COMPSCI 125 Spring 2005 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 3: IO *Standard Output *Formatting Decimal Numbers *Wrapper Classes *Standard Input *GregorianCalendar

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Standard Output *The showMessageDialog method is intended for displaying short one-line messages, not for a general-purpose output mechanism. *Using System.out, we can output multiple lines of text to the standard output window - the console.

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. System.out *System.out is a PrintStream object. *Two useful methods the print method takes a String as its argument and prints it to the console the println method takes a String as its argument and prints it to the console with a newline character appended to the end *Both methods will continue printing from the end of the currently displayed output. *Both methods will do the necessary type conversion if we pass numerical data.

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Overloaded + Operator *The + operator can be used in 2 ways adding numbers concatenating Strings *What happens if we mix its arguments int x = 1; int y = 2; String output = “test” + x + y; String output = x + y + “test”;

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Output Example Printing a prompt on same line as input will be typed: System.out.print( "Enter radius: "); Echo printing the input using print and println: System.out.print( "The radius is "); System.out.println( radius); Printing results (using +): System.out.println( "Area : " + area);

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. DecimalFormat *Since floating point numbers can't be represented exactly in computer memory, they often print out with lots of digits after the decimal point. This can make the output look messy *The DecimalFormat class lets you print numbers with a fixed number of digits after the decimal place. *Use a "picture" string to show what you want the number to look like "0.000" means 3 digits after the decimal point *Use the format method with the number you want formatted as an argument

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. DecimalFormat Example *Create the DecimalFormat with a String DecimalFormat df = new DecimalFormat("0.000"); *Call the format method with the number as an argument double fp; /* assign a value to fp */ System.out.print( df.format(fp));

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Getting Numerical Input Values *Java provides primitive data types for working with numbers. *Wrapper classes exist that allow you to make an object out of a number. There is a wrapper class for each primitive type: Integer, Double, … *These classes provide methods we can use to convert an appropriate String object to a numerical value. Integer.parseInt() converts a String like "23" to an int Double.parseDouble() converts a String like "2.3" to a double

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Wrapper classes *Allow you to create an object from a primitive value *Are used to perform necessary type conversions Converting a String object to a numerical value Converting a numeric value to a String. *There is a wrapper class for each of the java primitive types

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Getting Numeric Input Data Data from the keyboard often comes into the program as a String String radiusStr = JOptionPane.showInputDialog(null, "Enter radius:"); A string containing numeric data needs to be converted before it can be used in calculations radius = Double.parseDouble(radiusStr);

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Standard Input *Using input dialogs is tedious for large amounts of input. And, not very user-friendly *The technique of using System.in to input data is called standard input. *System.in is an InputStream which handles byte data. *Using the intermediate InputStreamReader object allows us to read a single character at a time. *The BufferedReader class allows us to read one line at a time (in String form) We could read numbers with BufferedReader and wrapper classes

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. The Scanner Class *Java1.5 has added a new input class that makes numeric input simpler. java.util.Scanner *Use System.in to create a Scanner Scanner kbd = new Scanner( System.in); Note: All the Scanner examples in the text need to be corrected.

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Scanner input methods nextInt gets an int from the input int i = kbd.nextInt(); nextDouble double d = kbd.nextDouble(); nextByte, nextShort, nextLong and nextFloat work similarly next() gets a String reads characters until the next whitespace

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Reading line-by-line with Scanner *By default, Scanner uses white space to delimit (separate) Strings *To read an entire line need to replace the delimiter Ask System class what it is for your OS String delim = System.getProperties( "line.separator"); Tell the Scanner to use it kbd.useDelimiter( delim); Then read the line using next String line = kbd.next();

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. The GregorianCalendar Class *The GregorianCalendar class is useful in manipulating calendar information such as year, month, and day. *You can create a GregorianCalendar for any date by giving the year, month and day cal = new GregorianCalendar(2001, 8, 11); Caution: the first month of the year, January, is represented by 0.

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. GregorianCalendar *getTime method returns a Date *get(Calendar.property) allows you to determine year, month, date, day of week, hours, minutes, … *Use SimpleDateFormat or DateFormat to output the date in the desired format *DateFormat has a parse method that takes a String in the proper format and returns a Date

COMPSCI 125 Spring 2005 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Extra Material *BufferedReader is covered later in your text

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Input classes *How the sequence of I/O objects adds greater capabilities. InputStream has the capability to read bytes InputStreamReader has the capablity to read characters (2 bytes) BufferedReader can read an entire line

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Using BufferedReader *Declaration BufferedReader kbd *Creation kbd = new BufferedReader( new InputStreamReader( System.in)); *Use String input = kbd.readLine();

COMPSCI 125 Spring 2005 ©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. Handling Input Errors *Calling the readLine method of a BufferedReader can result in an error condition called an exception. *The programmer is required to handle this potential error condition. *For now we choose to ignore these problems by adding the clause throws IOException to the method declaration whenever a method includes a call to the readLine method. public static void main(String [] args) throws IOException {... String input = bufReader.readLine();... }