如果我運行該程序的輸出是:數組初始化 - 性能差異
init: 1239.0
toCharArray: 1343.6
arraycopy: 1583.8
第一個問題是:
爲什麼init()
比toCharArray()
快?這裏有編譯器優化嗎? (我使用Java 1.8.0_20)
第二個問題是:
爲什麼toCharArray()
比arraycopy()
快?我從這裏複製arraycopy()
:String.toCharArray()
。
public class MyClass {
private static char[] SRC = { 'a', 'b', 'c', 'd', 'e', 'f', 'g' };
public static void main(String[] args) {
long start = 0;
int init = 0, toCharArray = 0, arraycopy = 0;
for (int j = 0; j < 5; j++) {
start = System.currentTimeMillis();
for (int i = 0; i < 100000000; i++)
init();
init += (System.currentTimeMillis() - start);
start = System.currentTimeMillis();
for (int i = 0; i < 100000000; i++)
toCharArray();
toCharArray += (System.currentTimeMillis() - start);
start = System.currentTimeMillis();
for (int i = 0; i < 100000000; i++)
arraycopy();
arraycopy += (System.currentTimeMillis() - start);
}
System.out.println("init: " + init/5.0);
System.out.println("toCharArray: " + toCharArray/5.0);
System.out.println("arraycopy: " + arraycopy/5.0);
}
private static void init() {
char[] c = { 'a', 'b', 'c', 'd', 'e', 'f', 'g' };
doSomething(c);
}
private static void toCharArray() {
char[] c = "abcdefg".toCharArray();
doSomething(c);
}
private static void arraycopy() {
char[] c = new char[SRC.length];
System.arraycopy(SRC, 0, c, 0, SRC.length);
doSomething(c);
}
private static void doSomething(char[] c) {
for (int i = 0; i < c.length; i++)
c[i] = ' ';
}
}
EDIT
卡尺結果:
init: min=11.90, 1st qu.=12.27, median=12.48, mean=12.44, 3rd qu.=12.54, max=13.16
toCharArray: min=13.10, 1st qu.=13.21, median=13.39, mean=13.49, 3rd qu.=13.78, max=14.27
arraycopy: min=15.42, 1st qu.=15.49, median=15.51, mean=15.51, 3rd qu.=15.55, max=15.58
這些結果可以忽略,因爲你沒有加熱JVM。要麼學習如何微觀Java或使用像Caliper這樣的測試庫。 – 2014-09-19 22:49:45
是的,測試是有缺陷的,如果你切換到CharArray()和arraycopy()我打賭結果會有所不同 – 2014-09-19 23:03:29
我改變了順序:arraycopy:1584.2 toCharArray:1364.0。但是,謝謝你的提示,我會嘗試卡尺! – Krayo 2014-09-19 23:13:34