4 WORKING WITH DATA TYPES AND OPERATIONS

Transcription

1 WORKING WITH NUMERIC VALUES 27 4 WORKING WITH DATA TYPES AND OPERATIONS WORKING WITH NUMERIC VALUES This application will declare and display numeric values. To declare and display an integer value in an application: 1. Open the Eclipse IDE, if necessary. 2. Right-click at the helloworld project, create a new package named helloworld.variables. 3. Create a new class named DemoVariables in the helloworld.variables package. 4. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { } 5. Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following to declare a variable of type int named intno with a value of 296. int intno = 296; 6. Press Enter at the end of the intno declaration statement then type the following two statements. The first statement uses the print()method to output The int is and leaves the insertion point on the same output line. The second statement uses the println()method to output the value of intno and advances the insertion point to a new line. System.out.print( The int is ); System.out.println(intNo); 7. Save the file application. If necessary correct any errors and then save again. 8. Run the application. The output should appear as shown in Figure 4.1.

2 28 Java Programming Workbook Figure 4.1 Output of the DemoVariables application ADDING VARIABLES TO A CLASS To declare two or more variables in the application: 1. Return to the DemoVariables.java file in Eclipse. Rename the class DemoVariables2. 2. Position the insertion point at the end of the line that contains the intno declaration, press Enter and then type the following variable declarations on separate lines: short shortno = 19; long longno = L; 3. Position the insertion point at the end of the line that contains the println()method that displays the intno value, press Enter and then type the following statements to display the values of the two new variables: System.out.print( The short is ); System.out.println(shortNo); System.out.print( The long is ); System.out.println(longNo); 4. Save the application using the filename DemoVariables2.java. If necessary correct any errors and then save again. 5. Run the application. The output should appear as shown in Figure 4.2.

3 CONCATENATING STRING 29 Figure 4.2 Output of the DemoVariables2 application CONCATENATING STRING To change the two print methods into a single statement: 1. Open the DemoVariables2.java file and rename the class DemoVariables3. 2. Use the mouse to select the two statements that print The int is and the value of intno, and then press Delete to delete them. In place of the deleted statements, type the following println() statement: System.out.println( The int is + intno); 3. Select the two statements that produce output for the short variable, press Delete to delete them and then type the following statement: System.out.println( The short is + shortno); 4. Finally select the two statement that produce output for the long variable, delete them and then type the following statement: System.out.println( The long is + longno); 5. Save the file as DemoVariables3.java then run the application.

4 30 Java Programming Workbook Figure 4.3 Output of the DemoVariables3 application INTEGER ARITHMETIC OPERATORS You use arithmetic operators to perform calculations with values in your programs. A value used on either side of an operator is an operand. In the expression , 20 and 9 are operands. Operator Description Example + Addition , the result is 29 Subtraction 20 9, the result is 11 * Multiplication 20 * 9, the result is 180 / Division 20 / 9, the result is 2 (not 2.22) % Modulus 20 % 9, the result is 2 (that is, 20 / 9 with a remainder of 2) (remainder)

5 USING ARITHMETIC STATEMENTS 31 USING ARITHMETIC STATEMENTS To use some arithmetic statements in an application: 1. Open the DemoVariables3.java file in Eclipse and change the class to DemoVariables4. 2. Position the insertion point at the end of the last line of the current variable declarations, press Enter and then type the following declarations: int value1 = 43, value2 = 10, sum, difference, product, quotient, modulus; 3. Position the insertion point after the statement that prints the longno variable, press Enter and then type the following statements on separate lines: sum = value1 + value2; difference = value1 value2; product = value1 * value2; quotient = value1 / value2; modulus = value1 % value2; 4. After the calculations you just added, type the following output statements: System.out.println( Sum is + sum); System.out.println( Difference is + difference); System.out.println( Product is + product); System.out.println( Quotient is + quotient); System.out.println( Modulus is + modulus); 5. Save the application as DemoVariables4.java. Run the application. Your output should look like Figure 4.4. Analyze the output and confirm that the arithmetic is correct. USING FLOATING-POINT VARIABLES To add floating-point variables to the application: 1. Open the DemoVariables4.java file in Eclipse and change the class to DemoVariables5. 2. Position the insertion point after the line that declares the integer variables, press Enter and then type the following to add some new floating-point variables to the class: double doubnum1 = 2.3, doubnum2 = 14.8, doubresult; 3. Press Enter and then type the following statements to perform arithmetic and produce output: doubresult = doubnum1 + doubnum2; System.out.println( The sum of the doubles is + doubresult); doubresult = doubnum1 * doubnum2; System.out.println( The product of the doubles is + doubresult);

