Java Input and Output

Java Input  Input is any information needed by your program to complete its execution  So far we have been using InputBox for this  Examples of other Input:  Direct keyboard input  Mouse  Network  Disk  Microphone  …

Java Output  Output is any information that the program must convey to the user  So far we have been using OutputBox and System.out.println()  Examples of other Output:  printer  Network  Disk  Speakers  …

Java I/O  We have been using the javabook2 package because its easier  Its easier because it eliminates a lot of errors:  Ex. – NumberFormatException occurs if user enters letters when a number is expected  getInteger() checks for this  It is also easier because it is simple  Getting numbers, Strings, etc. is straightforward  These actions become much more complex without javabook  Standard java classes are not as simple because they offer added flexibility. Flexibility comes at the cost of increasing complexity.

Java Console Input  Remember Console window is the black window that is automatically launched when you run your program  We haven’t used this to get input so far  There are three parts to getting console input:  Prompt the user  Get this input  Convert the input

Java Console Input  Prompt the user  The user must be told to enter information  The user will only know what type of information to enter if you tell them

Java Console Input  Get the input:  Several standard classes are helpful  java.io.InputStream – Stores information about the connection between an input device and your program  java.io.InputStreamReader – used to translate data bytes received from InputStream objects into a stream of characters  Java.io.BufferedReader – used to buffer (store) input received from an InputStreamReader object (stores input as strings).

Java Console Input  None of these classes has a method as simple as getString  BufferedReader has readLine() which returns a string  So let’s make a BufferedReader:  Look at the constructor online  We need a Reader object

Java Console Input  InputStreamReader extends the Reader class  Extends means:  Robin extends bird  Robin is a bird,  bird is not necessarily a robin  If you need a bird, a robin will do  We will use an InputStreamReader for our reader (Because we want an InputStream)  Look at constructor for InputStreamReader  We need an InputStream object

Java Console Input  The System class in the java.lang package (which is automatically imported) automatically creates an InputStream object for us called in  We can access it with System.in  This InputStream is connected to the keyboard  We can use it to create our InputStreamReader object, which we can use to create our BufferedReader object

Java Console Input  Put it all together:  1. Use System.in to create an InputStreamReader object  2. Use the InputStreamReader object to create a BufferedReader object  3. Display a prompt to the user for the desired data  4. Use the BufferedReader object to read a line of text from the user  5. Convert/use the input received

Java Console Input // 1. Use System.in to create an InputStreamReader object InputStreamReader isr = new InputStreamReader(System.in); // 2. Use the InputStreamReader object to create a // BufferedReader object BufferedReader stdin = new BufferedReader(isr); // 3. Display a prompt to the user for the desired data System.out.print(“Johnny 5 needs more input:”); // 4. Use the BufferedReader object to read a line of text from the user String input = stdin.readLine(); // 5. Convert/use the input received System.out.println(“You typed: “ + input);

Java Console Input  Questions??

Java Console Input // 1. Use System.in to create an InputStreamReader object InputStreamReader isr = new InputStreamReader(System.in); // 2. Use the InputStreamReader object to create a // BufferedReader object BufferedReader stdin = new BufferedReader(isr); // 3. Display a prompt to the user for the desired data System.out.print(“Johnny 5 needs more input:”); // 4. Use the BufferedReader object to read a line of text from the user String input = stdin.readLine(); // 5. Convert/use the input received System.out.println(“You typed: “ + input);

Java Console Input Don’t forget to import java.io.*; Shorter version: BufferedReader stdin = new BufferedReader( new InputStreamReader (System.in) );

Java Console Input  Converting input is sometimes necessary:  String input = stdin.readLine(); // user enters “123” int number = Integer.parseInt(input); This can cause Exceptions Look at the Integer class online

Java Console Input  Questoins??

Java Console Output  We have used System.out.println() as an alternative to OutbutBox and Message box of javabook2  This console output displays a String of characters  System.out is an instance of the PrintStream class

Java Console Output  A Stream object is used to store information needed to connect a computer program to an input or output device  Just like a Reader object adds functionality to an InputStream, a Printer object adds functionality to an OutputStream  Console output is easy in java because printer methods (print and println) can handle many types of input

Java Console Output  int x = 3;  char a = ‘a’;  boolean r = true;  String phrase = “Cat’s meow”;  System.out.println(x);  System.out.println(a);  System.out.println(r);  System.out.println(phrase);

java.lang.System  java.lang.System automatically creates three streams for your program:  System.in - InputStream  System.out - PrintStream  System.err - PrintStream

Java Console Output  Questions????

Printing objects  In java, you can print anything: Wanderer w = new Wanderer(“Gil”,Color.green); System.out.println(w) This prints:  This is a memory location (it is not very useful)

Printing objects  In java, all objects have a toString( ) method  It is inherited from the Object object  You can override it by writing your own toString( ) for your class public String toString( ){ String coords = “(“ + myLoc.getX() + “,” + myLoc.getY() + “)”; return myName + “ is at “ + coords; }

Printing objects  Now, Wanderer w = new Wanderer(“Gil”,Color.green); System.out.println(w) Will print: Gil is at (11,9)  This is Useful!  It can help you display output easily and debug your program