nodes are called content branches. Those that contain a ViewPlatform object ( the object that specifies the user's position and orientation ( are called view branches. Note that branch graphs describe what should be rendered, not the order in which to render objects. Neither node order nor node placement within a graph determines an order of execution or rendering. The path between a graph's root node and any leaf of that root node uniquely determines how to draw (or process) that path's leaf node. The almost total lack of implied order (the Ordered group node provides the only exception) means that Java 3D can execute behaviours and render objects in any order best suited to the system's underlying architecture, including parallel systems. Branch graphs Java 3D branch graphs are trees where each node in the branch graph has only one parent. Though this may seem restrictive, Java 3D does support sharing common scene graphs through an extra scene-graph mechanism. Leaf nodes called Link nodes can link to (reference) specially defined shareable subgraphs. The nodes in a branch graph fall into one of two categories: group or leaf nodes. Group nodes act as the glue that holds a branch graph together. Group nodes combine other nodes (group and leaf nodes) into one common unit. They position and orient those nodes, and, in a limited way, they control rendering order. Leaf nodes represent items in a branch graph such as objects, lights, and behaviours. Leaf nodes do not refer to nodes but only other Java 3D objects like NodeComponent objects. The Java 3D-view model consists of two intertwined components: the virtual world, as represented by the ViewPlatform object, and the physical world, as represented by the View object and its associated objects. The View object and its associated objects describe the user's display environment in considerable detail. They specify how many displays occupy that environment, whether it consists of a desktop monitor, three back projected walls or two displays in a head-mount. They specify the availability (or not) of a head tracker and its location relative to the display(s). They specify the user's height and interpupilary distance. They also specify numerous policies that determine how to interpret various changes in the many parameters. The view model imposes a clean separation between the virtual and the physical world. At the same time, it builds a bridge between the two worlds by defining a one-to-one-and-onto correspondence between the virtual space surrounding the ViewPlatform and the physical space as specified by the View objects. A point in one space has a corresponding point in the other space. A camera-based model provides applications with a variety of controls over the camera's parameters such as its location and orientation, its aspect ratio and field-of-view, and others. These controls work well in many settings. Unfortunately, they fail to provide sufficient flexibility in some cases, and in other cases, they allow an inappropriate level of control. For example, in a portal (or cave-like) environment, where the user stands inside a multiwalled environment with multiple display surfaces, the camera model proves insufficient, since it can only provide control over one display surface at a time. A different problem exists in an immersive environment, where users wear head-mounted displays. The optical elements in the head-mount determine the application's permissible field-of-view. Any deviation from that value will create a significant mismatch in perceived movement versus the movement shown in the display. Such a mismatch induces nausea rather quickly. This example demonstrates a case where the application should not control a particular parameter. The central concept in the view model is co-existence. The viewer exists in the physical world as well as in the virtual world. By allowing sufficient control over describing the user's physical environment, Java 3D can map movements of the ViewPlatform or the user's head onto the corresponding, appropriate, view computations. The View object and its associated objects describe the viewer's initial position relative to display screens in the display environment. To prevent confusion, we call the individual display screens image plates. The viewer's eyepoints and the environment's image plates have well defined locations in the physical environment. Because of Java 3D's co-existence concept, the viewer's eyepoints and the image plates' position and orientation have well defined locations in the virtual environment. Since we know the location of the eyepoints and the image plates in the virtual world, it's relatively straightforward to construct the appropriate view frustum for computing a projective 3D image on a per-image-plate basis. If head tracking is available, we can straightforwardly translate head movements into new eyepoints relative to the image plates. If head tracking is not available, the frustums only need to be constructed once per image plate. Co-existence is essentially a mapping from the physical space containing the viewer and the image plates and the virtual space surrounding the ViewPlatform and back again. This mapping provides the means to translate movement or selection within one space directly into a similar movement or selection within the other space. Think of the ViewPlatform as the cart in an amusement park ride. Wherever the ViewPlatform takes you (the viewer), you go. However, just as you can turn your head to look around while enjoying a ride, you can look around as the ViewPlatform navigates around the virtual environment. By tracking the viewer's head position and orientation, we can instantaneously convert that head movement into the corresponding movement within the ViewPlatform's co-ordinate system. This virtual head position and orientation give us sufficient information to automatically generate the corresponding view frustums. If a head tracker is not available, then the user experiences the application's navigation as if fixed rigidly in a seat without the ability to move head or limb. If a developer doesn't know the exact structure of the user's environment, how can the developer