6 32 Java Programming Workbook 4. Save the file as DemoVariables5.java, compile it and then run the program. The output is shown in Figure 4.5. Figure 4.4 Output of the DemoVariables4 application Figure 4.5 Output of the DemoVariables5 application

7 USING BOOLEAN VARIABLES 33 COMPARISON OPERATORS A comparison operator compares two items. An expression containing a comparison operator has a Boolean value. Operator Description True example False example < Less than 5 < < 5 > Greater than 10 > 5 5 > 10 == Equal to 10 == 10 5 == 10 <= Less than or equal to 5 <= 5 10 <= 5 >= Greater than or equal to 10 >= 5 5 >= 10!= Not equal to 5!= 10 5!= 5 USING BOOLEAN VARIABLES To add Boolean variables to an application: 1. Open the DemoVariables5.java file in Eclipse and change the class name to DemoVariables6. 2. Position the insertion point at the end of the line with the double variable declarations, press Enter and then type the following on one line to add two new Boolean variables to the application: boolean isprogrammingfun = (10 > 5), isprogramminghard = (10 <= 5); 3. Next, add some print statement to display the values. Press Enter and then type the following statements: System.out.println( The value of isprogrammingfun is + isprogrammingfun); System.out.println( The value of isprogramminghard is + isprogramminghard); 4. Save the file as DemoVariables6.java and then run the application. The output as in Figure 4.6.

8 34 Java Programming Workbook Figure 4.6 Output of the DemoVariables6 application OPERATOR PRECEDENCE The Java compiler follows a set of rules called operator precedence to determine the order in which the operations should be performed. The order is the same as in ordinary mathematics. Operator Hierarchy Associativity Operation ( ) Left to right parentheses for explicit grouping * / % Left to right multiplication, division, modulus + Left to right addition, subtraction USING OPERATOR PRECEDENCE To develop a better understanding of the rules of operator precedence, consider the evaluation of a second-degree polynomial ( y = ax 2 + bx +c ). To implement operator precedence in the above application: 1. Open the DemoVariables6.java file in Eclipse and change the class name to DemoVariables7.

9 USING INCREMENT AND DECREMENT OPERATORS Position the insertion point at the end of the line with the Boolean variable declarations, press Enter and then type the following on one line to add a new integer variable to the application: int poly = (2 * 5 * 5) + (3 * 5) + 7; 3. Next, add some print statement to display the values. Press Enter and then type the following statements: System.out.println( The value of the second-degree polynomial is + poly); 4. Save the file as DemoVariables7.java and then run the application. The output as in Figure 4.7. Figure 4.7 Output of the DemoVariables7 application USING INCREMENT AND DECREMENT OPERATORS To implement increment and decrement operators in an application: 1. Open the DemoVariables7.java file in Eclipse and change the class name to DemoVariables8. 2. Position the insertion point at the end of the line with the integer variable declarations for polynomial example before, press Enter and then type the following on one line to add four new integer variables to the application: int num1 = 1, num2 = 2, num3, num4; 3. Then press Enter and then type the following statements on separate lines: num3 = ++num2; num4 = num1++; num3++; 4. After the calculations you just added, type the following output statements:

10 36 Java Programming Workbook System.out.println( The value of num1 is + num1); System.out.println( The value of num2 is + num2); System.out.println( The value of num3 is + num3); System.out.println( The value of num4 is + num4); 5. Save the file as DemoVariables8.java and then run the application. The output as in Figure 4.8. Figure 4.8 Output of the DemoVariables8 application USING NUMERIC TYPE CONVERSION When you are performing arithmetic involving two operands of different types, Java will automatically (or implicitly) converts the operand based on the following order: 1. double 2. float 3. long 4. int To use numeric type conversion in an application: 1. Create a new class named NumericConversion in the helloworld.variables package. 2. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { }

