Why Java Is Platform Independent

Getting output from java program is two step procedure, compilation and interpretation. Output of java compiler is not executable code but it is compiled code. JVM is an interpreter for byte code. Byte code is set of instructions which are to be executed by java run time system called as Java Virtual Machine (JVM). Each JVM has its own executable format so this byte code (.class file) becomes platform independent as this byte code runs on JVM and not on actual OS.

So once we have compiled java program, it can be executed on any platform. The details of the JVM differ from platform to platform and each OS has its own JVM format.  So in java it is “write once; run anywhere, anytime”.

So java is platform independent and JVM is highly platform dependent.

Why Java Is Preferred?

The developers preferred java over traditional programming languages like C and C++.Java is preferred due to its unique feature platform independent, robustness and more secure.

·         Simple: Java inherits most of the syntax of C and many of the OOPs features of C++. In addition to this is has its own add in features which are easy for professional programmers to learn.

·         Object Oriented: Java uses OOPs concepts for faster programming. It has most of the packages and interfaces built-in which are easily reused and extended. The class and object structure provides data encapsulation, abstraction, inheritance and polymorphism.

·         Robust:  Java is robust because of two reasons memory management and exception handling. In traditional programming C,C++ dynamic memory management is very difficult as memory allocation and de-allocation must be done by programmer but java does this dynamic memory allocation and de-allocation without any extra efforts. Exceptional conditions may arise in traditional programming which may leads to abnormal termination of programs but java provides exception handling which helps to handle and eliminate these run time errors.

·         Secure: All java programs run over JVM platform and not on Operating system. So any OS related updates or modification does not affect execution of java program. Java also provides packages and access modifiers which in turn provides security. Java applets are executed inside sandbox which creates an environment which provides restricted access to OS resources and allows user to run untrusted code from unknown source safely. Java uses byte code verifier module which checks java code automatically before executing it. Java provides run time array boundary check which is not provided in traditional programming. In traditional programming memory references can be manipulated but in java you cannot manipulate references which interns provides secure memory access.

·         Platform Independent: Traditional languages C, C++ were designed to compile for specific platform (processors, OS). In java executable code runs over JVM platform which is independent of OS. So once we have byte code (.class file) we can execute it on any OS. So java code needs to be compile only once and we can execute it any number of times across different platforms (processors, OS). So it is “write once, run anywhere, anytime”. 

·      Multithreaded: Java provides multithreaded programming which allows doing many things simultaneously. Each thread performs separate task. For example using text editor with spell check feature enabled which allows editing and spelling check simultaneously.

Why Java is preferred over C and C++

When it comes on selecting a programming language, we need to consider few parameters like security, robustness, ease in programming and few other parameters, because of below mentioned parameters java is preferred over C++ :

·         Robust:  Java is robust because of two reasons memory management and exception handling. In C,C++ programming dynamic memory management is very difficult as memory allocation and de-allocation must be done by programmer but Java does this dynamic memory allocation and de-allocation without any extra efforts.

·         No Pointers: Use of pointer is avoided in Java. In C++ programming memory references can be manipulated but in Java, you cannot manipulate references which in turn provide secure memory access.

·         Platform Independence and Portability: Traditional languages C, C++ were designed to compile for specific platform (processors, OS). In Java executable code runs over JVM platform which is independent of OS. So once we have byte code (.class file) we can execute it on any OS. So Java code needs to be compiled only once and we can execute it any number of times across different platforms (processors, OS). So Java provides platform independence and portability.

·         Secure: Java provides sandbox security model for applet execution which creates an environment which provides restricted access to OS resources and allows user to run un-trusted code from unknown source safely. All Java programs run over JVM platform and not on Operating system. So any OS related updates or modifications do not affect execution of Java program.  Java uses byte code verifier module which checks Java code automatically before executing it.

·         Multithreading : its support to multithreading made java enabled for multitasking which allows available resources to be used optimally. Java has built in support for multithreading but in C++, multithreading is entirely dependent on Operating System.

Why java is Secure

When it comes across the security issue of programming languages, Java is surplus over traditional programming languages like C and C++. Java is more secure than C and C++ for the following reasons:

·         Its JVM and not OS: All Java programs run over JVM platform and not on Operating system. So any OS related updates or modification does not affect execution of Java program.

