Tuesday, October 16, 2012

Java: Variable, Identifiers and Data Types


Variable, Identifiers and Data Types

Variables are used for data that change during program execution. All variables have a name, a type, and a scope. The programmer assigns the names to variables, known as identifiers. An Identifier must be unique within a scope of the Java program. Variables have a data type, that indicates the kind of value they can store. Variables declared inside of a block or method are called local variables; They are not automatically initialized. The compiler will generate an error as a result of the attempt to access the local variables before a value has been assigned.
public class localVariableEx {
  public static int a;
  public static void main(String[] args) {
    int b;
    System.out.println("a : "+a);
    System.out.println("b : "+b);  //Compilation error
}}
Note in the above example, a compilation error results in where the variable is tried to be accessed and not at the place where its declared without any value.
The data type indicates the attributes of the variable, such as the range of values that can be stored and the operators that can be used to manipulate the variable. Java has four main primitive data types built into the language. You can also create your own composite data types.
Java has four main primitive data types built into the language. We can also create our own data types.
  • Integer: byte, short, int, and long.
  • Floating Point: float and double
  • Character: char
  • Boolean: variable with a value of true or false.
  • The following chart (Taken from Sun Java Site) summarizes the default values for the java built in data types. Since I thought Mentioning the size was not important as part of learning Java, I have not mentioned it in the below table. The size for each Java type can be obtained by a simple Google search.
    Data TypeDefault Value (for fields)Range
    byte0-127 to +128
    short0-32768 to +32767
    int0
    long0L
    float0.0f
    double0.0d
    char‘\u0000′0 to 65535
    String (object)null
    booleanfalse
    When we declare a variable we assign it an identifier and a data type.
    For Example
    String message = “hello world”
    In the above statement, String is the data type for the identifier message. If you don’t specify a value when the variable is declared, it will be assigned the default value for its data type.
    Identifier Naming Rules
    • Can consist of upper and lower case letters, digits, dollar sign ($) and the underscore ( _ ) character.
    • Must begin with a letter, dollar sign, or an underscore
    • Are case sensitive
    • Keywords cannot be used as identifiers
    • Within a given section of your program or scope, each user defined item must have a unique identifier
    • Can be of any length.

    Java Comments


    Comments

    Comments are descriptions that are added to a program to make code easier to understand. The compiler ignores comments and hence its only for documentation of the program.
    Java supports three comment styles.

    Block style comments begin with /* and terminate with */ that spans multiple lines.
    Line style comments begin with // and terminate at the end of the line. (Shown in the above program)
    Documentation style comments begin with /** and terminate with */ that spans multiple lines.

    /** This class is a Hello World Program used to introduce
    the Java Language*/
    public class HelloWorld {
    public static void main(String[] args) {
    System.out.println(“Hello World”); //Prints output to console

    }
    }




    They are generally created using the automatic documentation generation tool, such as javadoc. (Shown in the above program)
    name of this compiled file is comprised of the name of the class with .class as an extension.

    Java Keywords


    Keywords

    There are certain words with a specific meaning in java which tell (help) the compiler what the program is supposed to do. These Keywords cannot be used as variable names, class names, or method names. Keywords in java are case sensitive, all characters being lower case.
    Keywords are reserved words that are predefined in the language; see the table below (Taken from Sun Java Site). All the keywords are in lowercase.
        abstract    default    if            private      this
     boolean     do         implements    protected    throw
     break       double     import        public       throws
     byte        else       instanceof    return       transient
     case        extends    int           short        try
     catch       final      interface     static       void
     char        finally    long          strictfp     volatile
     class       float      native        super        while
     const       for        new           switch
     continue    goto       package       synchronized


    /** This class is a Hello World Program used to introduce
    the Java Language*/
    public class HelloWorld {
    public static void main(String[] args) {
    System.out.println(“Hello World”); //Prints output to console
    }
    }

    Some Tricky Observations: The words virtual, ifdef, typedef, friend, struct and union are all words related to
    the C programming language. const and goto are Java keywords. The word finalize is the name of a method
    of the Object class and hence not a keyword. enum and label are not keywords.

    Getting Started with Java


    Java Architecture


    The Java environment is composed of a number of system components. You use these components at compile time to create the Java program and at run time to execute the program. Java achieves its independence by creating programs designed to run on the Java Virtual Machine rather than any specific computer system.