write an application that can adapt to that environment? Java 3D provides a set of Universe utility classes that construct a virtual universe and all associated objects needed for a fully functioning view branch. The universe construction classes read the local machine's configuration properties and the user's configuration properties and, using that information, constructs the appropriate view-branch objects reflecting those configuration values. The developer only needs to manoeuvre the location and orientation of the ViewPlatform object to get the desired effect. The view model operates as a highly optimised constraint system. It has two grounding policies. One policy is used in environments where the user's eye can move relative to the display surface. This policy supports non-head-tracked, flat-screen display environments as well as all environments where the user can move relative to any display surfaces - environments such as immersive workbenches, multi-walled, back-projected rooms, or even desktop, head-tracked environments. The second policy is used in environments where the user's eye does not move relative to the display surface. This policy handles head-mounted displays, augmented-reality environments, and boom-mounted displays. The Java 3D API specification describes both constraint systems in detail. The view model assumes that a Java 3D application will at times execute in an environment with a head tracker. In such environments, the view-model's constraint system automatically incorporates the head-tracker's latest position and orientation information when computing view frustums. The head tracker information comes via the input device interface. Input devices in Java 3D Supporting head tracking requires Java 3D to provide access to six-degrees-of-freedom (6DOF) tracker information. Different tracking devices work differently and require that the computer interact with them in different ways. A device driver that operates a Polhemus device will definitely not operate an Ascension device. A Magellan driver will not operate a Spaceball. As new devices become available, they too will require custom device drivers. Rather than trying to support all possible 6DOF input devices, Java 3D defines an InputDevice interface that tracker vendors or developers can use to support a particular 6DOF device. The InputDevice interface requires the implementer of a device driver to define nine methods. The nine methods provide device-specific semantics for open, close and read operations as well as a minimal number of state-setting and state-query methods. A Java 3D environment can have any number of InputDevices. Moreover, InputDevices need not be real physical devices. They can be virtual devices. It's perfectly reasonable to write a software-input device that translates the manipulation of a virtual trackball via mouse movements into 6DOF values. Or, alternatively, to develop an InputDevice driver that reads time-stamped 6DOF values from a file or a network connection and then regenerates that data as if it arrived from a real tracker. Each input device has associated with it a fixed number of Sensor objects. A Sensor object represents one source of 6DOF data associated with that device. Whenever an input device driver generates new data, it does so for all of its Sensor objects. A Sensor object contains a fixed number of SensorRead objects arranged as a circular buffer. This circular buffer contains the "n" most recent SensorRead values for that Sensor. Having multiple SensorRead values makes it possible to perform data averaging and prediction on a particular stream of input values. SensorRead objects contain a time stamp, a 6DOF value, and an integer array of button values. The time stamp provides essential information for averaging or predicting 6DOF values. The button values let a device driver encode not only simple button-up and button-down states, but also a trigger or a knob. Sensors can also be used to support fewer degree-of-freedom devices such as conventional joysticks. Using abstract sensors Application developers do not use input devices directly. Java 3D abstracts away input devices by providing an array of available Sensors. This array of Sensor objects belongs under the Physical Environment object (one of the objects associated with the View object). The array consists of pointers to the objects associated with InputDevices. This array of Sensors lets an application developer require some number of 6DOF inputs and then have users provide the device drivers available on their systems. Typically, a Universe construction utility enumerates InputDevices and the Sensors associated with each device, then assigns each Sensor it finds to a slot in the array of Sensors. Since application code never references devices, only Sensors, the Universe utility can rearrange sensors at will as long as the appropriate sensors are associated with the corresponding appropriate Sensor index used by the application. The Universe construction utility reads a device and application configuration profile and, using that information, assigns a particular Sensor on a particular device to a particular Sensor index. In essence, this facility provides an input device/sensor patch panel. Finally, as a last resort, if an application requires 6DOF inputs and the user does not have tracking hardware, the Universe construction utility can construct virtual sensors. Conclusion Java programmers can quickly and easily define graphics programs using Java 3D's scene graph classes. An expanded view model lets applications seamlessly operate in a variety of single and multiple display, non-head-tracked and head-tracked, display environments. This view model relies on the flexible InputDevice interface that Java 3D provides to remove most of the vagaries of hardware trackers.(c) Sun Microsystems 1999 Compiled by Rachel Hodgkins

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Java 3D proves a natural choice for any Java programmer wanting to write an interactive 3D graphics program The Java 3D application programmer's interface (API) provides a very flexible platform for building a broad range of graphics applications.

