What is messaging in OOP

Objects interact by sending and receiving messages. A message is a request to perform an action, followed by a set of arguments that may be needed when performing an action. In OOP, sending a message (calling a method) is the only way to transfer control to an object. If an object is to "reply" to this message, then it must have a method corresponding to the message. Also, objects, using their methods, can themselves send messages to other objects. Messaging is done with dynamic calls, resulting in extreme late binding.

Let's say you want to create a physical model describing colliding balls of different sizes. The traditional approach to solving this problem is approximately the following: a set of data describing each ball (for example, its coordinates, mass and acceleration) is determined; each ball is assigned a unique identifier (for example, an array is organized, the index value of which corresponds to the ball number), which will distinguish each of the balls from all the others. Finally, a subroutine is written called, say, bounce; this procedure should, based on the ball number and its initial parameters, modify the data describing the ball accordingly. In contrast to the traditional approach, the object-oriented version of the program models each of the balls by means of an object. In this case, the object corresponding to a specific ball contains not only its parameters, but also the entire code describing the behavior of the ball under various interactions. So, each ball will have its own bounce() method. Instead of calling bounce with an argument defining, say, ball #3, you would need to send a message to the ball #3 object telling it to perform a collision.

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more