Open Mind Tree

The Basic Java Application

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A PROGRAM IS A SEQUENCE of instructions that a computer can execute to perform some task. A simple enough idea, but for the computer to make any use of the instructions, they must be written in a form that the computer can use. This means that programs have to be written in programming languages. Programming languages differ from ordinary human languages in being completely unambiguous and very strict about what is and is not allowed in a program. The rules that determine what is allowed are called the syntax of the language. Syntax rules specify the basic vocabulary of the language and how programs can be constructed using things like loops, branches, and subroutines. A syntactically correct program is one that can be successfully compiled or interpreted; programs that have syntax errors will be rejected (hopefully with a useful error message that will help you fix the problem).
So, to be a successful programmer, you have to develop a detailed knowledge of the syntax of the programming language that you are using. However, syntax is only part of the story. It's not enough to write a program that will run -- you want a program that will run and produce the correct result! That is, the meaning of the program has to be right. The meaning of a program is referred to as its semantics. A semantically correct program is one that does what you want it to.
Furthermore, a program can be syntactically and semantically correct but still be a pretty bad program. Using the language correctly is not the same as using it well. For example, a good program has "style." It is written in a way that will make it easy for people to read and to understand. It follows conventions that will be familiar to other programmers. And it has an overall design that will make sense to human readers. The computer is completely oblivious to such things, but to a human reader, they are paramount. These aspects of programming are sometimes referred to as pragmatics.
When I introduce a new language feature, I will explain the syntax, the semantics, and some of the pragmatics of that feature. You should memorize the syntax; that's the easy part. Then you should get a feeling for the semantics by following the examples given, making sure that you understand how they work, and maybe writing short programs of your own to test your understanding. And you should try to appreciate and absorb the pragmatics -- this means learning how to use the language feature well, with style that will earn you the admiration of other programmers.
Of course, even when you've become familiar with all the individual features of the language, that doesn't make you a programmer. You still have to learn how to construct complex programs to solve particular problems. For that, you'll need both experience and taste. You'll find hints about software development throughout this textbook.

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We begin our exploration of Java with the problem that has become traditional for such beginnings: to write a program that displays the message "Hello World!". This might seem like a trivial problem, but getting a computer to do this is really a big first step in learning a new programming language (especially if it's your first programming language). It means that you understand the basic process of:

1. getting the program text into the computer,
2. compiling the program, and
3. running the compiled program.

The first time through, each of these steps will probably take you a few tries to get right. I won't go into the details here of how you do each of these steps; it depends on the particular computer and Java programming environment that you are using. See Section 2.6 for information about creating and running Java programs in specific programming environments. But in general, you will type the program using some sort of text editor and save the program in a file. Then, you will use some command to try to compile the file. You'll either get a message that the program contains syntax errors, or you'll get a compiled version of the program. In the case of Java, the program is compiled into Java bytecode, not into machine language. Finally, you can run the compiled program by giving some appropriate command. For Java, you will actually use an interpreter to execute the Java bytecode. Your programming environment might automate some of the steps for you -- for example, the compilation step is often done automatically -- but you can be sure that the same three steps are being done in the background.
Here is a Java program to display the message "Hello World!". Don't expect to understand what's going on here just yet; some of it you won't really understand until a few chapters from now:

// A program to display the message
// "Hello World!" on standard output

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

The command that actually displays the message is:

System.out.println("Hello World!");

