The Internet helped catapult Java to the forefront of programming, and Java, in turn, has had a profound effect on the Internet. The reason for this is quite simple: Java expands the universe of objects that can move about freely in cyberspace. In a network, there are two very broad categories of objects that are transmitted between the server and your personal computer:
passive information and dynamic, active programs. For example, when you read your e-mail, you are viewing passive data. Even when you download a program, the program’s code is still only passive data until you execute it. However, a second type of object can be transmitted to your computer: a dynamic, self-executing program. Such a program is an active agent on the client computer, yet it is initiated by the server. For example, a program might be provided by the server to properly display the data that it is sending.
As desirable as dynamic, networked programs are, they also present serious problems in the areas of security and portability. Prior to Java, cyberspace was effectively closed to half of the entities that now live there. As you will see, Java addresses those concerns and, in doing so, has defined a new form of program: the applet.
Java Applets
An applet is a special kind of Java program that is designed to be transmitted over the Internet and automatically executed by a Java-compatible Web browser. Furthermore, an applet is downloaded on demand, just like an image, sound file, or video clip. The important difference is that an applet is an intelligent program, ot just an animation or media file. In other words, an applet is a program that can react to user input and dynamically change—not just run the same animation or sound over and over.
As exciting as applets are, they would be nothing more than wishful thinking if Java
were not able to address the two fundamental problems associated with them: security and portability. Before continuing, let’s define what these two terms mean relative to the Internet.
Security
As you are almost certainly aware, every time you download a “normal” program, you are risking a viral infection. Prior to Java, most users did not download executable programs frequently, and those that did, scanned them for viruses prior to execution. Even so, most users still worried about the possibility of infecting their systems with a virus or allowing a malicious program to run wild in their systems. (A malicious program might gather private information, such as credit card numbers, bank account balances, and passwords by searching the contents
of your computer’s local file system.) Java answers these concerns by providing a firewall between a networked application and your computer.
When using a Java-compatible web browser, it is possible to safely download Java applets without fear of viral infection. The way that Java achieves this is by confining a Java program to the Java execution environment and not allowing it access to other parts of the computer. (You will see how this is accomplished, shortly.) Frankly, the ability to download applets with confidence that no harm will be done to the client computer is the single most important aspect
of Java.
Portability
As discussed earlier, many types of computers and operating systems are connected to the Internet. For programs to be dynamically downloaded to all of the various types of platforms, some means of generating portable executable code is needed. As you will soon see, the same mechanism that helps ensure security also helps create portability. Indeed, Java’s solution to these two problems is both elegant and efficient.
Recently a new language called C# has come on the scene. Created by Microsoft to support its .NET Framework, C# is closely reated to Java. In fact, many of C#’s features were directly adapted from Java. Both Java and C# share the same general C++-style syntax, support distributed programming, and utilize the same object model. There are, of course, differences between Java and C#, but the overall “look and feel” of these languages is very similar. This means that if you already know C#, then learning Java will be especially easy. Conversely, if C# is in your future, then your knowledge of Java will come in handy.
Given the similarity between Java and C#, one might naturally ask, “Will C# replace
Java?” The answer is No. Java and C# are optimized for two different types of computing
environments. Just as C++ and Java will co-exist for a long time to come, so will C# and Java.
Java is directly related to both C and C++. Java inherits its syntax from C. Its object model is adapted from C++. Java’s relationship with C and C++ is important for several reasons. First, many programmers are familiar with the C/C++ syntax. This makes it easy for a C/C++ programmer to learn Java and, conversely, for a Java programmer to learn C/C++.
Second, Java’s designers did not “reinvent the wheel.” Instead, they further refined an already highly successful programming paradigm. The modern age of programming began with C. It moved to C++, and now to Java. By inheriting and building upon that rich heritage, Java provides a powerful, logically consistent programming environment that takes the best of the past and adds new features required by the online environment. Perhaps most important, because of their similarities, C, C++, and Java define a common, conceptual framework for the professional programmer. Programmers do not face major rifts when switching from one language to another.
One of the central design philosophies of both C and C++ is that the programmer is in
charge! Java also inherits this philosophy. Except for those constraints imposed by the Internet environment, Java gives you, the programmer, full control. If you program well, your programs reflect it. If you program poorly, your programs reflect that, too. Put differently, Java is not a language with training wheels. It is a language for professional programmers.
Java has one other attribute in common with C and C++: it was designed, tested, and refined by real, working programmers. It is a language grounded in the needs and experiences of the people who devised it. There is no better way to produce a top-flight professional programming language.
Because of the similarities between Java and C++, especially their support for objectoriented programming, it is tempting to think of Java as simply the “Internet version of C++.” However, to do so would be a mistake. Java has significant practical and philosophical differences.
Although Java was influenced by C++, it is not an enhanced version of C++. For example, it is neither upwardly nor downwardly compatible with C++. Of course, the similarities with C++ are significant, and if you are a C++ programmer, you will feel right at home with Java. Another point: Java was not designed to replace C++. Java was designed to solve a certain set of problems. C++ was designed to solve a different set of problems. Both will coexist for many years to come.
