![ The Java Virtual Machine (JVM)
java code byte code (machine independent)
[ just-in-time compilation native code ] execution
We need an interpreter(JVM) to run Java programs.
• The bytecode is machine-independent and can be executed on any
machine that has a Java interpreter, which is the Java Virtual
Machine (JVM).
11
Features of Java : Interpreted Language
compile](https://image.slidesharecdn.com/lecture2-170603105042/85/Introduction-to-JAVA-11-320.jpg)
![ Subjected to intense scrutiny by security experts with
[potentially serious ] bugs found and fixed.
• become a big news if new bugs found!!
• A recent security flaw(2012/08) (0-day flaw)
One of the best mainstream platforms with the strongest
security guarantee.
15
Features of Java : Secure](https://image.slidesharecdn.com/lecture2-170603105042/85/Introduction-to-JAVA-15-320.jpg)
![How JAVA code are structured
A java application consists of
• many packages
• Some from JRE (built-ins)+ some from 3rd party + your own ]
A package consists of
• 0 or more classes
• 0 or more subpackages
A java source file xxx.java is called a compilation unit,
which may
• contain multiple java class definitions and
• may be compiled into multiple java byte code (***.class ).
24](https://image.slidesharecdn.com/lecture2-170603105042/85/Introduction-to-JAVA-24-320.jpg)
Introduction to JAVA
- 1. Introduction to JAVA Md. Tanvir Hossain Student, Dept of CSE Daffodil International University 1
- 2. Features of Java : Simple Intentionally created to be syntactically similar to C/C++ Eliminate traditionally troublesome features of C/C++ • Pointer arithmetic (no troublesome * and &) • Multiple inheritance • Implicit type coercions // if (1) {x = 1 ;} is not well-formed • Explicit memory management • Preprocessor 2
- 3. Features of Java : Simple Eliminate features of C/C++ • struct, typedef,union • generic, enum (both recovered since jdk5.0) • (Programmer controlled) operator overloading (but preserving method overloading) Features included as part of base language: • Threads • Exception handling • reflection 3
- 4. Features of Java : Object Oriented Java is inherently object-oriented. • Although many object-oriented languages began strictly as procedural languages, Java was designed from the start to be object-oriented. Object-oriented programming (OOP) is a popular programming approach that is replacing traditional procedural programming techniques. 4
- 5. Features of Java: Object Oriented One of the central issues in software development is how to reuse code. • Object-oriented programming provides great flexibility, modularity, clarity, and reusability through encapsulation, inheritance, and polymorphism. 5
- 6. What is object-oriented ? Systems are built from sets of classes Classes are instantiated at runtime to give objects Objects communicate via messages passing (i.e., method or function invocation) (In java) everything is part of a class 6
- 7. Supported OO Concepts: • Data abstraction and Encapsulation • Inheritance • Polymorphism – dynamic binding - overriding – overloading – generics Logical cluster of classes == package 7 Features of Java: Object Oriented
- 8. Features of Java : Distributed Distributed computing involves several computers working together on a network. Java is designed to make distributed computing easy. Since networking capability is inherently integrated into Java, writing network programs is like sending and receiving data to and from a file. 8
- 9. Features of Java : Distributed Network programming is supported by built in JDK class library: • TCP sockets ,UDP packets ,IP addresses • URLs ,RMI (Remote Method Invocation) • Web Service Security features designed into language 9
- 10. Language implementation • Compiler – produce machine code – Ex: Fortran, Cobol, C, C++ • Interpreter – execute source or compiled instructions on a virtual machine – Ex: Scheme, Haskell, Python., Java Hybrid compilation/interpretation : Java 10 Features of Java : Interpreted Language
- 11. The Java Virtual Machine (JVM) java code byte code (machine independent) [ just-in-time compilation native code ] execution We need an interpreter(JVM) to run Java programs. • The bytecode is machine-independent and can be executed on any machine that has a Java interpreter, which is the Java Virtual Machine (JVM). 11 Features of Java : Interpreted Language compile
- 12. characteristics of Java • Strongly-typed language (cf Smalltalk and VisualBasic) • Compile-time and runtime checking • No pointer arithmetic • Exception handling • Automatic memory management 12 Features of Java : Robust