This command is an example of a subroutine call statement. It uses a "built-in subroutine" named System.out.println to do the actual work. Recall that a subroutine consists of the instructions for performing some task, chunked together and given a name. That name can be used to "call" the subroutine whenever that task needs to be performed. A built-in subroutine is one that is already defined as part of the language and therefore automatically available for use in any program.
When you run this program, the message "Hello World!" (without the quotes) will be displayed on standard output. Unfortunately, I can't say exactly what that means! Java is meant to run on many different platforms, and standard output will mean different things on different platforms. However, you can expect the message to show up in some convenient place. (If you use a command-line interface, like that in Sun Microsystem's Java Development Kit, you type in a command to tell the computer to run the program. The computer will type the output from the program, Hello World!, on the next line. In an integrated development environment such as Eclipse, the output might appear somewhere in one of the environment's windows.)
You must be curious about all the other stuff in the above program. Part of it consists of comments. Comments in a program are entirely ignored by the computer; they are there for human readers only. This doesn't mean that they are unimportant. Programs are meant to be read by people as well as by computers, and without comments, a program can be very difficult to understand. Java has two types of comments. The first type, used in the above program, begins with // and extends to the end of a line. The computer ignores the // and everything that follows it on the same line. Java has another style of comment that can extend over many lines. That type of comment begins with /* and ends with */.
Everything else in the program is required by the rules of Java syntax. All programming in Java is done inside "classes." The first line in the above program (not counting the comments) says that this is a class named HelloWorld. "HelloWorld," the name of the class, also serves as the name of the program. Not every class is a program. In order to define a program, a class must include a subroutine named main, with a definition that takes the form:

public static void main(String[] args) {
      statements
}

When you tell the Java interpreter to run the program, the interpreter calls this main() subroutine, and the statements that it contains are executed. These statements make up the script that tells the computer exactly what to do when the program is executed. The main() routine can call subroutines that are defined in the same class or even in other classes, but it is the main() routine that determines how and in what order the other subroutines are used.
The word "public" in the first line of main() means that this routine can be called from outside the program. This is essential because the main() routine is called by the Java interpreter, which is something external to the program itself. The remainder of the first line of the routine is harder to explain at the moment; for now, just think of it as part of the required syntax. The definition of the subroutine -- that is, the instructions that say what it does -- consists of the sequence of "statements" enclosed between braces, { and }. Here, I've used statements as a placeholder for the actual statements that make up the program. Throughout this textbook, I will always use a similar format: anything that you see in this style of text (green and in boldface) is a placeholder that describes something you need to type when you write an actual program.
As noted above, a subroutine can't exist by itself. It has to be part of a "class". A program is defined by a public class that takes the form:

public class program-name {

    optional-variable-declarations-and-subroutines
    
    public static void main(String[] args) {
       statements
    }
    
    optional-variable-declarations-and-subroutines

}

The name on the first line is the name of the program, as well as the name of the class. (Remember, again, that program-name is a placeholder for the actual name!) If the name of the class is HelloWorld, then the class must be saved in a file called HelloWorld.java. When this file is compiled, another file named HelloWorld.class will be produced. This class file, HelloWorld.class, contains the translation of the program into Java bytecode, which can be executed by a Java interpreter. HelloWorld.java is called the source code for the program. To execute the program, you only need the compiled class file, not the source code.
The layout of the program on the page, such as the use of blank lines and indentation, is not part of the syntax or semantics of the language. The computer doesn't care about layout -- you could run the entire program together on one line as far as it is concerned. However, layout is important to human readers, and there are certain style guidelines for layout that are followed by most programmers. These style guidelines are part of the pragmatics of the Java programming language.
Also note that according to the above syntax specification, a program can contain other subroutines besides main(), as well as things called "variable declarations." You'll learn more about these later, but not until Chapter 4.

Variables and the Primitive Types

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NAMES ARE FUNDAMENTAL TO PROGRAMMING. In programs, names are used to refer to many different sorts of things. In order to use those things, a programmer must understand the rules for giving names to things and the rules for using the names to work with those things. That is, the programmer must understand the syntax and the semantics of names.
According to the syntax rules of Java, a name is a sequence of one or more characters. It must begin with a letter or underscore and must consist entirely of letters, digits, and underscores. ("Underscore" refers to the character '_'.) For example, here are some legal names:

