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Introduction to Java Programming Key Benefits of Java • Java is “write once, run anywhere” – architecture neutral – portable across different platforms – Due to Java Virtual Machine (JVM) • Security features – highly configurable security levels prevent any piece of Java code doing harm to the host system Key Benefits of Java • Network-centric platform – easy to work with resources across a network and to create network based applications • Object Oriented – an interacting collection of independent software components – dynamic extensible programs Key Benefits of Java • Internationalisation – uses 16 bit Unicode characters that represents the phonetic and ideographic character sets of the entire world • Performance – although an interpreted language Java programs run almost as fast as native C, C++ programs • Simple and easy to develop – powerful & well designed set of APIs JVM 1001100101001 … … class myCode { … … … … } Compiled by Java compiler myCode.class Bytecode Interpreted by JVM myCode.java Source Code Application runs JVM • JVM provides the run time environment for the bytecode (Java Runtime Environment JRE) – executes the bytecode and causes native machine code instructions to execute on the CPU that the JVM is on each target platform needs an implementation of the JVM Basic Program structure • Basic class definition class className { } // field declarations … // method declarations … • Each program needs a main method to tell the program where to start executing accessible to all classes (info hiding) Simple Java Program public class HelloWorld{ // no fields indicates class main method method returns nothing comman d line args // public static void main (String [] args){ invoking a member System.out.println("Hello World.."); } } Objects • An object includes state (fields) and behaviour (methods) • A class object is the blueprint for the instance objects • All code must be included in a class – no inline functions like C++ An Example Class a business class predefined public class Student { Java class // member fields Stringnoname; return reference to // constructor type the object itself name) { public Student(String this.name=name; } // methods void printDetails(){ String concatenation System.out.println("\nName: “ + name); } } Instantiation • Class definition creates “class object” at runtime • To instantiate “instance objects” use new operator ClassName myInstance = new ClassName(); where ClassName() is a constructor Note: no need to allocate the memory for the object like in C++ Using a Class in a Program the program control class source file called myProg.java public class myProg { public static void main(String args []){ case sensitive // instantiate a Student object Student student= new Student("Joe Bloggs"); // invoke printDetails method student.printDetails(); } } Using the JDK • Create source files for each class in your program • The name of source file should be the same as the name of class public class myCode { … … … … } myCode.java Source File Compiling your source code • Compile each class source file into bytecode (class files) • To compile a java source file javac myCode.java • This creates a classfile called myCode.class 1001101001110101011 … … … … myCode.class Class File To run your program • To start your program running you run the bytecode of the program control class • The program control class has the main method • To run bytecode – pass it to the JVM java classFileName e.g. java myProg note no .class included