    After you write a Java program, you use a compiler that reads the statements in the program and translates them into a machine independent format called bytecode.
    Bytecode files, which are very compact, are easily transported through a distributed system like the Internet.
    The compiled Java code (resulting byte code) will be executed at run time.
    Java programs can be written and executed in two ways:

    Stand-alone application (A Java Swing Application)
    Applet which runs on a web browser (Example: Internet Explorer)

    Java source code

    A Java program is a collection of one or more java classes. A Java source file can contain more than one class definition and has a .java extension. Each class definition in a source file is compiled into a separate class file. The name of this compiled file is comprised of the name of the class with .class as an extension. Before we proceed further in this section, I would recommend you to go through the ‘Basic Language Elements’.

    Below is a java sample code for the traditional Hello World program. Basically, the idea behind this Hello World program is to learn how to create a program, compile and run it. To create your java source code you can use any editor( Text pad/Edit plus are my favorites) or you can use an IDE like Eclipse.

    public class HelloWorld {
    public static void main(String[] args) {
    System.out.println("Hello World");
    }//End of main
    }//End of HelloWorld Class

    Output
    Hello World

    ABOUT THE PROGRAM

    I created a class named “HelloWorld” containing a simple main function within it. The keyword class specifies that we are defining a class. The name of a public class is spelled exactly as the name of the file (Case Sensitive). All java programs begin execution with the method named main(). main method that gets executed has the following signature : public static void main(String args[]).Declaring this method as public means that it is accessible from outside the class so that the JVM can find it when it looks for the program to start it. It is necessary that the method is declared with return type void (i.e. no arguments are returned from the method). The main method contains a String argument array that can contain the command line arguments. The brackets { and } mark the beginning and ending of the class. The program contains a line ‘System.out.println(“Hello World”);’ that tells the computer to print out on one line of text namely ‘Hello World’. The semi-colon ‘;’ ends the line of code. The double slashes ‘//’ are used for comments that can be used to describe what a source code is doing. Everything to the right of the slashes on the same line does not get compiled, as they are simply the comments in a program.

    Java Main method Declarations

    class MainExample1 {public static void main(String[] args) {}}
    class MainExample2 {public static void main(String []args) {}}
    class MainExample3 {public static void main(String args[]) {}}

    All the 3 valid main method’s shown above accepts a single String array argument.

    Compiling and Running an Application

    To compile and run the program you need the JDK distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class libraries and packages, and tools. Download an editor like Textpad/EditPlus to type your code. You must save your source code with a .java extension. The name of the file must be the name of the public class contained in the file.

    Steps for Saving, compiling and Running a Java

    Step 1:Save the program With .java Extension.
    Step 2:Compile the file from DOS prompt by typing javac <filename>.
    Step 3:Successful Compilation, results in creation of .class containing byte code
    Step 4:Execute the file by typing java <filename without extension>

    Java Development Kit

    The Java Developer’s Kit is distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class libraries and packages, and tools

    Javadoc

    The javadoc tool provided by Sun is used to produce documentation for an application or program,

    Jar Files

    A jar file is used to group together related class files into a single file for more compact storage, distribution, and transmission.

    Monday, October 15, 2012

    Java Best Interview Questions


    Interview Questions on Java

    What if the main method is declared as private?
    The program compiles properly but at runtime it will give “Main method not public.” message.

    What is meant by pass by reference and pass by value in Java?
    Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.

    If you’re overriding the method equals() of an object, which other method you might also consider?
    hashCode()

    What is Byte Code?
    Or
    What gives java it’s “write once and run anywhere” nature?
    All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.

    Expain the reason for each keyword of public static void main(String args[])?
    public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.
    static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static.
    void: main does not return anything so the return type must be void
    The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line.

    What are the differences between == and .equals() ?
    Or
    what is difference between == and equals
    Or
    Difference between == and equals method
    Or
    What would you use to compare two String variables – the operator == or the method equals()?
    Or
    How is it possible for two String objects with identical values not to be equal under the == operator?
    The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.
    == compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.
     public class EqualsTest {
    
     public static void main(String[] args) {
    
      String s1 = "abc";
      String s2 = s1;
      String s5 = "abc";
      String s3 = new String("abc");
      String s4 = new String("abc");
      System.out.println("== comparison : " + (s1 == s5));
      System.out.println("== comparison : " + (s1 == s2));
      System.out.println("Using equals method : " + s1.equals(s2));
      System.out.println("== comparison : " + s3 == s4);
      System.out.println("Using equals method : " + s3.equals(s4));
     }
    }
    Output
    == comparison : true
    == comparison : true
    Using equals method : true
    false
    Using equals method : true

    What if the static modifier is removed from the signature of the main method?
    Or
    What if I do not provide the String array as the argument to the method?
    Program compiles. But at runtime throws an error “NoSuchMethodError”.

