Friday 30 December 2011

Arrays

Arrays

An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. You've seen an example of arrays already, in the main method of the "Hello World!" application. This section discusses arrays in greater detail.
Illustration of an array as 10 boxes numbered 0 through 9; an index of 0 indicates the first element in the array
An array of ten elements

Each item in an array is called an element, and each element is accessed by its numerical index. As shown in the above illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.
The following program, ArrayDemo, creates an array of integers, puts some values in it, and prints each value to standard output.
class ArrayDemo {
    public static void main(String[] args) {
        // declares an array of integers
        int[] anArray;

        // allocates memory for 10 integers
        anArray = new int[10];
           
        // initialize first element
        anArray[0] = 100;
        // initialize second element
        anArray[1] = 200;
        // etc.
        anArray[2] = 300;
        anArray[3] = 400;
        anArray[4] = 500;
        anArray[5] = 600;
        anArray[6] = 700;
        anArray[7] = 800;
        anArray[8] = 900;
        anArray[9] = 1000;

        System.out.println("Element at index 0: "
                           + anArray[0]);
        System.out.println("Element at index 1: "
                           + anArray[1]);
        System.out.println("Element at index 2: "
                           + anArray[2]);
        System.out.println("Element at index 3: "
                           + anArray[3]);
        System.out.println("Element at index 4: "
                           + anArray[4]);
        System.out.println("Element at index 5: "
                           + anArray[5]);
        System.out.println("Element at index 6: "
                           + anArray[6]);
        System.out.println("Element at index 7: "
                           + anArray[7]);
        System.out.println("Element at index 8: "
                           + anArray[8]);
        System.out.println("Element at index 9: "
                           + anArray[9]);
    }
} 
The output from this program is:
Element at index 0: 100
Element at index 1: 200
Element at index 2: 300
Element at index 3: 400
Element at index 4: 500
Element at index 5: 600
Element at index 6: 700
Element at index 7: 800
Element at index 8: 900
Element at index 9: 1000
In a real-world programming situation, you'd probably use one of the supported looping constructs to iterate through each element of the array, rather than write each line individually as shown above. However, this example clearly illustrates the array syntax. You'll learn about the various looping constructs (for, while, and do-while) in the Control Flow section.

Declaring a Variable to Refer to an Array

The above program declares anArray with the following line of code:
// declares an array of integers
int[] anArray;
Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written as type[], where type is the data type of the contained elements; the square brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in the naming section. As with variables of other types, the declaration does not actually create an array — it simply tells the compiler that this variable will hold an array of the specified type.
Similarly, you can declare arrays of other types:
byte[] anArrayOfBytes;
short[] anArrayOfShorts;
long[] anArrayOfLongs;
float[] anArrayOfFloats;
double[] anArrayOfDoubles;
boolean[] anArrayOfBooleans;
char[] anArrayOfChars;
String[] anArrayOfStrings;
You can also place the square brackets after the array's name:
// this form is discouraged
float anArrayOfFloats[];
However, convention discourages this form; the brackets identify the array type and should appear with the type designation.

Creating, Initializing, and Accessing an Array

One way to create an array is with the new operator. The next statement in the ArrayDemo program allocates an array with enough memory for ten integer elements and assigns the array to the anArray variable.
// create an array of integers
anArray = new int[10];
If this statement were missing, the compiler would print an error like the following, and compilation would fail:
ArrayDemo.java:4: Variable anArray may not have been initialized.
The next few lines assign values to each element of the array:
// initialize first element
anArray[0] = 100;
// initialize second element
anArray[1] = 200;
// etc.
anArray[2] = 300;
Each array element is accessed by its numerical index:
System.out.println("Element 1 at index 0: "
                   + anArray[0]);
System.out.println("Element 2 at index 1: "
                   + anArray[1]);
System.out.println("Element 3 at index 2: "
                   + anArray[2]);
Alternatively, you can use the shortcut syntax to create and initialize an array:
int[] anArray = { 
    100, 200, 300,
    400, 500, 600, 
    700, 800, 900, 1000
};
Here the length of the array is determined by the number of values provided between { and }.
You can also declare an array of arrays (also known as a multidimensional array) by using two or more sets of square brackets, such as String[][] names. Each element, therefore, must be accessed by a corresponding number of index values.
In the Java programming language, a multidimensional array is simply an array whose components are themselves arrays. This is unlike arrays in C or Fortran. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemo program:
class MultiDimArrayDemo {
    public static void main(String[] args) {
        String[][] names = {
            {"Mr. ", "Mrs. ", "Ms. "},
            {"Smith", "Jones"}
        };
        // Mr. Smith
        System.out.println(names[0][0] +
            names[1][0]);
        // Ms. Jones
        System.out.println(names[0][2] +
            names[1][1]);
    }
}
The output from this program is:
Mr. Smith
Ms. Jones
Finally, you can use the built-in length property to determine the size of any array. The code
System.out.println(anArray.length);
will print the array's size to standard output.