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Java 3D proves a natural choice for any Java programmer wanting to write an interactive 3D graphics program

The Java 3D application programmer's interface (API) provides a very flexible platform for building a broad range of graphics applications. Developers have already used Java 3D to build applications in a variety of domains including mechanical CAD, molecular visualisation, scientific visualisation, animation previews, geographic information systems, business graphics, 3D logos and educational offerings. Virtual reality applications have included immersive workbench applications, headtracked shutter-glass-based desktop applications and portals (a cave-like room with multiple back-projected walls).

Writing a VR program requires substantial programming effort. A developer must either write code to handle the various input and display devices that the application might encounter or, alternatively, the developer will need to rely on a programming API designed to support VR applications. A typical VR application must track a user's head position and orientation (so that it can generate images of the virtual world as if the user's head were in that position and orientation). Additionally, the application might need to track other parts of the user's body, such as an arm, a hand or a leg, so that it can make user interaction with that environment easier by including a virtual view of that body part. The application must use these tracker inputs in placing objects within a view and in specifying the appropriate position and orientation in generating 3D images. An API that enables VR applications must support generating 3D graphics, handling input trackers and continuously integrating that tracker information into the rendering loop. The Java 3D API includes specific features for automatically incorporating head tracker inputs into the image generation process and for accessing other tracker information to control other features. It does this through a new view model that separates the user's environment from the application code. The API also explicitly defines the values returned by six-degrees-of-freedom detectors as primitive Java 3D objects, called Sensors. Together, the new view and input models make it quite easy to turn interactive 3D graphics applications into VR-enabled applications. A detailed description of the API appears in the Java 3D specification. A Java 3D program Creating a Java 3D application, or applet, involves constructing a virtual universe and inserting one or more scene graphs into that universe. A virtual universe consists of superstructure objects including one Universe object, one but possibly more Locale objects, and one or more scene graphs containing node objects arranged in tree structures. The scene graphs, which are called branch graphs because their root node must always be a BranchGroup node, contain the objects to render, the lights to apply, the behaviours to execute, the sounds to play and so on. Branch graphs containing content nodes are called content branches. Those that contain a ViewPlatform object ( the object that specifies the user's position and orientation ( are called view branches. Note that branch graphs describe what should be rendered, not the order in which to render objects. Neither node order nor node placement within a graph determines an order of execution or rendering. The path between a graph's root node and any leaf of that root node uniquely determines how to draw (or process) that path's leaf node. The almost total lack of implied order (the Ordered group node provides the only exception) means that Java 3D can execute behaviours and render objects in any order best suited to the system's underlying architecture, including parallel systems. Branch graphs Java 3D branch graphs are trees where each node in the branch graph has only one parent. Though this may seem restrictive, Java 3D does support sharing common scene graphs through an extra scene-graph mechanism. Leaf nodes called Link nodes can link to (reference) specially defined shareable subgraphs. The nodes in a branch graph fall into one of two categories: group or leaf nodes. Group nodes act as the glue that holds a branch graph together. Group nodes combine other nodes (group and leaf nodes) into one common unit. They position and orient those nodes, and, in a limited way, they control rendering order. Leaf nodes represent items in a branch graph such as objects, lights, and behaviours. Leaf nodes do not refer to nodes but only other Java 3D objects like NodeComponent objects. The Java 3D-view model consists of two intertwined components: the virtual world, as represented by the ViewPlatform object, and the physical world, as represented by the View object and its associated objects. The View object and its associated objects describe the user's display environment in considerable detail. They specify how many displays occupy that environment, whether it consists of a desktop monitor, three back projected walls or two displays in a head-mount. They specify the availability (or not) of a head tracker and its location