メソッドの監視にはある程度のオーバーヘッドがかかることに注意してください。VisualVM の場合、これはかなり高くなります。頻繁に呼び出されるがほとんど実行しないメソッドがある場合、大量の CPU を使用しているように見えることがあります。たとえば、私は Integer.hashCode() が大ヒットしたことがあります。;)
私のマシンでは、フロアの所要時間は 5.6 ns 未満ですが、キャストの所要時間は 2.3 ns です。あなたのマシンでこれを試してみてください。
特殊なケースを処理する必要がない限り、プレーン キャストの方が高速です。
// Rounds to zero, instead of Negative infinity.
public static double floor(double a) {
return (long) a;
}
public static void main(String... args) {
int size = 100000;
double[] a = new double[size];
double[] b = new double[size];
double[] c = new double[size];
for (int i = 0; i < a.length; i++) a[i] = Math.random() * 1e6;
for (int i = 0; i < 5; i++) {
timeCast(a, b);
timeFloor(a, c);
for (int j = 0; j < size; j++)
if (b[i] != c[i])
System.err.println(a[i] + ": " + b[i] + " " + c[i]);
}
}
public static double floor(double a) {
return a < 0 ? -(long) -a : (long) a;
}
private static void timeCast(double[] from, double[] to) {
long start = System.nanoTime();
for (int i = 0; i < from.length; i++)
to[i] = floor(from[i]);
long time = System.nanoTime() - start;
System.out.printf("Cast took an average of %.1f ns%n", (double) time / from.length);
}
private static void timeFloor(double[] from, double[] to) {
long start = System.nanoTime();
for (int i = 0; i < from.length; i++)
to[i] = Math.floor(from[i]);
long time = System.nanoTime() - start;
System.out.printf("Math.floor took an average of %.1f ns%n", (double) time / from.length);
}
版画
Cast took an average of 62.1 ns
Math.floor took an average of 123.6 ns
Cast took an average of 61.9 ns
Math.floor took an average of 6.3 ns
Cast took an average of 47.2 ns
Math.floor took an average of 6.5 ns
Cast took an average of 2.3 ns
Math.floor took an average of 5.6 ns
Cast took an average of 2.3 ns
Math.floor took an average of 5.6 ns
