Getting Input in Java

Creating Scanner objects

The easiest way to get input is to create an instance of the scanner class.

• If you want the input to come from the user's keyboard (i.e., typed into the Interactions window in DrJava, or in the Java Console) it's really easy easy:

    // Put this import at the top
    import java.util.Scanner;
  
    // ...
        Scanner kbdScanner = new Scanner(System.in);
  

• If you want the input to come from a particular file, it's a little more work because you have to "open" the file before you can connect it to a scanner object:

    // Put these imports at the top
    import java.util.Scanner;
    import java.io.File;
    import java.io.FileNotFoundException;  
    
    // ... assume that we want to read input from a file named "myfile.in"
      
       Scanner fileScanner = null;
  
       try { // in case the file's not there
          fileScanner = new Scanner(new File("myfile.in"));
       } catch (FileNotFoundException e) {
          System.out.println("File " + filename + " not found.");
          System.out.println( e );
       }
  

Using Scanner objects

• scanner.nextLine(), which returns the next line of input from a file (or prompts for a line of keyboard input)
• scanner.nextInt(), which returns the next integer appearing in the input (or prompts for a number at the keyboard).

Putting it All Together

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Arrays;

class change
{
  public static void main( String[] args )
  {
    // get keyboard input Object
    Scanner kbd = new Scanner(System.in);
    // get input from keyboard
    System.out.print( "Enter a filename: " );
    String filename = kbd.nextLine();
    // trim off any whitespace (optional)
    filename = filename.trim();
    System.out.println("Now trying to open " + filename);
    
    // get file input Object via the file:
    Scanner file = null;
    try { // in case the file's not there
      file = new Scanner(new File(filename));
    } catch (FileNotFoundException e) {
      System.out.println("File " + filename + " not found.");
      System.out.println( e );
    }
    // get integer at the start of the file:
    int num_coins = file.nextInt();
    System.out.println("Read the # of coins = " + num_coins);
    
    // create an array, coins, to hold the coins:
    int[] coins = new int[num_coins];
    
    // here, you'll need to loop in order to read each coin in...
    
    // It's useful for debugging to be able to print an array 
    System.out.println("The array, coins, is " + Arrays.toString( coins ));
    
    // It's also useful to be able to get more input from the keyboard:
    System.out.println("Input an integer and I'll add 1: ");
    int amt = kbd.nextInt(); // it will crash if it's not an int!
    System.out.println("You entered " + amt + ", one less than " + (amt+1));
  }
}