    Why oracle Type 4 driver is named as oracle thin driver?
    Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string uses the TCP/IP port address, not the TNSNAMES entry for the database name.

    What is the difference between final, finally and finalize? What do you understand by the java final keyword?
    Or
    What is final, finalize() and finally?
    Or
    What is finalize() method?
    Or
    What is the difference between final, finally and finalize?
    Or
    What does it mean that a class or member is final?
    final – declare constant
    finally – handles exception
    finalize – helps in garbage collection
    Variables defined in an interface are implicitly final. A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.

    What is the Java API?
    The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.
    What is the GregorianCalendar class?
    The GregorianCalendar provides support for traditional Western calendars.

    What is the ResourceBundle class?
    The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.

    Why there are no global variables in Java?
    Global variables are globally accessible. Java does not support globally accessible variables due to following reasons:
    • The global variables breaks the referential transparency
    • Global variables creates collisions in namespace.
    How to convert String to Number in java program?
    The valueOf() function of Integer class is is used to convert string to Number. Here is the code example:
    String numString = “1000″;
    int id=Integer.valueOf(numString).intValue();
    What is the SimpleTimeZone class?
    The SimpleTimeZone class provides support for a Gregorian calendar.

    What is the difference between a while statement and a do statement?
    A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once.

    What is the Locale class?
    The Locale class is used to tailor a program output to the conventions of a particular geographic, political, or cultural region.

    Describe the principles of OOPS.
    There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.

    Explain the Inheritance principle.
    Inheritance is the process by which one object acquires the properties of another object. Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places

    What is implicit casting?
    Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios.
    Example
    int i = 1000;
    long j = i; //Implicit casting

    Is sizeof a keyword in java?
    The sizeof operator is not a keyword.

    What is a native method?
    A native method is a method that is implemented in a language other than Java.

    In System.out.println(), what is System, out and println?
    System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object.

    What are Encapsulation, Inheritance and Polymorphism
    Or
    Explain the Polymorphism principle. Explain the different forms of Polymorphism.
    Polymorphism in simple terms means one name many forms. Polymorphism enables one entity to be used as a general category for different types of actions. The specific action is determined by the exact nature of the situation.
    Polymorphism exists in three distinct forms in Java:
    • Method overloading
    • Method overriding through inheritance
    • Method overriding through the Java interface

    What is explicit casting?
    Explicit casting in the process in which the complier are specifically informed to about transforming the object.
    Example
    long i = 700.20;
    int j = (int) i; //Explicit casting
    What is the Java Virtual Machine (JVM)?
    The Java Virtual Machine is software that can be ported onto various hardware-based platforms

    What do you understand by downcasting?
    The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy

    What are Java Access Specifiers?
    Or
    What is the difference between public, private, protected and default Access Specifiers?
    Or
    What are different types of access modifiers?
    Access specifiers are keywords that determine the type of access to the member of a class. These keywords are for allowing
    privileges to parts of a program such as functions and variables. These are:
    • Public : accessible to all classes
    • Protected : accessible to the classes within the same package and any subclasses.
    • Private : accessible only to the class to which they belong
    • Default : accessible to the class to which they belong and to subclasses within the same package

    Which class is the superclass of every class?
    Object.

    Name primitive Java types.
    The 8 primitive types are byte, char, short, int, long, float, double, and boolean.

    What is the difference between static and non-static variables?
    Or
    What are class variables?
    Or
    What is static in java?
    Or
    What is a static method?
    A static variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static variables i.e. there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class. These are declared outside a class and stored in static memory. Class variables are mostly used for constants. Static variables are always called by the class name. This variable is created when the program starts and gets destroyed when the programs stops. The scope of the class variable is same an instance variable. Its initial value is same as instance variable and gets a default value when its not initialized corresponding to the data type. Similarly, a static method is a method that belongs to the class rather than any object of the class and doesn’t apply to an object or even require that any objects of the class have been instantiated.
    Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method with a non-static method. In other words, you can’t change a static method into an instance method in a subclass.
    Non-static variables take on unique values with each object instance.
    What is the difference between the boolean & operator and the && operator?
    If an expression involving the boolean & operator is evaluated, both operands are evaluated, whereas the && operator is a short cut operator. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. If the first operand evaluates to false, the evaluation of the second operand is skipped.

    How does Java handle integer overflows and underflows?
    It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

    What if I write static public void instead of public static void?
    Program compiles and runs properly.

    What is the difference between declaring a variable and defining a variable?
    In declaration we only mention the type of the variable and its name without initializing it. Defining means declaration + initialization. E.g. String s; is just a declaration while String s = new String (“bob”); Or String s = “bob”; are both definitions.