11 USING NUMERIC TYPE CONVERSION 37 INCREMENT AND DECREMENT OPERATORS In computer programming it is quite common to want to increase or decrease the value of an integer type by 1. Java provides the increment and decrement operators that add 1 to a variable and subtract 1 from a variable, respectively. Operator Name Description ++i Pre-increment Add 1 to i The value is returned after the increment is made i = 1; j = ++i; Then j will hold 2 and i will hold 2. --i Pre-decrement Subtract 1 from i The value is returned after the decrement is made i = 1; j = --i; Then j will hold 0 and i will hold 0. i++ Post-increment Add 1 to i The value is returned before the increment is made i = 1; j = i++; Then j will hold 1 and i will hold 2. i-- Post-decrement Subtract 1 from i The value is returned before the decrement is made i = 1; j = i--; Then j will hold 1 and i will hold Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following statement: int hoursworked = 37; double payrate = 4.50; double grosspay = hoursworked * payrate; System.out.println("Gross pay is "+ grosspay);

12 38 Java Programming Workbook 4. Save the application as NumericConversion.java. Run the application. Your output should look like Figure 4.9. Analyze the program and the output. Then confirm that the conversion is according to the order. 5. Now change the type of variable grosspay from double to int. What is the output? Figure 4.9 Output of the NumericConversion application You can purposely (or explicitly) override the automatic conversion imposed by Java by performing type cast. Type casting forces a value of one data type to be used as a value of another type. To use type casting in an application: 1. Open the NumericConversion.java file in Eclipse and change the class name to NumericConversion2. 2. Position the insertion point at the beginning of the line with the grosspay variable declaration. Change the declaration to the following statement: int grosspay = (int)(hoursworked * payrate); 3. Save the application as NumericConversion2.java. Run the application. Your output should look like Figure USING CHARACTER VARIABLES To use a character variable in an application: 1. Open the DemoVariables8.java file in Eclipse and change the class name to DemoVariables9.

13 USING CHARACTER VARIABLES 39 Figure 4.10 Output of the NumericConversion2 application 2. Position the insertion point after the line that declares the integer variables for increment and decrement example before, press Enter and then type the following statements. The first declares two character grades; the second display them. char mygrade = A, myfriendsgrade = C ; System.out.println( Our grades are + mygrade + and + myfriendsgrade); 3. Save the file as DemoVariables9.java and then test the application. Your output should look like Figure Figure 4.11 Output of the DemoVariables9 application

14 40 Java Programming Workbook COMMON ESCAPE SEQUENCES Escape sequence Description \b Backspace; moves the cursor on space to the left \t Tab; moves the cursor to the next tab stop \n Newline or linefeed; moves the cursor to the beginning of the next line \r Carriage return; moves the cursor to the beginning of the current line \ Double quotation mark; prints a double quotation mark \ Single quotation mark; prints a single quotation mark \\ Backslash; prints a backslash character The escape sequence \n (newline), \ (double quote) and \\ (backslash) operate as expected in the dialog boxes. Others do not work in the GUI environment. USING ESCAPE SEQUENCES Next you will add statements to your DemoVariables9.java file to use the \n and \t escape sequences. To use the escape sequences in an application: 1. Open the DemoVariables9.java file in Eclipse and change the class name to DemoVariables Position the insertion point after the statement that prints the values of mygrade and myfriendsgrade variables. Press Enter to start a new line, and then type the following statements: System.out.println( \nthis is on one line\nthis is on another ); System.out.println( This shows\thow\ttabs\twork ); 3. Save the file as DemoVariables10.java and then test the application. Your output should look like Figure 4.12.

15 Do it yourself 41 Figure 4.12 Output of the DemoVariables10 application Do it yourself 4.1 Based on the algorithm, write a Java program to calculate the area of a circle. Assign an integer values to the variable radius for example, int radius = 2. Save the class as Circle.java. Algorithm: 1. Read in the radius 2. Compute the area using the following formula: area = radius x radius x Display the area 4.2 Write a Java program that declares variables to represent the length and width of a room. Assign appropriate values to the variables for example, length = 20 and width = 15. Then compute and display the floor space in square feet. Display explanatory text with the value for example, The floor space is 300 square feet. Save the class as Room.java. [Note: Floor space = length * width.] 4.3 Write a program that converts pounds into kilograms. Assign appropriate values to the variable in pounds, converts it to kilograms, and displays the result. Save the class as Weight.java. [Note: One pound is kilograms.]

