Using Classes BCIS 3680 Enterprise Programming

Overview 2  Using Classes  Using premade classes for input and output  Display output: System, JOptionPane classes  Format output: Decimal Format, NumberFormat classes  Handling Input: Wrapper classes, e.g., Integer, Double  Processing: Math class

Organization of Classes 3  Related classes are organized into packages.  Java SE comes with a number of commonly used packages.  To use Class A in Class B, you need to have an import statement for Class A at the beginning of Class B.  The only exception is the classes in the java.lang package, which can be used by all other classes without an import java.lang; statement.  Two ways to write the import statement:  Specifically import the name of the particular class you want to use, e.g., import java.util.Scanner;.  Import the entire package the class belongs to, e.g., import java.util.*;.  All other classes in that package are accessible now.

Packages of “Premade” Classes 4  Classes are grouped in packages according to functionality by using the package keyword.  The following packages ship with the Java language: PackageCategories of Classes java.langBasic functionality common to many programs, such as the String class and Math class java.awtGraphics classes for drawing and using colors javax.swingUser-interface components java.textClasses for formatting numeric output java.utilThe Scanner class, the Random class and other miscellaneous classes java.ioClasses for reading from and writing to files

The System Class 5  The System class is in the java.lang package, so it does not need to be imported.  Two useful static constants (variables with constant value) of the System class:  in represents the standard input device (the keyboard by default).  out represents the standard output device (the Java console by default).  Examples: Scanner scan = new Scanner(System.in); System.out.println("Hello");

Using System.out Example: System.out.print("The answer is "); System.out.println(3); The output is: The answer is 3 Return typeSignature voidprint( anyDataType argument ) prints argument to the standard output device voidprintln( anyDataType argument ) prints argument to the standard output device followed by a newline character

Using Dialog Boxes 7  The JOptionPane class is in the javax.swing package.  You must import it.  Static methods are provided for input and output dialog boxes.  For input dialog boxes, return value is a String, so numeric input needs to be converted (using methods such as parseInt() or parseDouble() ).

JOptionPane static Methods 8 Return valueSignature StringshowInputDialog( Component parent, Object prompt, String title, int messageType ) pops up an input dialog box, where prompt asks the user for input. voidshowMessageDialog( Component parent, Object message, String title, int messsageType ) pops up an output dialog box with message displayed.

Message Dialog Box 9  Displays a message and then waits until the user press OK: showMessageDialog(Component parent, Object message, String title, int messageType)  parent – use null for projects  mesage – the message you want to display. A string is fine as it is an object.  title – the text you want to appear on the title bar.  messageType – indicates the nature of the message box. It is of the int type but you don’t have to enter an integer. Instead, you may enter one of the following easy-to-read words and Java will translate it into an integer for you.  JOptionPane.INFORMATION_MESSAGE: Standard info icon is used.  JOptionPane.PLAIN_MESSAGE: No icon is displayed.  JOptionPane.QUESTION_MESSAGE: Question mark icon is displayed.  JOptionPane.WARNING_MESSAGE: Warning icon is displayed.

Input Dialog box 10  Displays a displays a text field into which the user can enter a string.  showInputDialog(Component parent, Object prompt, String title, int messsageType)  parent – use null for projects  prompt – the prompt you want to display. A string is fine as it’s an object.  title – the text you want to appear on the title bar.  messageType – indicates the nature of the message box. The options are:  JOptionPane.ERROR_MESSAGE  JOptionPane.INFORMATION_MESSAGE  JOptionPane.PLAIN_MESSAGE  JOptionPane.QUESTION_MESSAGE  JOptionPane.WARNING_MESSAGE

Formatting Numeric Output 11  The DecimalFormat and NumberFormat classes allow you to specify the display format of numbers - number of digits, number of places after the decimal, formatting elements such as dollar signs and percent signs, etc.  Both classes are in the java.text package.  Importing is needed.  Usage pattern:  Create a formatting object (either a DecimalFormat or a NumberFormat object).  Call the format method of the formatting object and pass the number to be formatted as the argument.  A formatting object may be used to format multiple numbers.  If numbers are to be formatted in different ways, then multiple formatting objects are needed.

DecimalFormat Class 12 Pattern characters: 0 required digit # optional digit, suppress if 0. decimal point, comma separator % multiply by 100 and display a percent sign DecimalFormat Constructor DecimalFormat( String pattern ) instantiates a DecimalFormat object with the format specified by pattern

The format( ) Method 13 Return typeSignature Stringformat( double number ) returns a formatted String representation of number

The Wrapper Classes 14  There are occasions when you want to work with a number as an object.  Numbers are primitive types. They are not objects.  Wrapper classes resolve the problem because they are objects so can be handled as such.  In the “core” of each wrapper object is a numeric value.

Wrapper Classes 15  Remember the naming convention:  Primitive types start with a lowercase letter; Class names start with an uppercase letter. Primitive Data TypeWrapper Class doubleDouble floatFloat longLong intInteger shortShort byteByte charCharacter booleanBoolean

Using Wrapper Classes 16  A handy use of wrapper class is a static method for converting textual input into numbers.  Usually, when a program obtains input from users, the input is read as strings. Even when the user enters a number, it’s still a string (of characters that stand for numbers).  Despite looking numeric ostensibly, such strings cannot be used in mathematic operations. They must be converted into primitive types first.  To perform the conversion, call the respective parseX() method of the wrapper class.  To convert a string into an integer, call parseInt().  To convert a string into a double, call parseDouble().

Integer Static Methods 17 Return valueSignature intparseInt( String s ) returns the String s as an int IntegervalueOf( String s ) returns the String s as an Integer object

Double Static Methods 18 Return valueSignature doubleparseDouble( String s ) returns the String s as a double DoublevalueOf( String s ) returns the String s as a Double object

Reading Numeric Input 19  The showInputDialog() method returns a string.  To convert the string to a numeric type, use the wrapper class methods.  Example: String input = JOptionPane.showInputDialog(null, "Please enter your age in years.", "Get Input", JOptionPane.QUESTION_MESSAGE); int age = Integer.parseInt(input);

Math Class 20  The Math class provides static constants and static methods for performing common calculations.  The Math class is in the java.lang package, so it does not need to be imported.  Methods in the Math class are static.

Methods of the Math Class 21 Return typeSignature dataTypeOfArgabs( dataType arg ) returns the absolute value of the argument arg, which can be a double, float, int or long. doublelog( double a ) returns the natural logarithm (in base e) of its argument. doublesqrt( double a ) returns the positive square root of a doublepow( double base, double exp ) returns the value of base raised to the power exp

The round() Method 22  Rounding rules:  Any factional part <.5 is rounded down  Any fractional part.5 and above is rounded up Return typeSignature longround( double a ) returns the closest integer to its argument a

The min() and max() Methods 23 Find smallest of three numbers: int smaller = Math.min( num1, num2 ); int smallest = Math.min( smaller, num3 ); Return typeSignature dataTypeOfArgsmin( dataType a, dataType b ) returns the smaller of the two arguments. The arguments can be doubles, floats, ints, or longs. dataTypeOfArgsmax( dataType a, dataType b ) returns the larger of the two arguments. The arguments can be doubles, floats, ints, or longs.