Copying Arrays

The System class has an arraycopy method that you can use to efficiently copy data from one array into another:
public static void arraycopy(Object src,
                       int srcPos,
                       Object dest,
                       int destPos,
                       int length)
The two Object arguments specify the array to copy from and the array to copy to. The three int arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.
The following program, ArrayCopyDemo, declares an array of char elements, spelling the word "decaffeinated". It uses arraycopy to copy a subsequence of array components into a second array:
class ArrayCopyDemo {
    public static void main(String[] args) {
        char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
       'i', 'n', 'a', 't', 'e', 'd' };
        char[] copyTo = new char[7];

        System.arraycopy(copyFrom, 2, copyTo, 0, 7);
        System.out.println(new String(copyTo));
    }
}
The output from this program is:
caffein

Java programming fundamentals

The Java programming language defines the following kinds of variables:
  • Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.
  • Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated. A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number of gears will apply to all instances. The code static int numGears = 6; would create such a static field. Additionally, the keyword final could be added to indicate that the number of gears will never change.
  • Local Variables Similar to how an object stores its state in fields, a method will often store its temporary state in local variables. The syntax for declaring a local variable is similar to declaring a field (for example, int count = 0;). There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method. As such, local variables are only visible to the methods in which they are declared; they are not accessible from the rest of the class.
  • Parameters You've already seen examples of parameters, both in the Bicycle class and in the main method of the "Hello World!" application. Recall that the signature for the main method is public static void main(String[] args). Here, the args variable is the parameter to this method. The important thing to remember is that parameters are always classified as "variables" not "fields".
Variable names are case-sensitive. A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "_". 

A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values. The eight primitive data types supported by the Java programming language are:
  • byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation.
  • short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.
  • int: The int data type is a 32-bit signed two's complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead.
  • long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided by int.
  • float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead. Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform.
  • double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.
  • boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.
  • char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).

    Default Values

    It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.
    The following chart summarizes the default values for the above data types.
    Data Type Default Value (for fields)
    byte 0
    short 0
    int 0
    long 0L
    float 0.0f
    double 0.0d
    char '\u0000'
    String (or any object)   null
    boolean false

    Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.



    Literals

    You may have noticed that the new keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. As shown below, it's possible to assign a literal to a variable of a primitive type:
    boolean result = true;
    char capitalC = 'C';
    byte b = 100;
    short s = 10000;
    int i = 100000;
    

    Integer Literals

    An integer literal is of type long if it ends with the letter L or l; otherwise it is of type int. It is recommended that you use the upper case letter L because the lower case letter l is hard to distinguish from the digit 1.
    Values of the integral types byte, short, int, and long can be created from int literals. Values of type long that exceed the range of int can be created from long literals. Integer literals can be expressed these number systems:
  • Decimal: Base 10, whose digits consists of the numbers 0 through 9; this is the number system you use every day
  • Hexadecimal: Base 16, whose digits consist of the numbers 0 through 9 and the letters A through F
  • Binary: Base 2, whose digits consists of the numbers 0 and 1 (you can create binary literals in Java SE 7 and later)
For general-purpose programming, the decimal system is likely to be the only number system you'll ever use. However, if you need to use another number system, the following example shows the correct syntax. The prefix 0x indicates hexadecimal and 0b indicates binary:
// The number 26, in decimal
int decVal = 26;
//  The number 26, in hexadecimal
int hexVal = 0x1a;
// The number 26, in binary
int binVal = 0b11010;

Floating-Point Literals

A floating-point literal is of type float if it ends with the letter F or f; otherwise its type is double and it can optionally end with the letter D or d.
The floating point types (float and double) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal) and D or d (64-bit double literal; this is the default and by convention is omitted).
double d1 = 123.4;
// same value as d1, but 
// in scientific notation
double d2 = 1.234e2;
float f1  = 123.4f;

Character and String Literals

Literals of types char and String may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as '\u0108' (capital C with circumflex), or "S\u00ED Se\u00F1or" (Sí Señor in Spanish). Always use 'single quotes' for char literals and "double quotes" for String literals. Unicode escape sequences may be used elsewhere in a program (such as in field names, for example), not just in char or String literals.
The Java programming language also supports a few special escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage return), \" (double quote), \' (single quote), and \\ (backslash).
There's also a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. There's little you can do with a null value beyond testing for its presence. Therefore, null is often used in programs as a marker to indicate that some object is unavailable.
Finally, there's also a special kind of literal called a class literal, formed by taking a type name and appending ".class"; for example, String.class. This refers to the object (of type Class) that represents the type itself.

