Java Notes

Static/Class methods

There are two types of methods.

Qualifying a static call

From outside the defining class, an instance method is called by prefixing it with an object, which is then passed as an implicit parameter to the instance method, eg, inputTF.setText("");

A static method is called by prefixing it with a class name, eg, Math.max(i,j);. Curiously, it can also be qualified with an object, which will be ignored, but the class of the object will be used.


Here is a typical static method.

class MyUtils {
    . . .
    //================================================= mean
    public static double mean(int[] p) {
        int sum = 0;  // sum of all the elements
        for (int i=0; i<p.length; i++) {
            sum += p[i];
        return ((double)sum) / p.length;
    }//endmethod mean
    . . .

The only data this method uses or changes is from parameters (or local variables of course).

Why declare a method static

The above mean() method would work just as well if it wasn't declared static, as long as it was called from within the same class. If called from outside the class and it wasn't declared static, it would have to be qualified (uselessly) with an object. Even when used within the class, there are good reasons to define a method as static when it could be.

Calling static methods

There are two cases.

Called from within the same class
Just write the static method name. Eg,
// Called from inside the MyUtils class
double avgAtt = mean(attendance);
Called from outside the class
If a method (static or instance) is called from another class, something must be given before the method name to specify the class where the method is defined. For instance methods, this is the object that the method will access. For static methods, the class name should be specified. Eg,
// Called from outside the MyUtils class.
double avgAtt = MyUtils.mean(attendance);
If an object is specified before it, the object value will be ignored and the the class of the object will be used.

Accessing static variables

Altho a static method can't access instance variables, it can access static variables. A common use of static variables is to define "constants". Examples from the Java library are Math.PI or Color.RED. They are qualified with the class name, so you know they are static. Any method, static or not, can access static variables. Instance variables can be accessed only by instance methods.

Alternate call

What's a little peculiar, and not recommended, is that an object of a class may be used instead of the class name to access static methods. This is bad because it creates the impression that some instance variables in the object are used, but this isn't the case.