Knowledge Base

From Java How to Program about abstract classes: Because they’re used only as superclasses in inheritance hierarchies, we refer to them as abstract superclasses. These classes cannot be used to instantiate objects, because abstract classes are incomplete. Subclasses must declare the “missing pieces” to become “concrete” classes, from which you can instantiate objects. Otherwise, these subclasses, too, will be abstract. To answer your question "What is the reason to use interfacs?": An abstract class’s purpose is to provide an appropriate superclass from which other classes can inherit and thus share a common design. As opposed to an interface: An interface describes a set of methods that can be called on an object, but does not provide concrete implementations for all the methods... Once a class implements an interface, all objects of that class have an is-a relationship with the interface type, and all objects of the class are guaranteed to provide the functionality...

Comparable A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances. Comparator A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances. This comparator class must implement the java.util.Comparator interface. Comparator provides a way for you to provide custom comparison logic for types that you have no control over. Comparable allows you to specify how objects that you are implementing get compared. Obviously, if you don't have control over a class (or you want to provide multiple ways to compare objects that you do have control over) then use Comparator. Otherwise you can use Comparable. Other Sources: http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html http://www.programcreek.com/2011/12/examples-to-demonstrate-comparable-vs-...

Interfaces: A good place to look at Java Interfaces would be the collections framework. java.util.List //interface java.util.ArrayList //Concrete class java.util.LinkedList //Concrete class So you can write code like this: List l = new ArrayList(); l.add(..) //do something else. If in future you want to change the implementation with say LinkedList or you own AwesomeList which implements List interface, all you have to do is change the very first line to: List l = new MyAwesomeList(); or List l = new LinkedList(); The rest of the code would follow through. More Examples: Use interfaces to define an application programming contract (blueprint, interface) which "3rd-party" vendors have to fully adhere and implement. This way the endusers can just code against the API contract and easily switch of the concrete implementation "under the hoods" without changing the code. The JDBC API is an excellent example. It exist of almost only interfaces...

Docker, a new container technology, is hotter than hot because it makes it possible to get far more apps running on the same old servers and it also makes it very easy to package and ship programs. Here's what you need to know about it. All the noise is happening because companies are adopting Docker at a remarkable rate. At OSCon in July, I ran into numerous businesses that were already moving their server applications from virtual machines (VM) to containers. Indeed, James Turnbull, Docker's VP of services and support, told me at the conference that three of the largest banks that had been using Docker in beta were moving it into production. That's a heck of a confident move for any 1.0 technology, but it's almost unheard of in the safety-first financial world. At the same time, Docker, an open-source technology, isn't just the darling of Linux powers such as Red Hat and Canonical. Proprietary software companies such as  Microsoft have also embraced D...

As should you know by now, XML by itself only serves to deliver data. You must provide other files to format and validate the XML code. This is where the DTD and schema come into play. The advancement of technology has provided many ways to validate XML. The DTD is just one. Document Type Definitions provide rules the computer can follow when processing the code. The DTD and schema are the precursors to XSL. Although still used in some formats, learning to write these files is not a requirement to create XML platforms, but it is indispensable to know what they are and how they function. These are terms you will stumble across in your studies frequently and understanding the basic design will be an advantage.What is a DTDThis is an explanation of document type definitions and the role they play in XML design. DTD code comes in two forms, internal and external. This article provides a description and explanation of how DTD files work.The DTD: Elements, Entities, Attributes, and Notation...

When beginning a study of XML, some may encounter an unfamiliar term that leaves them asking the question, “What is an XML schema (XSD)?” An XML schema is a road map for the XML document similar to a Document Type Definition (DTD). Created by the World Wide Web Consortium (W3C), schemas describe the elements and map out the presentation and nesting of XML documents. Essentially, the schema enables all applications to understand the flow of the page and validate the elements. Why Use An XML Schema? Step away from websites for a minute and consider house building. If the plumber, electrician and contractor all do their own thing, the result is a building with an uneven line, sinks in the bedroom and cable hookups in the bathroom. That does not happen because the architect draws a diagram that maps out the basic structure. With a blueprint, there is no guesswork. The plumber understands where the sinks go, and the electrician knows what rooms need what type of wiring. A schema is the...

XML namespaces are used for providing uniquely named elements and attributes in an XML document. XML Schema is an XML-based alternative to DTD.It describes the structure of an XML document. A schema describes the structure of the XML: tag names, parent/child relationships, cardinality, types, restrictions, etc. Namespaces are more like packages in Java: They give you a way to distinguish your tags from others. You can combine XML streams together. If you and someone else both have a <name> tag, you can have namespace foo, someone else can have namespace bar, and you can tell the difference with <foo:name> and <bar:name>.