N   n   rate  x15   quite_a_long_name   HelloWorld

No spaces are allowed in identifiers; HelloWorld is a legal identifier, but "Hello World" is not. Upper case and lower case letters are considered to be different, so that HelloWorld, helloworld,HELLOWORLD, and hElloWorLD are all distinct names. Certain names are reserved for special uses in Java, and cannot be used by the programmer for other purposes. These reserved words include: class,public, static, if, else, while, and several dozen other words.
Java is actually pretty liberal about what counts as a letter or a digit. Java uses the Unicode character set, which includes thousands of characters from many different languages and different alphabets, and many of these characters count as letters or digits. However, I will be sticking to what can be typed on a regular English keyboard.
The pragmatics of naming includes style guidelines about how to choose names for things. For example, it is customary for names of classes to begin with upper case letters, while names of variables and of subroutines begin with lower case letters; you can avoid a lot of confusion by following the same convention in your own programs. Most Java programmers do not use underscores in names, although some do use them at the beginning of the names of certain kinds of variables. When a name is made up of several words, such as HelloWorld or interestRate, it is customary to capitalize each word, except possibly the first; this is sometimes referred to as camel case, since the upper case letters in the middle of a name are supposed to look something like the humps on a camel's back.
Finally, I'll note that things are often referred to by compound names which consist of several ordinary names separated by periods. (Compound names are also called qualified names.) You've already seen an example: System.out.println. The idea here is that things in Java can contain other things. A compound name is a kind of path to an item through one or more levels of containment. The nameSystem.out.println indicates that something called "System" contains something called "out" which in turn contains something called "println". Non-compound names are called simple identifiers. I'll use the term identifier to refer to any name -- simple or compound -- that can be used to refer to something in Java. (Note that the reserved words are not identifiers, since they can't be used as names for things.)

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2.2.1  Variables

Programs manipulate data that are stored in memory. In machine language, data can only be referred to by giving the numerical address of the location in memory where it is stored. In a high-level language such as Java, names are used instead of numbers to refer to data. It is the job of the computer to keep track of where in memory the data is actually stored; the programmer only has to remember the name. A name used in this way -- to refer to data stored in memory -- is called a variable.
Variables are actually rather subtle. Properly speaking, a variable is not a name for the data itself but for a location in memory that can hold data. You should think of a variable as a container or box where you can store data that you will need to use later. The variable refers directly to the box and only indirectly to the data in the box. Since the data in the box can change, a variable can refer to different data values at different times during the execution of the program, but it always refers to the same box. Confusion can arise, especially for beginning programmers, because when a variable is used in a program in certain ways, it refers to the container, but when it is used in other ways, it refers to the data in the container. You'll see examples of both cases below.
(In this way, a variable is something like the title, "The President of the United States." This title can refer to different people at different times, but it always refers to the same office. If I say "the President is playing basketball," I mean that Barack Obama is playing basketball. But if I say "Sarah Palin wants to be President" I mean that she wants to fill the office, not that she wants to be Barack Obama.)
In Java, the only way to get data into a variable -- that is, into the box that the variable names -- is with an assignment statement. An assignment statement takes the form:

variable = expression;

where expression represents anything that refers to or computes a data value. When the computer comes to an assignment statement in the course of executing a program, it evaluates the expression and puts the resulting data value into the variable. For example, consider the simple assignment statement

rate = 0.07;

The variable in this assignment statement is rate, and the expression is the number 0.07. The computer executes this assignment statement by putting the number 0.07 in the variable rate, replacing whatever was there before. Now, consider the following more complicated assignment statement, which might come later in the same program:

interest = rate * principal;

Here, the value of the expression "rate * principal" is being assigned to the variable interest. In the expression, the * is a "multiplication operator" that tells the computer to multiply rate timesprincipal. The names rate and principal are themselves variables, and it is really the values stored in those variables that are to be multiplied. We see that when a variable is used in an expression, it is the value stored in the variable that matters; in this case, the variable seems to refer to the data in the box, rather than to the box itself. When the computer executes this assignment statement, it takes the value ofrate, multiplies it by the value of principal, and stores the answer in the box referred to by interest. When a variable is used on the left-hand side of an assignment statement, it refers to the box that is named by the variable.
(Note, by the way, that an assignment statement is a command that is executed by the computer at a certain time. It is not a statement of fact. For example, suppose a program includes the statement "rate = 0.07;". If the statement "interest = rate * principal;" is executed later in the program, can we say that the principal is multiplied by 0.07? No! The value of rate might have been changed in the meantime by another statement. The meaning of an assignment statement is completely different from the meaning of an equation in mathematics, even though both use the symbol "=".)

