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Concurrent and Distributed Programming Java for C/C++ Programmers Based on slides from Introduction to Software Engineering
,Why Use Java? Simple - Java has thrown out many of the complex features of C++ and C resulting in a simpler language (no pointers, no unions, no enumerations)
Object-oriented - Java is a single-root, single-inheritance object oriented language Multithreaded - Java has a built-in support for multithreading Distributed - Using Java RMI (remote method invocation) you can access objects on other machines almost as if they were local Portable - programs written in the Java language are platform independent ,The Java execution environment Like C and C++ programs, Java programs are compiled.
Unlike C and C++ programs, Java programs are not compiled down to a platform-specific machine language. Instead, Java programs are compiled down to a platform-independent language called bytecode. Bytecode is similar to machine language, but bytecode is not designed to run on any real, physical computer. Instead, bytecode is designed to be run by a program, called a Java Virtual Machine (JVM), which simulates a real machine. ,JVM – Java Virtual Machine JVM is an interpreter that translates Java bytecode into real machine language instructions that are executed on the underlying, physical machine
A Java program needs to be compiled down to bytecode only once; it can then run on any machine that has a JVM installed ,JVM – Cont. ,Running Java Programs // file HelloWorld.java public class HelloWorld { public staticvoid main(String[] args) { System.out.println(“Hello World !”); } } ﹥ javac HelloWorld.java The compilation phase: This command will produce the java bytecode file HelloWord.class ﹥ java HelloWorld The execution phase (on the JVM): This command will produce the output “Hello World!”
,Case Sensitivity Case sensitivity:
String is not the same as string
MAIN is not the same as main Java keywords are all lower case e.g. public class static void
,Naming Conventions Methods and variables start with a leading lowercase letter
next, push(), index, etc.
Classes starts with a leading upper-case letter String, StringBuffer, Vector, Calculator, etc.
,Naming Conventions – Cont. Constants (final) are all upper-case :
DEBUG, MAX_SCROLL_X, CAPACITY final double PI = 3.1415926;
Word separation in identifiers is done by capitalization (e.g maxValue), except for constants where underscore is used (e.g MAX_SCROLL_X) ,Comments C++ Like:// comment ../* this is a comment */
And Javadoc Comments:/** this is javadoc comment */ ,Flow control It is like C/C++: do/while switch int i=5; do { // act1 i--; } while(i!=0); if/else char c=IN.getChar(); switch(c) { case ‘a’: case ‘b’: // act1 break; default: // act2 } if(x==4) { // act1 } else { // act2 } for int j; for(int i=0;i﹤=9;i++) { j+=i; }
,Variables There are two types of variables in Java, primitive types (int, long, float etc.)and reference types (objects)
In an assignment statement, the value of a primitive typed variable is copied In an assignment statement, the pointer of a reference typed variable is copied ,Primitive Types The Java programming language guarantees the size, range, and behavior of its primitive types
* The default value for primitive typed variables is zero bit pattern
,Wrappers Java provides Objects which wrap primitive types.
Example: Integer n = new Integer(“4”); int m = n.intValue();
,Reference Types Reference types in Java are objects
An object has a set of data members (attributes) and a set of methods All reference typed variables are dynamically allocated from heap at runtime (and can’t be explicitly deallocated by the programmer) Referenced typed variables can’t be dereferenced (no reference * or derefernce & operators) The default value of reference typed variables is null ,Reference Types Java C++ MyObject x MyObject *x ( not init