- 13. Java compilers can detect many problems that would first show up at execution time in other languages. Java eliminated certain types of error-prone programming constructs found in other languages. Runtime exception-handling feature provides programming support for robustness. 13 Features of Java : Robust
- 14. Designed with security in mind. • Java implements several security mechanisms to protect your system against harm caused by stray programs. • Allow users to download untrusted code over a network and run it in a secure environment in which it cannot do any harm. Configurable security levels and restrictions. 14 Features of Java : Secure
- 15. Subjected to intense scrutiny by security experts with [potentially serious ] bugs found and fixed. • become a big news if new bugs found!! • A recent security flaw(2012/08) (0-day flaw) One of the best mainstream platforms with the strongest security guarantee. 15 Features of Java : Secure
- 16. Byte-codes are architecture neutral • Write once, run anywhere • With a Java Virtual Machine (JVM), you can write one program that will run on any platform. Performance suffers somewhat by using bytecodes 16 Features of Java : Architecture Neutral
- 17. Source code : • Primitive type sizes are explicit - not architecture dependent • Strings and characters are (16-bit) Unicode compliant – easier for internationalization. Byte code: • run everywhere where a JVM is available User experience : • GUI libraries give a native graphic library-independent mechanism for creating quality graphical interfaces (sort of) • "They gave us a library that is good for writing programs that look equally mediocre on the different systems." 17 Features of Java : Portable
- 18. Interpreting leads to quicker development cycle Depends what you compare it to • "Slightly faster than VB" - (Core Java, page 9) • JITC(Just-In-Time Compiler) help greatly in this respect • Sun’s Java HotSpot is Newest high performace JIT compiler. Can use native code for mission-critical performance sections of code • JNI: Java Native Interface • Sacrifice portability. 18 Features of Java : High Performance
- 19. Based on well-known 20 year old Hoare monitor synchronization Thread support built into language Thread synchronization primitives supplied Garbage collector runs permanently as a low priority background thread Issue: • Still hard to use right for general programmers: • New package java.util.concurrency and other extensions added since jdk5 allow easier and correct use of threads. 19 Features of Java : Multithreaded
- 20. Class linking, layout, name resolution and object references not resolved until run-time Runtime Type Information (RTTI) available • Can check the type of objects at run-time • java.reflect.* package Class class for dynamic instantiation • Can create objects of types unkown until runtime. String name = getSpecies(); // get a name of a class Object p = Class.forName(name).newInstance(); if(p instanceof Hen) {…} else if(p instanceof Coq) { …} else {… } 20 Features of Java : Dynamic
- 21. Green Project (1990) • need a small PL for developing consumer device operating software • Requirements: small size, robust, portable, extremely reliable ,real- time performance Oak // the resulting language • Originally used C++, then realized a new language was needed • Original requirements same as for current language Java (1993) // OAK was renamed Java • Intended market never eventuated (no technology/product sold) • WWW starting to takeoff • Language design "based on" many current OO languages (e.g., C++, Eiffel, Smalltalk, Cedar/Mesa, Objective C) • 1995/5/23 Sun launched Java JDK 1.0 released early 1996/1/23 ( 211 classes / 8 pkgs) 21 Early history of Java
- 22. Evolution of the Java Language 22
- 23. Java Jargon 23
- 24. How JAVA code are structured A java application consists of • many packages • Some from JRE (built-ins)+ some from 3rd party + your own ] A package consists of • 0 or more classes • 0 or more subpackages A java source file xxx.java is called a compilation unit, which may • contain multiple java class definitions and • may be compiled into multiple java byte code (***.class ). 24
- 25. Package and class naming A class or package has two names: • simpleName • fully qualified name = parentPackageName . simpleName • packageName = parentPackageName . simpleName • For package, we use only its full name. – Ex: – java.lang.String – javax.swing.JButton 25
- 26. Package and class naming The package/subpackage/(*.class, *.java) structure is not only logically analogous to directory/ subdirectory/ file in OS file system but in fact they are stored physically in file system in this way. • Namely, if package p is located in a directory d then subpackage p.q of p would be stored in subdirecty q of d, and class p.A of p would be stored in file named A.class or A.java. 26