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2.2.2  Types and Literals

A variable in Java is designed to hold only one particular type of data; it can legally hold that type of data and no other. The compiler will consider it to be a syntax error if you try to violate this rule. We say that Java is a strongly typed language because it enforces this rule.
There are eight so-called primitive types built into Java. The primitive types are named byte, short, int, long, float, double, char, and boolean. The first four types hold integers (whole numbers such as 17, -38477, and 0). The four integer types are distinguished by the ranges of integers they can hold. The float and double types hold real numbers (such as 3.6 and -145.99). Again, the two real types are distinguished by their range and accuracy. A variable of type char holds a single character from the Unicode character set. And a variable of type boolean holds one of the two logical values true or false.
Any data value stored in the computer's memory must be represented as a binary number, that is as a string of zeros and ones. A single zero or one is called a bit. A string of eight bits is called a byte. Memory is usually measured in terms of bytes. Not surprisingly, the byte data type refers to a single byte of memory. A variable of type byte holds a string of eight bits, which can represent any of the integers between -128 and 127, inclusive. (There are 256 integers in that range; eight bits can represent 256 -- two raised to the power eight -- different values.) As for the other integer types,

• short corresponds to two bytes (16 bits). Variables of type short have values in the range -32768 to 32767.
• int corresponds to four bytes (32 bits). Variables of type int have values in the range -2147483648 to 2147483647.
• long corresponds to eight bytes (64 bits). Variables of type long have values in the range -9223372036854775808 to 9223372036854775807.