Using Underscore Characters in Numeric Literals

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example. to separate groups of digits in numeric literals, which can improve the readability of your code.
For instance, if your code contains numbers with many digits, you can use an underscore character to separate digits in groups of three, similar to how you would use a punctuation mark like a comma, or a space, as a separator.
The following example shows other ways you can use the underscore in numeric literals:
long creditCardNumber = 
    1234_5678_9012_3456L;
long socialSecurityNumber = 
    999_99_9999L;
float pi =  3.14_15F;
long hexBytes = 0xFF_EC_DE_5E;
long hexWords = 0xCAFE_BABE;
long maxLong =
    0x7fff_ffff_ffff_ffffL;
byte nybbles = 0b0010_0101;
long bytes =
    0b11010010_01101001_10010100_10010010;
You can place underscores only between digits; you cannot place underscores in the following places:
  • At the beginning or end of a number
  • Adjacent to a decimal point in a floating point literal
  • Prior to an F or L suffix
  • In positions where a string of digits is expected
The following examples demonstrate valid and invalid underscore placements (which are highlighted) in numeric literals:
// Invalid: cannot put underscores
// adjacent to a decimal point
float pi1 = 3_.1415F;
// Invalid: cannot put underscores 
// adjacent to a decimal point
float pi2 = 3._1415F;
// Invalid: cannot put underscores 
// prior to an L suffix
long socialSecurityNumber1 
    = 999_99_9999_L;

// This is an identifier, not 
// a numeric literal
int x1 = _52;
// OK (decimal literal)
int x2 = 5_2;
// Invalid: cannot put underscores
// At the end of a literal
int x3 = 52_;
// OK (decimal literal)
int x4 = 5_______2;

// Invalid: cannot put underscores
// in the 0x radix prefix
int x5 = 0_x52;
// Invalid: cannot put underscores
// at the beginning of a number
int x6 = 0x_52;
// OK (hexadecimal literal)
int x7 = 0x5_2; 
// Invalid: cannot put underscores
// at the end of a number
int x8 = 0x52_;




Thursday 29 December 2011

Java Fundamentals

This is Menaka...I would like to create a technical blog on java, which iam starting with the following drops on it...




Java

Design and architecture decisions drew from a variety of languages such as Eiffel, SmallTalk, Objective C, and Cedar/Mesa. The result is a language platform that has proven ideal for developing secure, distributed, network-based end-user applications in environments ranging from network-embedded devices to the World-Wide Web and the desktop.
Java technology is both a programming language and a platform.
The Java programming language is a high-level language that can be characterized by all of the following buzzwords:
  • Simple
  • Object oriented
  • Distributed
  • Multithreaded
  • Dynamic
  • Architecture neutral
  • Portable
  • High performance
  • Robust
  • Secure
  • The two principal products in the Java SE platform are: Java Development Kit (JDK) and Java SE Runtime Environment (JRE).
  •  The JDK is a superset of the JRE, and contains everything that is in the JRE, plus tools such as the compilers and debuggers necessary for developing applets and applications. The Java Runtime Environment (JRE) provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language.

Architecture Neutral and Portable

Java technology is designed to support applications that will be deployed into heterogeneous network environments. In such environments, applications must be capable of executing on a variety of hardware architectures. Within this variety of hardware platforms, applications must execute atop a variety of operating systems and interoperate with multiple programming language interfaces. To accommodate the diversity of operating environments, the Java CompilerTM product generates bytecodes--an architecture neutral intermediate format designed to transport code efficiently to multiple hardware and software platforms. The interpreted nature of Java technology solves both the binary distribution problem and the version problem; the same Java programming language byte codes will run on any platform.
Architecture neutrality is just one part of a truly portable system. Java technology takes portability a stage further by being strict in its definition of the basic language. Java technology puts a stake in the ground and specifies the sizes of its basic data types and the behavior of its arithmetic operators. Your programs are the same on every platform--there are no data type incompatibilities across hardware and software architectures.
The architecture-neutral and portable language platform of Java technology is known as the Java virtual machine. It's the specification of an abstract machine for which Java programming language compilers can generate code. Specific implementations of the Java virtual machine for specific hardware and software platforms then provide the concrete realization of the virtual machine. The Java virtual machine is based primarily on the POSIX interface specification--an industry-standard definition of a portable system interface. Implementing the Java virtual machine on new architectures is a relatively straightforward task as long as the target platform meets basic requirements such as support for multithreading.

 In the Java programming language, all source code is first written in plain text files ending with the .java extension. Those source files are then compiled into .class files by the javac compiler. A .class file does not contain code that is native to your processor; it instead contains bytecodes — the machine language of the Java Virtual Machine1 (Java VM). The java launcher tool then runs your application with an instance of the Java Virtual Machine.