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Static Fields

Static Fields

Suppose we have a calculation that calls the Math.cos() a lot, but does it many times with the same argument. If the cos() funtion is time consuming, We might do something like this to optimize our code.

CosCache.java
1	public class CosCache {
2	static double PI = 4.0*Math.acos(1.0/Math.sqrt(2.0));
3	static double lastCosArg = 0.0;
4	static double lastCosValue = 1.0;
5
6	static double cos(double arg) {
7	if(arg != lastCosArg) {
8	lastCosArg = arg;
9	lastCosValue = Math.cos(lastCosArg);
10	}
11	return lastCosValue;
12	}
13
14	public static void main(String[] args) {
15	PI = 3.14; // make sure PI is set!
16	double sum = 0;
17	for(int i=0;i<10;i++) {
18	if(i == 74) {
19	sum += cos(.7*PI);
20	} else {
21	sum += cos(.3*PI);
22	}
23	}
24	System.out.println("sum="+sum);
25	}
26	}
$ javac CosCache.java
$ java CosCache sum=5.88171730331375

Here we've introduced three static fields.

PI - This is supposed to hold the numerical value of pi.
lastCosArg - This holds the last value we passed to the cos() function.
lastCosValue - This is always equal to Math.cos(lastCosArg)

There is, of course, a small bug in this program. When we initialized we supplied a highly accurate value of PI by using 4.0*Math.acos(1/Math.sqrt(2.0)) (Note: acos() is the arc cosine).

One can prevent this sort of bug by making PI final. Final means that the value of PI cannot be changed.

CosCache2.java
1	public class CosCache2 {
2	final static double PI = 4.0*Math.acos(1.0/Math.sqrt(2.0));
3	static double lastCosArg = 0.0;
4	static double lastCosValue = 1.0;
5
6	static double cos(double arg) {
7	if(arg != lastCosArg) {
8	lastCosArg = arg;
9	lastCosValue = Math.cos(lastCosArg);
10	}
11	return lastCosValue;
12	}
13
14	public static void main(String[] args) {
15	PI = 3.14; // make sure PI is set!
16	double sum = 0;
17	for(int i=0;i<10;i++) {
18	if(i == 74) {
19	sum += cos(.7*PI);
20	} else {
21	sum += cos(.3*PI);
22	}
23	}
24	System.out.println("sum="+sum);
25	}
26	}
$ javac CosCache2.java CosCache2.java:15: cannot assign a value to final variable PI PI = 3.14; // make sure PI is set! ^ 1 error