Common looping statements:
for loop
while
do while
Three components of a looping statement:
1. Initial value
2. Condition
3. Change of state
while loop
A while loop executes a statement, or a block of statements wrapped in curly braces, repeatedly until the condition specified by the Boolean expression reach the false condition.
The boolean expression is evaluated before any code in the following block has executed. When the boolean expression evaluates to true, the statements will execute. Once the statements have executed, control returns to the beginning of the while loop to check the boolean expression again.
Syntax in creating a while (expression) loop.
while(expression)
Statements...
}
Example:
int counter=0; //initial value
while(counter<5) style="font-weight: bold;">do while Loop
A do while loop is just like a while loop, except that it checks its condition at the end of the loop. This means that the do loop is guaranteed to execute at least one time. On the other hand, a while loop evaluates its Boolean expression at the beginning and there is generally no guarantee that the statements inside the loop will be executed, unless you program the code to explicitly do so.
Syntax in creating a do while (expression) loop.
do{
Statement...
}while(expression)
Example:
int counter=0;
do{
Console.WriteLine(counter);
counter++;
}while(counter<5);
Program Output:
0
1
2
3
4
The for Loop
A for loop works like a while loop, except that the syntax of the for loop includes initialization and condition modification. for loops are appropriate when you know exactly how many times you want to perform the statements within the loop. The contents within the for loop parentheses hold three sections separated by semicolons (
The initializer list is a comma separated list of expressions. These expressions are evaluated only once during the lifetime of the for loop. This is a one-time operation, before loop execution. This section is commonly used to initialize an integer to be used as a counter.
Once the initializer list has been evaluated, the for loop gives control to its second section, the boolean expression. There is only one boolean expression, but it can be as complicated as you like as long as the result evaluates to true or false. The boolean expression is commonly used to verify the status of a counter variable.
When the boolean expression evaluates to true, the statements within the curly braces of the for loop are executed. After executing for loop statements, control moves to the top of loop and executes the iterator list, which is normally used to increment or decrement a counter. The iterator list can contain a comma separated list of statements, but is generally only one statement.
for(int counter = 0; counter<5; counter++)
{
Console.WriteLine(counter);
}
Program Output:
0
1
2
3
4
In Java, literals refer to fixed values that are represented in their human-readable form. For example, the number 100 is a literal. Literals are also commonly called constants. For the most
part, literals, and their usage, are so intuitive that they have been used in one form or another by all the preceding sample programs. Now the time has come to explain them formally.
Java literals can be of any of the primitive data types. The way each literal is represented depends upon its type. As explained earlier, character constants are enclosed in single quotes. For example, 'a' and ' %' are both character constants.
Integer constants are specified as numbers without fractional components. For example, 10 and –100 are integer constants. Floating-point constants require the use of the decimal point followed by the number’s fractional component. For example, 11.123 is a floating-point constant.
Java also allows you to use scientific notation for floating-point numbers.
By default, integer literals are of type int. If you want to specify a long literal, append an l or an L. For example, 12 is an int, but 12L is a long. By default, floating-point literals are of type double. To specify a float literal, append an F or f to the constant. For example, 10.19F is of type float.
Although integer literals create an int value by default, they can still be assigned to variables of type char, byte, or short as long as the value being assigned can be represented by the target type. An integer literal can always be assigned to a long variable.
Integer Literals
Integer literals come in different formats: decimal (base 10), hexadecimal (base 16), and octal (base 8). In using integer literals in our program, we have to follow some special notations.
For decimal numbers, we have no special notations. We just write a decimal number as it is. For hexadecimal numbers, it should be preceded by “0x” or “0X”.
For octals, they are preceeded by “0”. For example, consider the number 12. It's decimal representation is 12, while in hexadecimal, it is 0xC, and in octal, it is equivalent to 014.
Integer literals default to the data type int. An int is a signed 32-bit value. In some cases, you may wish to force integer literal to the data type long by appending the “l” or “L” character. A long is a signed 64-bit value. We will cover more on data types later.
Floating-Point Literals
Floating point literals represent decimals with fractional parts. An example is 3.1415. Floating point literals can be expressed in standard or scientific notations. For example, 583.45 is in standard notation, while 5.8345e2 is in scientific notation. Floating point literals default to the data type double which is a 64-bit value. To use a smaller precision (32-bit) float, just append the “f” or “F” character.
Character Literals
Character Literals represent single Unicode characters. A Unicode character is a 16-bit character set that replaces the 8-bit ASCII character set. Unicode allows the inclusion of symbols and special characters from other languages. To use a character literal, enclose the character in single quote delimiters. For example, the letter a, is represented as ‘a’.
To use special characters such as a newline character, a backslash is used followed by the character code. For example, ‘\n’ for the newline character, ‘\r’ for the carriage return, ‘\b’ for backspace.
String Literals
String literals represent multiple characters and are enclosed by double quotes. An example of a string literal is, “Hello World”.
Boolean Literals
Boolean literals have only two values, true or false.
Reference: Jedi Courseware
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