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| Chapter 1 Self Test
1. What is bytecode and why is it important to Java’s use for Internet programming? 2. What are the three main principles of objectoriented programming? 3. Where do Java programs begin execution? 4. What is a variable? 5. Which of the following variable names is invalid? A. count B. $count C. count27 D. 67count 6. How do you create a singleline comment? How do you create a multiline comment? 7. Show the general form of the if statement. Show the general form of the for loop. 8. How do you create a block of code? 9. The moon’s gravity is about 17 percent that of earth’s. Write a program that computes your effective weight on the moon. 10. Adapt Try This 12 so that it prints a conversion table of inches to meters. Display 12 feet of conversions, inch by inch. Output a blank line every 12 inches. (One meter equals approximately 39.37 inches.) 11. If you make a typing mistake when entering your program, what sort of error will result? 12. Does it matter where on a line you put a statement? Chapter 2 Introducing Data Types and Operators Key Skills & Concepts ● Know Java’s primitive types ● Use literals ● Initialize variables ● Know the scope rules of variables within a method ● Use the arithmetic operators History Topics Tutorials Offers & Deals Highlights Settings Support Sign Out A ● Use the relational and logical operators ● Understand the assignment operators ● Use shorthand assignments ● Understand type conversion in assignments ● Cast incompatible types ● Understand type conversion in expressions t the foundation of any programming language are its data types and operators, and Java is no exception. These elements define the limits of a language and determine the kind of tasks to which it can be applied. Fortunately, Java supports a rich assortment of both data types and operators, making it suitable for any type of programming. Data types and operators are a large subject. We will begin here with an examination of Java’s foundational data types and its most commonly used operators. We will also take a closer look at variables and examine the expression. WHY DATA TYPES ARE IMPORTANT Data types are especially important in Java because it is a strongly typed language. This means that all operations are typechecked by the compiler for type compatibility. Illegal operations will not be compiled. Thus, strong type checking helps prevent errors and enhances reliability. To enable strong type checking, all variables, expressions, and values have a type. There is no concept of a “typeless” variable, for example. Furthermore, the type of a value determines what operations are allowed on it. An operation allowed on one type might not be allowed on another. JAVA’S PRIMITIVE TYPES Java contains two general categories of builtin data types: objectoriented and non objectoriented. Java’s objectoriented types are defined by classes, and a discussion of classes is deferred until later. However, at the core of Java are eight primitive (also called elemental or simple) types of data, which are shown in Table 21 . The term primitive is used here to indicate that these types are not objects in an objectoriented sense, but rather, normal binary values. These primitive types are not objects because of efficiency concerns. All of Java’s other data types are constructed from these primitive types. Table 21 Java’s Builtin Primitive Data Types Java strictly specifies a range and behavior for each primitive type, which all implementations of the Java Virtual Machine must support. Because of Java’s portability requirement, Java is uncompromising on this account. For example, an int is the same in all execution environments. This allows programs to be fully portable. There is no need to rewrite code to fit a specific platform. Although strictly specifying the range of the primitive types may cause a small loss of performance in some environments, it is necessary in order to achieve portability. Integers Java defines four integer types: byte, short, int, and long, which are shown here: As the table shows, all of the integer types are signed positive and negative values. Java does not support unsigned (positiveonly) integers. Many other computer languages support both signed and unsigned integers. However, Java’s designers felt that unsigned integers were unnecessary. NOTE Technically, the Java runtime system can use any size it wants to store a primitive type. However, in all cases, types must act as specified. The most commonly used integer type is int. Variables of type int are often employed to control loops, to index arrays, and to perform generalpurpose integer math. When you need an integer that has a range greater than int, use long. For example, here is a program that computes the number of cubic inches contained in a cube that is one mile by one mile, by one mile: Here is the output from the program: Clearly, the result could not have been held in an int variable. The smallest integer type is byte. Variables of type byte are especially useful when working with raw binary data that may not be directly compatible with Java’s other builtin