Here are presented notions of java, object oriented programming (OOP) and XML. The purpose is to permit the non-informatician reader to know what 'package', 'interface', 'abstract' ... mean.
Definitions are only given in JEphem context (some terms have a more general or deeper meaning).
Class : a class is a set of methods and fields.
Instanciating a class : if a class is not declared abstract, it can be instantiated through a constructor : an object is created.
Example : creation of an object of type Body : Body b = new Body(MERCURY);
Each object can store its own data.
static methods or fields : when a class is instanciated, the methods or fields are bound to a particular instance of the class.
Static methods and fields are shared by all the instances of the class.
Abstract classes : Sometimes, classes don't need to be instanciated. For example, there is no interest in creating an instance of Pluto99. So it is declared abstract and contains only static methods.
In this usage, creating a class is just a way to organize the code.
Interface : they contain constants or declaration of methods without implementation. In JEphem, I used interfaces in two different ways :
- to store constants, like
jephem.astro.SolarSystemConstants ;
- to force certain classes to have certain characteristics
package : in java, the classes are grouped into packages. An API is generally organized into packages and sub-packages.
The fully qualified name of a class (which must be unique) is composed by its package name followed by its class name.
A widely adopted convention consists in naming packages only with lower cases.
Ex : class VSOP87.java is jephem.astro.planets.vsop87.VSOP87
The file hierarchy must reflect the package hierarchy. See the page about source code for an example.
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XML means eXtended Markup Language. It's a protocol to write documents understandable by humans AND machines.
For this, tags are employed. A tag is delimited by '<' and '>', and can have attributes.
Example : <myTag attribute1 = "attribute value">
Informatic trail
