JRE and Multiple JVM

What is JRE (Java Run time environment) ?

·  JRE is a  Java Runtime Environment. It is a software that provide an environment to run the Java program.

·  To run  the java program we need to install JRE on our machine.

·  JRE consists of  JVM , core classes(class libraries), and supporting files.

i.e. JRE = JVM + Java Packages Classes(like util, math, lang, awt,swing etc)+runtime libraries.

JRE  does not contain any development tools such as compiler, debugger, etc.

· Compiler will convert the java code in to byte code and  JVM runs that complied program, and it uses the class libraries, and other supporting files provided in JRE.

Java application launcher opens a JRE, loads the class, and invokes its main method.

· The JRE is available for multiple computer platforms, including Mac, Windows, and Unix.

· If the JRE is not installed on a computer, Java programs may not be recognized by the operating system and will not run.

   JRE software is available as both a standalone environment and a Web browser plug-in, which allows Java applets to be run within a Web browser.

JRE for different version of Java

Adding  different feature, fixing bug etc to each version of java and will release new versions

We can see below,JRE download has not grown significantly in recent memory.

                                 Java 1.4.2         Java 5 (1.5)                    Java 6 (1.6)

JRE                           15.10 MB          16.13 MB                       15.42 MB  

SDK                          49.33 MB          51.82 MB                       72.90 MB

Multiple JVM 

Each instance of the Java execution will create separate JVM. If you start any two Java-based processes on your computer, they will each be running in their own JVM. You can run more than one java process, whether of the same JVM version or different ones.

Two JVMs cannot run and share memory. Each has its own heap, in different areas of RAM.

You would need to use sockets, multiple agents, RMI or similar to access code on different JVMs. 

Advantages and Disadvantages of Inheritance in Java

What is the use of Inheritance in Java? Examples where you used the concept of Inheritance in your project?

Inheritance: Code reuse using subclass and super class relationship. Subclass inherits all the members (fields, methods, and nested classes) from its superclass.
Inheritance is unidirectional, it is expressed using  “is a “ relationship. The car is a Vehicle, but all the vehicles are not a car.

Example:

 Benefits:

             

Minimize the amount of duplicate code in an application

If  duplicate code (variable and methods) exists in two related classes, we can refactored that hierarchy by moving that common code up to the common superclass.

Better organization of code

Moving of common code to superclass results in better organization of code.

Code more flexible change

Inheritance can also make application code more flexible to change because classes that inherit from a common superclass can be used interchangeably. If the return type of a method is superclass.

Where to use inheritance:

  

Before using inheritance first compare the relationship for the two classes. Use inheritance only if there is a parent child relationship is there.

A way to test the relationship is to ask a "is-a" question.

Real time example:

Consider the above example. Here if you ask ‘is a’ question between house and building. House “is a” building , but house is not a bathroom. So here we have a parent -> child relationship between building and house.

Disadvantage:

The inheritance relationship is a, tightly coupled relationship , there will be tight bonding between parent and child. If we change code of parent class it will get affects to the all the child classes which is inheriting the parent code.

Examples from projects:

If you took an example from banking. All the account will have balance amount enquire,withdraw and deposit amount.

So here we can create a parent class Account with

public class Account {

   private double balance;

   public Account(double startingBalance)

   {

getBalance () {…}

debitCash(double amount) {}           

creditCash(double amount){}

}

public class OverdraftAccount extends Account {

   public OverdraftAccount(double balance)

   {

     super(balance);

   }

 }

/* Note:  Constructors of super class are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass using ‘super’ keyword.(As the first line of the subclass constructor) */

this.overdraftLimit = limit;

     this.overdraftFee = overdraftFee;

                       

   public double getAvailableFunds()

   {

     return super.getBalance() + overdraftLimit;

   }

 }