    What type of parameter passing does Java support?
    In Java the arguments (primitives and objects) are always passed by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.

    Explain the Encapsulation principle.
    Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. Objects allow procedures to be encapsulated with their data to reduce potential interference. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.

    What do you understand by a variable?
    Variable is a named memory location that can be easily referred in the program. The variable is used to hold the data and it can be changed during the course of the execution of the program.

    What do you understand by numeric promotion?
    The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integral and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

    What do you understand by casting in java language? What are the types of casting?
    The process of converting one data type to another is called Casting. There are two types of casting in Java; these are implicit casting and explicit casting.

    What is the first argument of the String array in main method?
    The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. If we do not provide any arguments on the command line, then the String array of main method will be empty but not null.

    How can one prove that the array is not null but empty?
    Print array.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print array.length.

    Can an application have multiple classes having main method?
    Yes. While starting the application we mention the class name to be run. The JVM will look for the main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

    When is static variable loaded? Is it at compile time or runtime? When exactly a static block is loaded in Java?

    Static variable are loaded when classloader brings the class to the JVM. It is not necessary that an object has to be created. Static variables will be allocated memory space when they have been loaded. The code in a static block is loaded/executed only once i.e. when the class is first initialized. A class can have any number of static blocks. Static block is not member of a class, they do not have a return statement and they cannot be called directly. Cannot contain this or super. They are primarily used to initialize static fields.

    Can I have multiple main methods in the same class?
    We can have multiple overloaded main methods but there can be only one main method with the following signature :
    public static void main(String[] args) {}
    No the program fails to compile. The compiler says that the main method is already defined in the class.

    Explain working of Java Virtual Machine (JVM)?
    JVM is an abstract computing machine like any other real computing machine which first converts .java file into .class file by using Compiler (.class is nothing but byte code file.) and Interpreter reads byte codes.

    How can I swap two variables without using a third variable?
    Add two variables and assign the value into First variable. Subtract the Second value with the result Value. and assign to Second variable. Subtract the Result of First Variable With Result of Second Variable and Assign to First Variable. Example:
    int a=5,b=10;a=a+b; b=a-b; a=a-b;
    An other approach to the same question
    You use an XOR swap.
    for example:
    int a = 5; int b = 10;
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;

    What is data encapsulation?
    Encapsulation may be used by creating ‘get’ and ‘set’ methods in a class (JAVABEAN) which are used to access the fields of the object. Typically the fields are made private while the get and set methods are public. Encapsulation can be used to validate the data that is to be stored, to do calculations on data that is stored in a field or fields, or for use in introspection (often the case when using javabeans in Struts, for instance). Wrapping of data and function into a single unit is called as data encapsulation. Encapsulation is nothing but wrapping up the data and associated methods into a single unit in such a way that data can be accessed with the help of associated methods. Encapsulation provides data security. It is nothing but data hiding.

    What is reflection API? How are they implemented?
    Reflection is the process of introspecting the features and state of a class at runtime and dynamically manipulate at run time. This is supported using Reflection API with built-in classes like Class, Method, Fields, Constructors etc. Example: Using Java Reflection API we can get the class name, by using the getName method.

    Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why
    Yes, the JVM maintains a cache by itself. It creates the Objects on the HEAP, but references to those objects are on the STACK.
    What is phantom memory?
    Phantom memory is false memory. Memory that does not exist in reality.

    Can a method be static and synchronized?
    A static method can be synchronized. If you do so, the JVM will obtain a lock on the java.lang.
    Class instance associated with the object. It is similar to saying:
    synchronized(XYZ.class) {
    }

    What is difference between String and StringTokenizer?
    A StringTokenizer is utility class used to break up string.
    Example:
    StringTokenizer st = new StringTokenizer(“Hello World”);
    while (st.hasMoreTokens()) {
    System.out.println(st.nextToken());
    }
    Output:
    Hello
    World

    About Java : Basics


    About Java


    Java is a simple and yet powerful object oriented programming language and it is in many respects similar to C++. Java was given birth at Sun Microsystems, Inc. in 1991. Java was conceived by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridan at Sun Microsystems, Inc. It was developed to provide a platform-independent programming language.

    Platform independent

    Unlike many other programming languages including C and C++ when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by Java virtual Machine (JVM) on whichever platform it is being run.
    Java Virtual Machine

    What is the Java Virtual Machine? What is its role?