16 42 Java Programming Workbook 4.4 Write a class that displays the following heading on one line: First Name Last Name Matric Number Phone Number Display your first name, last name, matric number and phone number on the second line, below the appropriate column headings. Save the class as Escape.java. 4.5 Write a program that prints a box and a diamond as follows. Save the class as Box.java. ********* * * * * * * * * * * * * * * * * * * * * * ********* * 4.6 Using the escape sequences you learned before, write a program that calculates the squares and cubes of the numbers from 0 to 3 and uses tabs to print the following table of values. Save the class as Number.java. number square cube Identify and fix the error in the following code: public class Salary { public static void main(string[] args) { int paydate; System.out.println( Salary will be pay on + paydate + January ); }

17 CORRECTING ERRORS 43 CORRECTING ERRORS You might make typing errors as you enter Java statements into your text editor or Eclipse IDE. When you issue the command to compile the class containing errors, the Java compiler produces one or more error messages. Basically there are three types of errors that you will contend with when writing programs: Syntax errors (or semantic errors) Runtime errors Logic errors The errors become more difficult to find and fix as you move down the above list. SYNTAX ERRORS Syntax errors represent grammar errors in the use of the programming language. Common examples are: Misspelled variable and function names Missing semicolons Improperly matches parentheses, square brackets and curly braces Incorrect format in selection and loop statements To show syntax error in a program: 1. Create a new package named helloworld.errors under the helloworld project in Eclipse IDE. 2. Right-click at the helloworld.errors package and create a new class named ShowSyntaxErrors. 3. Position the insertion point after the opening curly brace, press Enter, and then type the following: public class ShowSyntaxErrors { public static void main(string[] args) { i = 30; System.out.println(i + 4); } } 4. Save the application and then test the application. The Errors in Workspace window will appear. Click Proceed. The output should look like the following Figure Rename the class name to ShowSyntaxErrorsFix. 6. To correct the errors, position the insertion point before the statement that declare the variable i to 30. Then type int and insert a space. 7. Save the application as ShowSyntaxErrorsFix.java and then test the application. Your output should look like Figure 4.14.

18 44 Java Programming Workbook Figure 4.13 Output of the ShowSyntaxErrors application Figure 4.14 Output of the ShowSyntaxErrorsFix application 8. You can now delete the ShowSyntaxErrors.java file. RUNTIME ERRORS Runtime errors occur when a program with no syntax errors asks the computer to do something that the computer is unable to reliably do. Common examples are: Trying to divide by a variable that contains a value of zero Trying to open a file that doesn't exist There is no way for the compiler to know about these kinds of errors when the program is compiled. To show runtime error in a program: 1. Right-click at the helloworld.errors package and create a new class named ShowRuntimeErrors. 2. Position the insertion point after the opening curly brace, press Enter, and then type the following:

19 CORRECTING ERRORS 45 public class ShowRuntimeErrors { public static void main(string[] args) { int i = 1 / 0; System.out.println(i); } } 3. Save the application and then test the application. The output should look like the following Figure Figure 4.15 Output of the ShowRuntimeErrors application LOGIC ERRORS Logic errors occur when there is a design flaw in your program. Common examples are: Multiplying when you should be dividing Adding when you should be subtracting Opening and using data from the wrong file Displaying the wrong message To show logic error in a program: 1. Right-click at the helloworld.errors package and create a new class named ShowLogicErrors. 2. Position the insertion point after the opening curly brace, press Enter, and then type the following: public class ShowLogicErrors { //Determine if a number is between 1 and 100 inclusively public static void main(string[] args) { int i = 100; System.out.println("The number is between 1 and 100, " + "inclusively? " + ((1 < i) && (i < 100))); } }

20 46 Java Programming Workbook 3. Save the application and then test the application. The output should look like the following Figure Analyze the program and the output. Figure 4.16 Output of the ShowLogicErrors application GETTING USER INPUT USING SCANNER CLASS To get input from a user using Scanner class in an application: 1. In the helloworld project, create a new class named ScannerInputs in the helloworld package. 2. Type the import statement after the package declaration line which allows you to use the Scanner class: import java.util.scanner; 3. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { } 4. Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following: Scanner scan = new Scanner(System.in); System.out.println( Enter your name ); String name = scan.nextline(); System.out.println( Enter first integer ); int first = scan.nextint(); System.out.println( Enter second integer ); int second = scan.nextint();

21 GETTING USER INPUT USING INPUT DIALOGS 47 System.out.print(name +, first integer is + first); System.out.println( and second integer is + second); 5. Save the file as ScannerInputs.java, compile and then test the application. 6. Highlight the console area and type in your name and press Enter. Do the same for the first and second integer. Your output should look like Figure Figure 4.17 Output of the ScannerInputs application GETTING USER INPUT USING INPUT DIALOGS To get input from a user using an input dialog box: 1. Create a new class named DialogInputs in the helloworld package. 2. Type the import statement that allows you to use the JOptionPane class: import javax.swing.joptionpane; 3. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { }

