method question

[Mitch Krah]

I have the following code:

 
class Arrays {
  public static void main(String[] args) {
  
    int min, max, sum;
    double avg;
  
    char arrayChar[] = {'A', 'b', 'C', 'j', 'k'};
    double arrayDouble[] = new double[4];
    Dog arrayOb[] = new Dog[4];
  
    min = max = arrayChar[0];
    sum = 0;
    avg = 0.0;
  
    for(int i =1; i < arrayChar.length; i++) {
      if(arrayChar[i] < min)
        min = arrayChar[i];
      if(arrayChar[i] > max)
        max = arrayChar[i];
  
      sum += arrayChar[i];
  
      avg = sum / (double)arrayChar.length;
     
      printResults(min, max, sum, avg);
      }
  }
   
  void printResults(int m, int x, int s, double a) {
      System.out.println("min: " + m + "\nmax: " + x + "\nsum: " + s +
                       "\navg: " + a);
      return;
    }
 
}

First he compiler made me put the printResults outside of main. I don't understand this??? Why can't I have a method inside of main? Next the compiler will not let me call printResults from inside of main. When I call the printResult method (as shown above), I get an error saying you cannot call a non-static method from a static context???

Pleasa help?

Thank you,
Mitch

[ edited to preserve formatting using the [code] and [/code] UBB tags -ds ]
[ December 05, 2004: Message edited by: Dirk Schreckmann ]

[Paul Santa Maria]

Hi -

You've actually asked a very important - and, in some ways, very subtle -question. You can get a good, detailed explanation here:

http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html

Bruce Eckel ("Thinking in Java, 3rd Ed") explains it as follows:

"[A static method is] the equivalent of a global function (from C)...
It means that there is no 'this' (no per-instance data) for that particular method.

"You cannot call non-static methods from inside static methods (although the reverse is possible", and you can call a static method for the class itself, without any object. In fact, that's primarily what a static method is for."

Please take a look at the JavaDoc for "java.lang.Math". You'll notice that most of the math functions (abs(), sin(), cos(), etc etc) are all static functions. The reason is simple - you shouldn't have to create an entire object just to get the absolute value of a number. You just call ... a "static method": "abs (x)".

'Hope that helps .. Paul Santa Maria

[Mike Gershman]

Try this:
static void printResults(int m, int x, int s, double a) {

[Paul Santa Maria]

With all due respect, you shouldn't just "try static printResults()..." as you need to understand the (important) difference between static and non-static methods and data.

For purposes of illustration, here's a slightly different example, using non-static methods:

/*
 * SAMPLE OUTPUT:
 * D:\temp>java Arrays =>
 *   min: 65
 *   max: 107
 *   sum: 378
 *   avg: 75.6
 */
class Arrays {
 
  /* 
   * Static main: create an instance of "Arrays" and use it 
   */
  public static void main(String[] args) {
    char arrayChar[] = {'A', 'b', 'C', 'j', 'k'};
    Arrays a = new Arrays (arrayChar);
    a.printResults ();
  }
 
  /* 
   * All the interesting stuff we previously did in "main()" 
   * now occurs here.  Things can now happen *differently*, 
   * on a *per-class* basis
   */
  public Arrays (char arrayChar[]) {
    min = max = arrayChar[0];
    sum = 0;
    avg = 0.0;
 
    for(int i =1; i < arrayChar.length; i++) {
      if(arrayChar[i] < min)
        min = arrayChar[i];
      if(arrayChar[i] > max)
        max = arrayChar[i];
      sum += arrayChar[i];
      avg = sum / (double)arrayChar.length;
    }  
  }
 
  /*
   * printResults is no longer static: it prints the results for whatever
   * values this particular instance of class "Arrays" happens to contain.
   */
  public void 
  printResults() {
    System.out.println("min: " + min + "\nmax: " + max + "\nsum: " + sum + "\navg: " + avg);
  }
 
  /*
   * Finally, we've moved the data *out* of main and *into* instance variables
   */
  int min, max, sum;
  double avg;
}

'Hope that helps .. PSM