relative to the display(s). They specify the user's height and interpupilary distance. They also specify numerous policies that determine how to interpret various changes in the many parameters. The view model imposes a clean separation between the virtual and the physical world. At the same time, it builds a bridge between the two worlds by defining a one-to-one-and-onto correspondence between the virtual space surrounding the ViewPlatform and the physical space as specified by the View objects. A point in one space has a corresponding point in the other space. A camera-based model provides applications with a variety of controls over the camera's parameters such as its location and orientation, its aspect ratio and field-of-view, and others. These controls work well in many settings. Unfortunately, they fail to provide sufficient flexibility in some cases, and in other cases, they allow an inappropriate level of control. For example, in a portal (or cave-like) environment, where the user stands inside a multiwalled environment with multiple display surfaces, the camera model proves insufficient, since it can only provide control over one display surface at a time. A different problem exists in an immersive environment, where users wear head-mounted displays. The optical elements in the head-mount determine the application's permissible field-of-view. Any deviation from that value will create a significant mismatch in perceived movement versus the movement shown in the display. Such a mismatch induces nausea rather quickly. This example demonstrates a case where the application should not control a particular parameter. The central concept in the view model is co-existence. The viewer exists in the physical world as well as in the virtual world. By allowing sufficient control over describing the user's physical environment, Java 3D can map movements of the ViewPlatform or the user's head onto the corresponding, appropriate, view computations. The View object and its associated objects describe the viewer's initial position relative to display screens in the display environment. To prevent confusion, we call the individual display screens image plates. The viewer's eyepoints and the environment's image plates have well defined locations in the physical environment. Because of Java 3D's co-existence concept, the viewer's eyepoints and the image plates' position and orientation have well defined locations in the virtual environment. Since we know the location of the eyepoints and the image plates in the virtual world, it's relatively straightforward to construct the appropriate view frustum for computing a projective 3D image on a per-image-plate basis. If head tracking is available, we can straightforwardly translate head movements into new eyepoints relative to the image plates. If head tracking is not available, the frustums only need to be constructed once per image plate. Co-existence is essentially a mapping from the physical space containing the viewer and the image plates and the virtual space surrounding the ViewPlatform and back again. This mapping provides the means to translate movement or selection within one space directly into a similar movement or selection within the other space. Think of the ViewPlatform as the cart in an amusement park ride. Wherever the ViewPlatform takes you (the viewer), you go. However, just as you can turn your head to look around while enjoying a ride, you can look around as the ViewPlatform navigates around the virtual environment. By tracking the viewer's head position and orientation, we can instantaneously convert that head movement into the corresponding movement within the ViewPlatform's co-ordinate system. This virtual head position and orientation give us sufficient information to automatically generate the corresponding view frustums. If a head tracker is not available, then the user experiences the application's navigation as if fixed rigidly in a seat without the ability to move head or limb. If a developer doesn't know the exact structure of the user's environment, how can the developer write an application that can adapt to that environment? Java 3D provides a set of Universe utility classes that construct a virtual universe and all associated objects needed for a fully functioning view branch. The universe construction classes read the local machine's configuration properties and the user's configuration properties and, using that information, constructs the appropriate view-branch objects reflecting those configuration values. The developer only needs to manoeuvre the location and orientation of the ViewPlatform object to get the desired effect. The view model operates as a highly optimised constraint system. It has two grounding policies. One policy is used in environments where the user's eye can move relative to the display surface. This policy supports non-head-tracked, flat-screen display environments as well as all environments where the user can move relative to any display surfaces - environments such as immersive workbenches, multi-walled, back-projected rooms, or even desktop, head-tracked environments. The second policy is used in