You don't have to remember these numbers, but they do give you some idea of the size of integers that you can work with. Usually, for representing integer data you should just stick to the int data type, which is good enough for most purposes.
The float data type is represented in four bytes of memory, using a standard method for encoding real numbers. The maximum value for a float is about 10 raised to the power 38. A float can have about 7 significant digits. (So that 32.3989231134 and 32.3989234399 would both have to be rounded off to about 32.398923 in order to be stored in a variable of type float.) A double takes up 8 bytes, can range up to about 10 to the power 308, and has about 15 significant digits. Ordinarily, you should stick to the double type for real values.
A variable of type char occupies two bytes in memory. The value of a char variable is a single character such as A, *, x, or a space character. The value can also be a special character such a tab or a carriage return or one of the many Unicode characters that come from different languages. When a character is typed into a program, it must be surrounded by single quotes; for example: 'A', '*', or 'x'. Without the quotes, A would be an identifier and * would be a multiplication operator. The quotes are not part of the value and are not stored in the variable; they are just a convention for naming a particular character constant in a program.
A name for a constant value is called a literal. A literal is what you have to type in a program to represent a value. 'A' and '*' are literals of type char, representing the character values A and *. Certain special characters have special literals that use a backslash, \, as an "escape character". In particular, a tab is represented as '\t', a carriage return as '\r', a linefeed as '\n', the single quote character as '\'', and the backslash itself as '\\'. Note that even though you type two characters between the quotes in '\t', the value represented by this literal is a single tab character.
Numeric literals are a little more complicated than you might expect. Of course, there are the obvious literals such as 317 and 17.42. But there are other possibilities for expressing numbers in a Java program. First of all, real numbers can be represented in an exponential form such as 1.3e12 or 12.3737e-108. The "e12" and "e-108" represent powers of 10, so that 1.3e12 means 1.3 times 10¹² and 12.3737e-108 means 12.3737 times 10^-108. This format can be used to express very large and very small numbers. Any numerical literal that contains a decimal point or exponential is a literal of type double. To make a literal of type float, you have to append an "F" or "f" to the end of the number. For example, "1.2F" stands for 1.2 considered as a value of type float. (Occasionally, you need to know this because the rules of Java say that you can't assign a value of type double to a variable of type float, so you might be confronted with a ridiculous-seeming error message if you try to do something like "x = 1.2;" when x is a variable of typefloat. You have to say "x = 1.2F;". This is one reason why I advise sticking to type double for real numbers.)
Even for integer literals, there are some complications. Ordinary integers such as 177777 and -32 are literals of type byte, short, or int, depending on their size. You can make a literal of type long by adding "L" as a suffix. For example: 17L or 728476874368L. As another complication, Java allows octal (base-8) and hexadecimal (base-16) literals. I don't want to cover base-8 and base-16 in detail, but in case you run into them in other people's programs, it's worth knowing a few things: Octal numbers use only the digits 0 through 7. In Java, a numeric literal that begins with a 0 is interpreted as an octal number; for example, the literal 045 represents the number 37, not the number 45. Hexadecimal numbers use 16 digits, the usual digits 0 through 9 and the letters A, B, C, D, E, and F. Upper case and lower case letters can be used interchangeably in this context. The letters represent the numbers 10 through 15. In Java, a hexadecimal literal begins with 0x or 0X, as in 0x45 or 0xFF7A.
Hexadecimal numbers are also used in character literals to represent arbitrary Unicode characters. A Unicode literal consists of \u followed by four hexadecimal digits. For example, the character literal'\u00E9' represents the Unicode character that is an "e" with an acute accent.
Java 7 introduces a couple of minor improvements in numeric literals. First of all, numeric literals in Java 7 can include the underscore character ("_"), which can be used to separate groups of digits. For example, the integer constant for one billion could be written 1_000_000_000, which is a good deal easier to decipher than 1000000000. There is no rule about how many digits have to be in each group. Java 7 also supports binary numbers, using the digits 0 and 1 and the prefix 0b (or OB). For example: 0b10110 or 0b1010_1100_1011.
For the type boolean, there are precisely two literals: true and false. These literals are typed just as I've written them here, without quotes, but they represent values, not variables. Boolean values occur most often as the values of conditional expressions. For example,

rate > 0.05

is a boolean-valued expression that evaluates to true if the value of the variable rate is greater than 0.05, and to false if the value of rate is not greater than 0.05. As you'll see in Chapter 3, boolean-valued expressions are used extensively in control structures. Of course, boolean values can also be assigned to variables of type boolean.
Java has other types in addition to the primitive types, but all the other types represent objects rather than "primitive" data values. For the most part, we are not concerned with objects for the time being. However, there is one predefined object type that is very important: the type String. A String is a sequence of characters. You've already seen a string literal: "Hello World!". The double quotes are part of the literal; they have to be typed in the program. However, they are not part of the actual string value, which consists of just the characters between the quotes. Within a string, special characters can be represented using the backslash notation. Within this context, the double quote is itself a special character. For example, to represent the string value

I said, "Are you listening!"

with a linefeed at the end, you would have to type the string literal:

"I said, \"Are you listening!\"\n"

You can also use \t, \r, \\, and Unicode sequences such as \u00E9 to represent other special characters in string literals. Because strings are objects, their behavior in programs is peculiar in some respects (to someone who is not used to objects). I'll have more to say about them in the next section.

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2.2.3  Variables in Programs

A variable can be used in a program only if it has first been declared. A variable declaration statement is used to declare one or more variables and to give them names. When the computer executes a variable declaration, it sets aside memory for the variable and associates the variable's name with that memory. A simple variable declaration takes the form:

type-name  variable-name-or-names;

The variable-name-or-names can be a single variable name or a list of variable names separated by commas. (We'll see later that variable declaration statements can actually be somewhat more complicated than this.) Good programming style is to declare only one variable in a declaration statement, unless the variables are closely related in some way. For example:

int numberOfStudents;
String name;
double x, y;        
boolean isFinished;
char firstInitial, middleInitial, lastInitial;

It is also good style to include a comment with each variable declaration to explain its purpose in the program, or to give other information that might be useful to a human reader. For example:

double principal;    // Amount of money invested.
double interestRate; // Rate as a decimal, not percentage.