22 48 Java Programming Workbook 4. Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following: String first = JOptionPane.showInputDialog(null, Enter first integer, Example 1, JOptionPane.QUESTION_MESSAGE); JOptionPane.showMessageDialog(null, First integer is + first, Example 1, JOptionPane.INFORMATION_MESSAGE); String second = JOptionPane.showInputDialog(null, Enter second integer, Example 2, JOptionPane.QUESTION_MESSAGE); JOptionPane.showMessageDialog(null, Second integer is + second, Example 2, JOptionPane.INFORMATION_MESSAGE); 5. Save the application and then test the application. Your outputs should look like Figure Figure 4.18 Outputs of the DialogInputs application CONVERTING STRINGS TO INTEGERS AND DOUBLES (WHEN USING INPUT DIALOGS) The input returned from the input dialog box is a string. To convert a string to an integer or double value, use the following: int intvalue = Integer.parseInt( ); double doublevalue = Double.parseDouble( ); To convert a String to an integer in a dialog box: 1. Open the DialogInputs.java in Eclipse and change the class name to DialogInputs2.

23 FORMATTED OUTPUT Position the insertion point after the statement that display the second integer in a dialog box, press Enter and then type the following: int firstint = Integer.parseInt(first); int secondint = Integer.parseInt(second); JOptionPane.showMessageDialog(null, "Sum of the two integers is "+ (firstint + secondint), "Example 3", JOptionPane.INFORMATION_MESSAGE); 3. Save the application as DialogInput2.java and then test the application. Your output should look like Figure Figure 4.19 Outputs of the DialogInputs2 application FORMATTED OUTPUT You can format the output of a program using the printf method. This method allows you to format numeric values in two useful ways: By specifying the number of decimal places to display. By specifying the field size in which to display values. The syntax to invoke this method is System.out.printf(format, items);

24 50 Java Programming Workbook To specify a number of decimal places to display in an application: 1. Create a new class named FormatOutput in the helloworld package. 2. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { } 3. Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following: int age = 23; double money = 123; System.out.println( Age is + age); System.out.println( Money is RM + money); System.out.println( After format: ); System.out.printf("Age is %d and money is RM%.2f, age, money); 4. Save and then test the application. Your output should look like Figure Figure 4.20 Output of the FormatOutput application

25 FORMATTED OUTPUT 51 To specify a field size with printf() in an application: 1. Create a new class named FormatOutput2 in the helloworld package. 2. Position the insertion point after the opening curly brace, press Enter, and then type the following main()method and its curly braces: public static void main(string[] args) { } 3. Position the insertion point after the opening curly brace in the main()method, press Enter and then type the following: int no1 = 1, no2 = 23, no3 = 456, no4 = 7890; System.out.printf( %5d\n, no1); System.out.printf( %5d\n, no2); System.out.printf( %5d\n, no3); System.out.printf( %5d\n, no4); 4. Save and then test the application. Your output should look like Figure Figure 4.21 Output of the FormatOutput2 application

26 52 Java Programming Workbook Do it yourself 4.8 Write a Java program that reads in two integers and display the sum of the two integers. Save the class as Sum.java. 4.9 Write a Java program that asks the user to enter two numbers using an input dialog and prints the sum, product, difference and quotient of the two numbers in a message dialog. Save the class as InputNumber.java Write a Java program that reads a Fahrenheit degree in double from an input dialog box, then converts it to Celsius and displays the result in a message dialog box. Save the class as Temperature.java. The formula for the conversion is: [Note: In Java, 5 / 9 is 0, so you need to write 5.0 / 9 in the program to obtain the correct result.] Celsius = (5.0/ 9) * (Fahrenheit 32) 4.11 What does the following code print? System.out.printf( %s\n%s\n, Welcome to, Java programming! ); CASE PROJECT When the MakingMoneyATM application first execute, it displays a Welcome! greeting. After displaying the greetings, the company has asked you to improve the application. The application should then prompt the users to enter their account number. Then the ATM will display the user s account number. Assign an appropriate value for the account number. Rewrite, recompile and retest the MakingMoneyATM class. Be certain to use appropriate comments in your class.