The do - while loop

Objectives

1. Introduce the do while loop.

Notes

• A tutorial/reference
• There is one more traditional loop in java.
• The while - do loop.
  • Syntax:

    do {
       body statements;
    } while (expression); 

  • Semantics

    body statements;
    while(expression) {
        body statements;
    } 

• This will always execute the body at least once.
• Let's use this to clean up the scanner initialization.
  • I want to read in the name of the file, so my program is more flexible.
  • But I want to handle a bad file name.
• Step 1, add an input scanner class variable.
• Ask for the file name and try to open the file.
• Put this into a loop.

•     private static void MakeScanners(){
         inputScanner = new Scanner(System.in);
         String fileName;
         boolean fileOpen = false;
         
         System.out.printf("Enter a file name => ");
         fileName = inputScanner.next();
         System.out.println();
         
         while (!fileOpen) {
            try {
                fileScanner = new Scanner(new File(fileName));
                fileOpen = true;
            } catch (FileNotFoundException  e) {
                System.out.printf("Error could not open the file %s\n",fileName);
                System.out.printf("Enter a file name => ");
                fileName = inputScanner.next();
                System.out.println();
            }
         }
  
      }
            
  } 

• The problem is the repeated input code.
  • I could fix this a couple of ways,
  • But I will use a do-while loop.

  •     private static void MakeScanners(){
           inputScanner = new Scanner(System.in);
           String fileName;
           boolean fileOpen = false;
    
           do {
               System.out.printf("Enter a file name => ");
               fileName = inputScanner.next();
               System.out.println();
    
              try {
                  fileScanner = new Scanner(new File(fileName));
                  fileOpen = true;
              } catch (FileNotFoundException  e) {
                  System.out.printf("Error could not open the file %s\n",fileName);
              }
           } while(!fileOpen);
        }