Class methods

Continuing through the textbook, we'll cover Chapter 10.

Using class methods

• constructor methods: for creating objects.
• instance methods: for sending messages to objects.
• class methods: methods not connected to any specific object.

The Math class includes several class methods, such as pow:
static double pow(double base, double exp)
Returns the result of raising base to the expth power.
The word static marks this method as a class method, as opposed to an instance method.

You use class methods without needing to create any instances of the class:

	io.println("The square root of 5 is " + Math.pow(5.0 , 0.5));

Defining class methods

• Doing the same thing numerous places in a program.
• Decomposing a large program into smaller, more manageable pieces.

As an example, let's define a factorial method, and use it in a program to calculate the number of ways one can choose r from n things:

	import pgss.*;

	public class Choose {
	    public static double fact(int k) {
	        double ret = 1; // this will be the factorial of k
	        for(int i = 1; i <= k; i++) {
		    ret *= i; // multiply the value of i into ret
		}
		return ret;
	    }

	    public static void main(String[] args) {
	        IOWindow io = new IOWindow();
		io.print("Choose how many of how many? ");
		int r = io.readInt();
		int n = io.readInt();
		io.println((Choose.fact(n)/Choose.fact(r)/Choose.fact(n-r))
		    + " choices");
		return;
	    }
	}

Parameters and variables

So the parameters of the method need to include any information it needs. For an example of a method with more than one parameter, suppose we add a class method to the Choose class to do the ``choose'' calculation:

	public static double choose(int n, int r) {
	    return fact(n) / fact(r) / fact(n - r);
	}

When such a method is called, the parameters are replaced by whatever is in those positions in the call:

	    io.println(Choose.choose(6, 2) + " choices");

In Java, methods use call by value: the values of the expressions in the call are copied into the method. So changing them inside the method does not affect the outside world. So in this example:

	import pgss.*;

	public class ZeroExample {
	    public static void setToZero(int n) {
	        n = 0;
	    }

	    public static void main(String[] args) {
	        IOWindow io = new IOWindow();
		int i = 1;
		setToZero(i);
		io.println(i);
	    }
	}

One fine point: arrays are a little trickier; the value of an array variable is a reference to the array, so you can affect the values of the array elements. If we change the above program to use arrays instead of integer parameters, we get a different result:

	import pgss.*;

	public class ZeroExample {
	    public static void setToZero(int[] arr) {
	        arr[0] = 0;
	    }

	    public static void main(String[] args) {
	        IOWindow io = new IOWindow();
		int[] a = new int[1];
		a[0] = 1;
		setToZero(a);
		io.println(a[0]);
	    }
	}