In this chapter, we will only use variables declared inside the main() subroutine of a program. Variables declared inside a subroutine are called local variables for that subroutine. They exist only inside the subroutine, while it is running, and are completely inaccessible from outside. Variable declarations can occur anywhere inside the subroutine, as long as each variable is declared before it is used in any expression. Some people like to declare all the variables at the beginning of the subroutine. Others like to wait to declare a variable until it is needed. My preference: Declare important variables at the beginning of the subroutine, and use a comment to explain the purpose of each variable. Declare "utility variables" which are not important to the overall logic of the subroutine at the point in the subroutine where they are first used. Here is a simple program using some variables and assignment statements:

/**
 * This class implements a simple program that
 * will compute the amount of interest that is
 * earned on $17,000 invested at an interest
 * rate of 0.07 for one year.  The interest and
 * the value of the investment after one year are
 * printed to standard output.
 */
 
public class Interest {
   
   public static void main(String[] args) {
   
       /* Declare the variables. */
   
       double principal;     // The value of the investment.
       double rate;          // The annual interest rate.
       double interest;      // Interest earned in one year.
       
       /* Do the computations. */
       
       principal = 17000;
       rate = 0.07;
       interest = principal * rate;   // Compute the interest.
       
       principal = principal + interest;
             // Compute value of investment after one year, with interest.
             // (Note: The new value replaces the old value of principal.)
             
       /* Output the results. */
             
       System.out.print("The interest earned is $");
       System.out.println(interest);
       System.out.print("The value of the investment after one year is $");
       System.out.println(principal);
                      
   } // end of main()
      
} // end of class Interest

This program uses several subroutine call statements to display information to the user of the program. Two different subroutines are used: System.out.print and System.out.println. The difference between these is that System.out.println adds a linefeed after the end of the information that it displays, while System.out.print does not. Thus, the value of interest, which is displayed by the subroutine call "System.out.println(interest);", follows on the same line after the string displayed by the previous System.out.print statement. Note that the value to be displayed bySystem.out.print or System.out.println is provided in parentheses after the subroutine name. This value is called a parameter to the subroutine. A parameter provides a subroutine with information it needs to perform its task. In a subroutine call statement, any parameters are listed in parentheses after the subroutine name. Not all subroutines have parameters. If there are no parameters in a subroutine call statement, the subroutine name must be followed by an empty pair of parentheses.
All the sample programs for this textbook are available in separate source code files in the on-line version of this text at http://math.hws.edu/javanotes/source. They are also included in the downloadable archives of the web site. The source code for the Interest program, for example, can be found in the file Interest.java.

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By the way, recall that one of the neat features of Java is that it can be used to write applets that can run on pages in a Web browser. Applets are very different things from stand-alone programs such as theInterest program, and they are not written in the same way. For one thing, an applet doesn't have a main() routine. Applets will be covered in Chapter 6. In the meantime, you will see applets in this text thatsimulate stand-alone programs. The applets you see are not really the same as the stand-alone programs that they simulate, since they run right on a Web page, but they will have the same behavior as the programs I describe. Here, just for fun, is an applet simulating the Interest program. To run the program, click on the button labeled "Run the Program". You will see the output from the program in the large white area of the applet:
This applet requires Java 5.0 (or higher). It will not work in a web browser that does not support Java or that uses an earlier version of Java.
I will include many applets like this one in the text to simulate my example programs. This example isn't very interesting, since it does exactly the same thing every time you run it. That's not true of most programs, and it won't be true of future examples. You don't need to know how these applets are written, but if you are curious about how I convert my programs into applets, you can look at the source code fileTextIOApplet.java.