Chapter 1 Introduction in Java

Chapter 1: Introduction     Introduction Main characteristics of Java Features of object oriented programming languages Phases to execute a java program Introduction Java is an object oriented language, which was generated by Sun Microsystems in 1995. Objects, or more precisely, the classes objects come from, are reusable software components. There are date objects, time objects, audio objects, video objects, automobile objects, people objects and so on. In fact, almost any noun can be represented as a software object in terms of attributes (e.g. name, color and size) and behaviors (e.g. calculating, moving and communicating).  A modular, object oriented design and implementation make software development more productive than was possible with earlier techniques like structured programming.  Object oriented programs are often easier to understand, correct and modify.  Java is the world’s most widely used object oriented programming language.  Java evolved from C++. C++ is a hybrid language – it is possible to program in either a C-like style, an object oriented style or both. However, java is a pure object oriented language, i.e. Java programs always consist of classes. Main characteristics of Java: 1. Platform independence and portability The Java complier translates Java source code into bytecodes. Bytecodes are executed by the Java Virtual Machine (JVM) – a part of the JDK and the foundation of the Java platform. A virtual machine (VM) is a software application that simulates a computer but hides the underlying operating system and hardware from the programs that interact with it. If the same VM is implemented on many computer platforms, 2 applications that it executes can be used on all those platforms. The JVM is one of the most widely used virtual machines. Machine code (binary code): is dependent on a specific computer hardware. Bytecodes: are platform independent instructions, so Java’s bytecodes are portable. The same bytecodes can execute on any platform containing a JVM that understands the version of Java in which the bytecodes were compiled. 2. Networking: Communication with other applications through the internet and the www. 3. Multithreading: which enables programmer to specify that activities should occur in parallel. 4. Richer file processing than is provided by C or C++. 5. Builds applications with GUI. 6. Uses multimedia capabilities (image, animation, video and audio) 7. Object-oriented programming language: in Java, the unit of programming is a class from which objects are instantiated (created). An object is an instance of a class. Features of OOP: 1. Encapsulation (information hiding): attributes and operations are encapsulated (wrapped) into objects. Objects communicate with other objects but they are not allowed to know how other objects are implemented. 2. Inheritance relationships: a form of SW reuse, in which a new class is created by absorbing characteristics of existing class and adding to them new characteristics of their own. Java doesn’t support multiple inheritance, it only supports single inheritance. 3. Abstraction: which provides an appropriate superclass from which other classes can inherit and thus share a common design. 3 4. Polymorphism: which enables us to “program in the general” rather than “program in the specific”. In particular, polymorphism enables us to write programs that process objects that share the same superclass (either directly or indirectly) as if they’re all objects of the superclass. Phases to execute a java program: Phase 1: creating a program This phase consists of editing a file with an editor program. You type a source code using the editor, make any necessary corrections and save the program on a secondary storage device. A file name ending with .java extension indicates that the file contains java source code. Many freeware and shareware editors are available online, including EditPlus, TextPad, jEdit. For organizations that develop substantial information systems, integrated development environments (IDEs) are available. Popular IDEs include Eclipse, NetBeans, JBuilder, JCreator, BlueJ, jGRASP. Phase 2: compiling a java program into bytecodes Compile the java program into Bytecodes using the command javac The compiler creates the bytecodes and store them in a file ending with .class Bytecodes are executed by JVM, JVM is invoked by the command java Phase 3: loading a program into memory The JVM’s class loader takes the .class files containing the program’s bytecodes and transfers then to primary memory. Phase 4: bytecode verification As classes are loaded, the bytecode verifier examines the bytecodes to ensure that they are valid and don’t violate java’s security restrictions. Phase 5: Execution JVM executes the program’s bytecodes. 4