    Java was designed with a concept of ‘write once and run everywhere’. Java Virtual Machine plays the central role in this concept. The Java Virtual Machine (JVM) is the environment in which Java programs execute. It is a software that is implemented on top of real hardware and operating system.
    When Java source code (.java files) is compiled, it is translated into Java bytecodes and then placed into (.class) files. The JVM executes Java bytecodes. So Java bytecodes can be thought of as the machine language of the JVM. A Java virtual machine can either interpret the bytecode one instruction at a time or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler. The JVM must be implemented on a particular platform before compiled Java programs can run on that platform.

    Java has powerful features. The following are some of them:-

    Java is object oriented

    Since Java is an object oriented programming language it has following advantages:
    Reusability of Code
    Emphasis on data rather than procedure
    Data is hidden and cannot be accessed by external functions
    Objects can communicate with each other through functions
    New data and functions can be easily added

    Java is Distributed

    With extensive set of routines to handle TCP/IP protocols like HTTP and FTP java can open and access the objects across net via URLs.

    Java is Multithreaded

    One of the powerful aspects of the Java language is that it allows multiple threads of execution to run concurrently within the same program A single Java program can have many different threads executing independently and continuously. Multiple Java applets can run on the browser at the same time sharing the CPU time.

    Java is Secure

    Java was designed to allow secure execution of code across network. To make Java secure many of the features of C and C++ were eliminated. Java does not use Pointers. Java programs cannot access arbitrary addresses in memory.

    Garbage collection

    Automatic garbage collection is another great feature of Java with which it prevents inadvertent corruption of memory. Similar to C++, Java has a new operator to allocate memory on the heap for a new object. But it does not use delete operator to free the memory as it is done in C++ to free the memory if the object is no longer needed. It is done automatically with garbage collector.

    Application of Java

    Java has evolved from a simple language providing interactive dynamic content for web pages to a predominant enterprise-enabled programming language suitable for developing significant and critical applications. Today, Java is used for many types of applications including Web based applications, Financial applications, Gaming applications, embedded systems, Distributed enterprise applications, mobile applications, Image processors, desktop applications and many more.

    Thursday, September 27, 2012

    Setting the JAVA_HOME Environment Variable(Class Path) in Windows

    Environment variables are global system variables accessible by all the processes running under the Operating System (OS). Environment variables are useful to store system-wide values such as the directories to search for the executable programs, the OS version, and the location of Windows binaries.After Installation of Java, you cannot compile programs until you have directory to your path environment variable if not already present, otherwise the tools won't be found when you try to compile and run programs. So you need to Set up Class path First. This tutorial explains how you can do that.If you want help on how to download or install Jdk, you can Read It Here

    Setting Java Class Path

    Classpath in Java is path to directory or list of directory which is used byClassLoaders to find and load class in Java program.
    Here is Step by Step guide for setting Java Classpath in Windows
    • Go to Environment variable window in Windows by pressing "Windows + Pause" > Advanced > Environment variable " or you can go from right click on my computer than choosing properties and then Advanced and then Environment variable this will open Environment variable window.How to se Java Classpath in windows
    • Now specify your environment variable CLASSPATH and put the value of your JAVA_HOME\lib and also include current directory by including (dot or period sign).
    • Now to check the value of Java classpath in windows type "echo %CLASSPATH" in your DOS command prompt and it will show you the value of directory which are included in CLASSPATH.

      You can also set classpath in windows by using DOS command like :

      set CLASSPATH=%CLASSPATH%;JAVA_HOME\lib;
       
    • This way you can set classpath in Windows XP, windows 2000 or Windows 7 and 8, as they all come with command prompt.

    Setting JAVA_HOME to JDK Installation Directory

    JRE (Java Runtime) is needed for running Java programs. JDK (Java Development Kit) is needed for writing and running Java programs. JDK includes JRE plus development tools such as Java compiler.

    Set JAVA_HOME to your JDK installation directory (e.g., "c:\Program Files\java\jdk1.6.0_xx"). JAVA_HOME is needed for running Tomcat and many other Java applications. JAVA_HOME is the directory you install your JDK.

    For windows users (in Vista / Windows 7), you can set the JAVA_HOME permanently via "Control Panel" > "System" > "Advanced system settings". Switch to "Advanced" tab > "Environment variables" and choose "System Variables" (for all users) or "User Variables" (for this login user only). To create JAVA_HOME, choose "New". In variable "Name", enter " JAVA_HOME" and in variable "Value", provide the JDK installation directory.

    You can optionally set JRE_HOME to the JRE base directory (e.g., "c:\Program Files\java\jre1.6.0_xx")

    After setting all above environment variable you can compile and run your Java application freely.