types. The short type creates a short integer. Variables of type short are appropriate when you don’t need the larger range offered by int. Ask the Expert Q : You say that there are four integer types: int, short, long, and byte. However, I have heard that char can also be categorized as an integer type in Java. Can you explain? A : The formal specification for Java defines a type category called integral types, which includes byte, short, int, long, and char. They are called integral types because they all hold wholenumber, binary values. However, the purpose of the first four is to represent numeric integer quantities. The purpose of char is to represent characters. Therefore, the principal uses of char and the principal uses of the other integral types are fundamentally different. Because of the differences, the char type is treated separately in this book. Floating-Point Types As explained in Chapter 1 , the floatingpoint types can represent numbers that have fractional components. There are two kinds of floatingpoint types, float and double, which represent single and doubleprecision numbers, respectively. Type float is 32 bits wide and type double is 64 bits wide. Of the two, double is the most commonly used, and many of the math functions in Java’s class library use double values. For example, the sqrt( ) method (which is defined by the standard Math class) returns a double value that is the square root of its double argument. Here, sqrt( ) is used to compute the length of the hypotenuse, given the lengths of the two opposing sides: The output from the program is shown here: One other point about the preceding example: As mentioned, sqrt( ) is a member of the standard Math class. Notice how sqrt( ) is called; it is preceded by the name Math. This is similar to the way System.out precedes println( ). Although not all standard methods are called by specifying their class name first, several are. Characters In Java, characters are not 8bit quantities like they are in many other computer languages. Instead, Java uses Unicode. Unicode defines a character set that can represent all of the characters found in all human languages. In Java, char is an unsigned 16bit type having a range of 0 to 65,535. The standard 8bit ASCII character set is a subset of Unicode and ranges from 0 to 127. Thus, the ASCII characters are still valid Java characters. A character variable can be assigned a value by enclosing the character in single quotes. For example, this assigns the variable ch the letter X: You can output a char value using a println( ) statement. For example, this line outputs the value in ch: Since char is an unsigned 16bit type, it is possible to perform various arithmetic manipulations on a char variable. For example, consider the following program: The output generated by this program is shown here: In the program, ch is first given the value X. Next, ch is incremented. This results in ch containing Y, the next character in the ASCII (and Unicode) sequence. Next, ch is assigned the value 90, which is the ASCII (and Unicode) value that corresponds to the letter Z. Since the ASCII character set occupies the first 127 values in the Unicode character set, all the “old tricks” that you may have used with characters in other languages will work in Java, too. Ask the Expert Q : Why does Java use Unicode? A : Java was designed for worldwide use. Thus, it needs to use a character set that can represent all the world’s languages. Unicode is the standard character set designed expressly for this purpose. Of course, the use of Unicode is inefficient for languages such as English, German, Spanish, or French, whose characters can be contained within 8 bits. But such is the price that must be paid for global portability. THE BOOLEAN TYPE The boolean type represents true/false values. Java defines the values true and false using the reserved words true and false. Thus, a variable or expression of type boolean will be one of these two values. Here is a program that demonstrates the boolean type: The output generated by this program is shown here: There are three interesting things to notice about this program. First, as you can see, when a boolean value is output by println( ), "true" or "false" is displayed. Second, the value of a boolean variable is sufficient, by itself, to control the if statement. There is no need to write an if statement like this: Third, the outcome of a relational operator, such as <, is a boolean value. This is why the expression 10 > 9 displays the value "true." Further, the extra set of parentheses around 10 > 9 is necessary because the + operator has a higher precedence than the >. Try This 21 How Far Away Is the Lightning? In this project, you will create a program that computes how far away, in feet, a listener is from a lightning strike. Sound travels approximately 1,100 feet per second through air. Thus, knowing the interval between the time you see a lightning bolt and the time the sound reaches you enables you to compute the distance to the lightning. For this project, assume that the time interval is 7.2 seconds. 