·         Sandbox Security Model: Java applets are executed inside sandbox which creates an environment, which provides restricted access to OS resources and allows user to run un-trusted code from unknown source safely.

·         No Pointer Manipulation: In traditional programming memory references can be manipulated but in Java we cannot manipulate references. Thus you cannot cause an object reference to point to an arbitrary memory location, which in turn provides secure memory access.

·         Byte Code Verifier:  Byte code verifier checks correctness of class files and API libraries. Java uses this byte code verifier module which checks Java code automatically before executing it.

·         Array Boundary Check: Java provides run time array boundary check which is not provided in traditional programming.

·         Packages and Access Modifiers: This combination of packages and access modifiers allows your class to have detailed knowledge of each other, but not expose that knowledge outside that package.

·         Run Time Exception Handling: Exceptional conditions may arise in traditional programming which may lead to abnormal termination of programs but Java provides exception handling which helps to handle and eliminate this abnormal termination.

·         Dynamic Memory Allocation and De-allocation: In traditional programming C,C++ dynamic memory management is very difficult as memory allocation and de-allocation must be done by programmer but Java does this dynamic memory allocation and de-allocation without any extra efforts. Java uses new operator to dynamically allocate memory and Garbage Collection technique to handle automatic memory de-allocation.

·         Two Level of Code Checking: Code is checked two times, first at compile time and then at run time which can be considered as more secure than single level of code checking.

Why main method is public static void in java

HelloWorld.java is a sample program which contains main method.

To Compile: javac HelloWorld.java

To Execute: java HelloWorld

Lets see what is meaning of public static and void keyword in main() declaration

·         public: The main method is called by JVM when we execute “Java HelloWorld”. But since the JVM is out of scope of main method of the class HelloWorld. to make it accessible by JVM, so main method is public in java.

·         static: static keyword allows us to access members without creating object of that class with the help of class name. When JVM calls main method, it does not have any instance of the class having main method. JVM calls main method internally using class name like HelloWorld.main().So main method is static in java.

·       void: when any method does not returns any value its return type must be void. As main method does not return any value, so main method is void in java.

Does Java support Multiple Inheritance?

Answer to this question is yes, java does not support multiple inheritance but by introducing the concept of interface it just gives the feeling of multiple inheritance but still it is not multiple inheritance.

Multiple inheritance means inheriting more than one class into single class. A class may implement more than one interface but it can inherit only one class. Interface does not contain method implementation but it contains abstract methods which need to be implemented by the class which uses that interface. So it just gives feeling of multiple inheritance but it is not multiple inheritance as the class which uses those interfaces must implement the methods present in those interfaces.

Multiple inheritance gives rise to diamond problem. Let us see that this problem is.

In the above diagram, two classes B and C are derived from same class A and class D is derived from two classes B and C using multiple inheritance. The problem with above type of inheritance is that when an instance of D is calling any method of class A, it is not clear whether to call version of method derived from class B or class C. You can see in the figure above that the classes essentially form the shape of a diamond, this problem is called the diamond problem.

In the designers' opinion, multiple inheritance causes more problems and confusions than it solves. The use of multiple inheritance results into diamond problem so for the sake of simplicity, multiple inheritance is not supported in java. 

Java is Compiled and Interpreted Both

It seems little confusing for java beginner to understand the exact difference between compilation and interpretation. Java is compiled language because java code is compiled to get byte code. Java is interpreted language because this byte code is interpreted into native code.

Actually getting output from java program is two steps process compilation and interpretation. The executable used for these steps are java compiler (javac) and java interpreter (java). First step is compilation in which the java compiler compiles the source code to generate byte code in the form of .class file. Actually this byte code is not machine code but it is just an intermediate code. In the second step java interpreter uses this byte code to generate native code for under lying operating system. The JIT (Just In Time) compiler comes into action in second interpretation step. The JIT reads the byte code to generate native code dynamically so that program runs fast.

Finally in short, java is compiled to byte code which is then used by JVM where it gets interpreted to native code. So java is compiled and interpreted both.