environments where the user's eye does not move relative to the display surface. This policy handles head-mounted displays, augmented-reality environments, and boom-mounted displays. The Java 3D API specification describes both constraint systems in detail. The view model assumes that a Java 3D application will at times execute in an environment with a head tracker. In such environments, the view-model's constraint system automatically incorporates the head-tracker's latest position and orientation information when computing view frustums. The head tracker information comes via the input device interface. Input devices in Java 3D Supporting head tracking requires Java 3D to provide access to six-degrees-of-freedom (6DOF) tracker information. Different tracking devices work differently and require that the computer interact with them in different ways. A device driver that operates a Polhemus device will definitely not operate an Ascension device. A Magellan driver will not operate a Spaceball. As new devices become available, they too will require custom device drivers. Rather than trying to support all possible 6DOF input devices, Java 3D defines an InputDevice interface that tracker vendors or developers can use to support a particular 6DOF device. The InputDevice interface requires the implementer of a device driver to define nine methods. The nine methods provide device-specific semantics for open, close and read operations as well as a minimal number of state-setting and state-query methods. A Java 3D environment can have any number of InputDevices. Moreover, InputDevices need not be real physical devices. They can be virtual devices. It's perfectly reasonable to write a software-input device that translates the manipulation of a virtual trackball via mouse movements into 6DOF values. Or, alternatively, to develop an InputDevice driver that reads time-stamped 6DOF values from a file or a network connection and then regenerates that data as if it arrived from a real tracker. Each input device has associated with it a fixed number of Sensor objects. A Sensor object represents one source of 6DOF data associated with that device. Whenever an input device driver generates new data, it does so for all of its Sensor objects. A Sensor object contains a fixed number of SensorRead objects arranged as a circular buffer. This circular buffer contains the "n" most recent SensorRead values for that Sensor. Having multiple SensorRead values makes it possible to perform data averaging and prediction on a particular stream of input values. SensorRead objects contain a time stamp, a 6DOF value, and an integer array of button values. The time stamp provides essential information for averaging or predicting 6DOF values. The button values let a device driver encode not only simple button-up and button-down states, but also a trigger or a knob. Sensors can also be used to support fewer degree-of-freedom devices such as conventional joysticks. Using abstract sensors Application developers do not use input devices directly. Java 3D abstracts away input devices by providing an array of available Sensors. This array of Sensor objects belongs under the Physical Environment object (one of the objects associated with the View object). The array consists of pointers to the objects associated with InputDevices. This array of Sensors lets an application developer require some number of 6DOF inputs and then have users provide the device drivers available on their systems. Typically, a Universe construction utility enumerates InputDevices and the Sensors associated with each device, then assigns each Sensor it finds to a slot in the array of Sensors. Since application code never references devices, only Sensors, the Universe utility can rearrange sensors at will as long as the appropriate sensors are associated with the corresponding appropriate Sensor index used by the application. The Universe construction utility reads a device and application configuration profile and, using that information, assigns a particular Sensor on a particular device to a particular Sensor index. In essence, this facility provides an input device/sensor patch panel. Finally, as a last resort, if an application requires 6DOF inputs and the user does not have tracking hardware, the Universe construction utility can construct virtual sensors. Conclusion Java programmers can quickly and easily define graphics programs using Java 3D's scene graph classes. An expanded view model lets applications seamlessly operate in a variety of single and multiple display, non-head-tracked and head-tracked, display environments. This view model relies on the flexible InputDevice interface that Java 3D provides to remove most of the vagaries of hardware trackers.(c) Sun Microsystems 1999 Compiled by Rachel Hodgkins

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Java 3D proves a natural choice for any Java programmer wanting to write an interactive 3D graphics program The Java 3D application programmer's interface (API) provides a very flexible platform for building a broad range of graphics applications.

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