1. Create a new file called Sound.java. 2. To compute the distance, you will need to use floatingpoint values. Why? Because the time interval, 7.2, has a fractional component. Although it would be permissible to use a value of type float, we will use double in the example. 3. To compute the distance, you will multiply 7.2 by 1,100. You will then assign this value to a variable. 4. Finally, you will display the result. Here is the entire Sound.java program listing: 5. Compile and run the program. The following result is displayed: 6. Extra challenge: You can compute the distance to a large object, such as a rock wall, by timing the echo. For example, if you clap your hands and time how long it takes for you to hear the echo, then you know the total roundtrip time. Dividing this value by two yields the time it takes the sound to go one way. You can then use this value to compute the distance to the object. Modify the preceding program so that it computes the distance, assuming that the time interval is that of an echo. LITERALS In Java, literals refer to fixed values that are represented in their humanreadable 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 literals are specified as numbers without fractional components. For example, 10 and –100 are integer literals. Floatingpoint literals require the use of the decimal point followed by the number’s fractional component. For example, 11.123 is a floating point literal. Java also allows you to use scientific notation for floatingpoint 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, floatingpoint 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. You can embed one or more underscores into an integer or floatingpoint literal. Doing so can make it easier to read values consisting of many digits. When the literal is compiled, the underscores are simply discarded. Here is an example: This specifies the value 123,451,234. The use of underscores is particularly useful when encoding things like part numbers, customer IDs, and status codes that are commonly thought of as consisting of subgroups of digits. Hexadecimal, Octal, and Binary Literals As you may know, in programming it is sometimes easier to use a number system based on 8 or 16 instead of 10. The number system based on 8 is called octal, and it uses the digits 0 through 7. In octal the number 10 is the same as 8 in decimal. The base 16 number system is called hexadecimal and uses the digits 0 through 9 plus the letters A through F, which stand for 10, 11, 12, 13, 14, and 15. For example, the hexadecimal number 10 is 16 in decimal. Because of the frequency with which these two number systems are used, Java allows you to specify integer literals in hexadecimal or octal instead of decimal. A hexadecimal literal must begin with 0x or 0X (a zero followed by an x or X). An octal literal begins with a zero. Here are some examples: As a point of interest, Java also allows hexadecimal floatingpoint literals, but they are seldom used. It is possible to specify an integer literal by use of binary. To do so, precede the binary number with a 0b or 0B. For example, this specifies the value 12 in binary: 0b1100. Character Escape Sequences Enclosing character constants in single quotes works for most printing characters, but a few characters, such as the carriage return, pose a special problem when a text editor is used. In addition, certain other characters, such as the single and double quotes, have special meaning in Java, so you cannot use them directly. For these reasons, Java provides special escape sequences, sometimes referred to as backslash character constants, shown in Table 22 . These sequences are used in place of the characters that they represent. Table 22 Character Escape Sequences For example, this assigns ch the tab character: The next example assigns a single quote to ch: String Literals Java supports one other type of literal: the string. A string is a set of characters enclosed by double quotes. For example, is a string. You have seen examples of strings in many of the println( ) statements in the preceding sample programs. In addition to normal characters, a string literal can also contain one or more of the escape sequences just described. For example, consider the following program. It uses the \n and \t escape sequences. The output is shown here: Ask the Expert Q : Is a string consisting of a single character the same as a character literal? For example, is "k" the same as 'k'? A : No. You must not confuse strings with characters. A character literal represents a single letter of type char. A string containing only one letter is still a string. Although strings consist of characters, they are not the same type. Notice how the \n escape sequence is used to generate a new line. You don’t need to use multiple println( ) statements to get multiline output. Just embed \n within a longer string at the points where you want the new lines to occur. A CLOSER LOOK AT VARIABLES A CLOSER LOOK AT VARIABLES Variables were introduced in Chapter 1 . Here, we will take a closer look at them. As you learned earlier, variables are declared using this form of statement, Yüklə 83 Mb. Dostları ilə paylaş:
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