Java introduction

Java introduction

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  Fundamentals of JAVA Dr.Kuppusamy .P Associate Professor / SCOPE
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  Content • Evolution ofJava • Java Architecture • Syntax of Java Program • Simple Java Program • Naming Convention • Comments Dr. Kuppusamy P
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  Evolution of Java •Java was the brainchild of: • James Gosling • Patrick Naughton • Chris Warth • Ed Frank • Frank Sheridan • The origin of Java is on fall of 1992, and was initially called Oak • Oak was renamed as Java in 1995 Goal • Platform-neutral language used for embedded devices software • Its portable Dr. Kuppusamy P
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  Java Architecture Dr. KuppusamyP
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  Java Architecture 1. Createa java source code with .java extension 2. Compile the source code using java compiler which will create bytecode file with .class extension 3. Class loader reads both the user defined and library classes into the memory for execution 4. Bytecode verifier validates all the bytecodes are valid and do not violate Java’s security restrictions 5. JVM reads bytecodes and translates into machine code for execution. While execution of the program the code interacts to the operating system and hardware Dr. Kuppusamy P
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  5 phases ofJava Programs 1. Edit - Use an editor to type Java program (Welcome.java) 2. Compile - translate Java program into an intermediate language called bytecodes, understood by Java interpreter (javac Welcome.java) 1. Use a compiler to create .class file, containing bytecodes (Welcome.class) 3. Loader – Use a class loader to read bytecodes from .class file into memory 4. Verifier - Use a Bytecode verifier to make sure bytecodes are valid and do not violate security restrictions 5. Execute - Java Virtual Machine (JVM) uses a combination of interpretation and just-intime compilation to translate bytecodes into machine language • Applications are run on user's machine, i.e. executed by interpreter with java command (java Welcome) Dr. Kuppusamy P
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  Java Virtual Machine(JVM) • The output of the compiler is bytecode • The bytecodes are executed by JVM • It is an interpreter which converts the byte code to machine specific instructions and executes • JVM is platform specific Dr. Kuppusamy P
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  Java Virtual Machine(JVM) • Most modern languages are designed to be compiled • Compilation is a one-time exercise and executes faster • Execution of compiled code over the Internet an impossibility • Executable code always generated to a CPU-OS combination • Interpreting a Java program into byte code facilitates its execution in a wide variety of environments Dr. Kuppusamy P
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  Java Virtual Machine(JVM) • Only the JVM needs to be implemented for each platform • Once the Java runtime package exists for a given system, any Java program can run on it • The JVM will differ from Platform to Platform and it is Platform Specific • All versions of JVM interpret the same Java byte code • Interpreted code runs much slower compared to executable code • The use of bytecode enables the Java runtime system to execute programs much faster • Java facilitates on-the-fly compilation of bytecode into native code Dr. Kuppusamy P
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  Adaptive optimizer (JITCompiler) • Another type of execution engine is an adaptive optimizer • Speed up the execution • The virtual machine starts by interpreting bytecodes • It also keeps a tab on the code that is running and identifies only the heavily used areas • The JVM compiles these heavily used areas of code into native code • The rest of the code, which is not heavily used continues to be interpreted and executed Preprocessor • Preprocessor is a javac plugin that plugs directly into your Java compiler and runs as part of javac. • Not needed to add intermediate build steps, manage source generation targets. • It’s effing fast and allows to easily build multiple targets from a single codebase. Dr. Kuppusamy P
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  Class Loader /Linker Class Loader / Linker • class loader is a part of the VM that is important from: • A security standpoint • Network mobility • The class loader loads a compiled Java source file (.class files represented as bytecode) into the Java Virtual Machine (JVM) • The bootstrap class loader (linker) is responsible for loading the classes, programmer defined classes as well as Java's API classes Java .class file 1. Java class file is designed for • platform independence • network mobility 2. The class file is compiled to a target JVM, but independent of underlying host platforms 3. The Java class file is a binary file that has the capability to run on any platform Dr. Kuppusamy P
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  Syntax of Javaprogram public class Classname { public static void main(String args[]) { Statements ….. } } Dr. Kuppusamy P Create source file: Classname .java Compile: javac Classname .java Execute: java Classname static – only one copy of memory is allotted (created) for the method main(). public – This main method can access by all members like objects, classes, etc void – main() is not returns anything String args[] – Input given in the console considered as string arguments
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  Simple Java program publicclass Mobilephone { public static void main(String args[]) { System.out.println(“Experience the PUBG..!”); } } Output: “Experience the PUBG..!” Dr. Kuppusamy P Create source file: Mobilephone .java Compile: javac Mobilephone .java Execute: java Mobilephone
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  Java program -Naming Conventions Class Names • Class names should be nouns, simple and descriptive • Noun could be in mixed case with the first letter of each internal word capitalized • Eg: class Student, class TestStudent Variable Names • The variables are in mixed case with a lowercase first letter • Variable names should not start with underscore _ or dollar sign $, even though both are allowed • Variable names should be small and meaningful • One-character variable names should be avoided except for temporary variables • Eg: int y, myWidth; Method Names • Methods should be verbs, in mixed case with the first letter lowercase, with the first letter of each internal word capitalized • Eg: void run(), void getColor() Dr. Kuppusamy P
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  Java program -Comments Block (Multi-line) Comments • Block comments are used to provide descriptions of files, methods, data structures and algorithms • Block comments may be used at the beginning of each file and before each method /* Here is a block comment */ Single line Comment • Single line comments can be written using // Single line Dr. Kuppusamy P
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  Number of VariablesDeclaration per Line • One declaration per line is recommended int height; int width; is preferred over int height, width; • Do not declare different types on the same line int height, width[]; // Not recommended Dr. Kuppusamy P
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  References Dr. Kuppusamy P HerbertSchildt, “Java: The Complete Reference”, McGraw-Hill Education, Tenth edition, 2017.