我为QuickSort创建了一些改进，并决定针对Java Arrays.sort()进行测试。
结果令人着迷：
在Java 6上：
在Java 7上：
正如你所看到的，我的算法在Java 6上的表现更好，但是我不明白它对Java 7的影响有多大。你可以找到原因吗？
编辑：我的算法如何运作：
public class QuickSort {
public static void sort(int[] source) {
int buffer[] = new int[source.length];
concatenate(source, buffer, 0, source.length);
}
private static void concatenate(int[] source, int[] buffer, int low, int high) {
int count = high - low;
int lowBuffer = low;
int highBuffer = high;
if (count < 2) {
return;
}
if (count < 12) {
networkSort(source, buffer, low, count);
return;
}
int pivotIndex = bestOfThree(source, low);
int value = source[pivotIndex];
for (int i = low; i < high; i++) {
if (i == pivotIndex) {
continue;
}
if (source[i] < value) {
buffer[lowBuffer] = source[i];
source[lowBuffer] = buffer[lowBuffer];
lowBuffer++;
}
else {
highBuffer--;
buffer[highBuffer] = source[i];
}
}
buffer[lowBuffer] = source[lowBuffer] = value;
for (int i = lowBuffer; i < high; i++) {
source[i] = buffer[i];
}
concatenate(source, buffer, lowBuffer + 1, high);
concatenate(source, buffer, low, lowBuffer);
}
private static int bestOfThree(int[] source, int low) {
int a = low;
int b = a + 1;
int c = a + 2;
int median = -1;
if (source[a] >= source[b] && source[a] >= source[c]) {
if (source[b] < source[c]) {
median = c;
}
else {
median = b;
}
}
else if (source[b] >= source[a] && source[b] >= source[c]) {
if (source[a] < source[c]) {
median = c;
}
else {
median = a;
}
}
else if (source[c] >= source[a] && source[c] >= source[b]) {
if (source[a] < source[b]) {
median = b;
}
else {
median = a;
}
}
return median;
}
private static int[][] networkSort = {
{ 0, 1 },
{ 1, 2, 0, 2, 0, 1 },
{ 0, 1, 2, 3, 0, 2, 1, 3, 1, 2 },
{ 0, 1, 3, 4, 2, 4, 2, 3, 1, 4, 0, 3, 0, 2, 1, 3, 1, 2 },
{ 1, 2, 4, 5, 0, 2, 3, 5, 0, 1, 3, 4, 2, 5, 0, 3, 1, 4, 2, 4, 1, 3, 2, 3 },
{ 1, 2, 3, 4, 5, 6, 0, 2, 3, 5, 4, 6, 0, 1, 4, 5, 2, 6, 0, 4, 1, 5, 0, 3, 2, 5, 1, 3, 2, 4, 2, 3 },
{ 0, 1, 2, 3, 4, 5, 6, 7, 0, 2, 1, 3, 4, 6, 5, 7, 1, 2, 5, 6, 0, 4, 3, 7, 1, 5, 2, 6, 1, 4, 3, 6, 2, 4, 3, 5, 3, 4 },
{ 0, 1, 3, 4, 6, 7, 1, 2, 4, 5, 7, 8, 0, 1, 3, 4, 6, 7, 2, 5, 0, 3, 1, 4, 5, 8, 3, 6, 4, 7, 2, 5, 0, 3, 1, 4, 5, 7, 2, 6, 1, 3, 4, 6, 2, 4, 5, 6, 2, 3 },
{ 4, 9, 3, 8, 2, 7, 1, 6, 0, 5, 1, 4, 6, 9, 0, 3, 5, 8, 0, 2, 3, 6, 7, 9, 0, 1, 2, 4, 5, 7, 8, 9, 1, 2, 4, 6, 7, 8, 3, 5, 2, 5, 6, 8, 1, 3, 4, 7, 2, 3, 6, 7, 3, 4, 5, 6, 4, 5 },
{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 3, 5, 7, 0, 2, 4, 6, 8, 10, 1, 2, 5, 6, 9, 10, 0, 4, 3, 7, 1, 5, 6, 10, 4, 8, 5, 9, 2, 6, 0, 4, 3, 8, 1, 5, 6, 10, 2, 3, 8, 9, 1, 4, 7, 10, 3, 5, 6, 8, 2, 4, 7, 9, 5, 6, 3, 4, 7, 8 }
};
private static int tmp;
private static void networkSort(int[] source, int[] buffer, int low, int count) {
int[] networkData = networkSort[count - 2];
for (int i = 0; i < networkData.length; i += 2) {
int index1 = low + networkData[i];
int index2 = low + networkData[i + 1];
if (source[index1] > source[index2]) {
tmp = source[index1];
buffer[index1] = source[index1] = source[index2];
buffer[index2] = source[index2] = tmp;
}
}
}
}
public abstract class Test {
protected int[][] buffer;
private final Random random = new Random();
public int numberOfTests = 100;
public int maxValue = 1000;
public int numberOfItems = 100;
protected void createBuffer() {
buffer = new int[numberOfTests][];
for (int i = 0; i < numberOfTests; i++) {
int[] list = new int[numberOfItems];
addRandomNumbers(list);
buffer[i] = list;
}
}
protected void createBuffer(int...parametes) {
buffer = new int[1][];
buffer[0] = parametes;
}
protected void addRandomNumbers(int[] list) {
for (int i = 0; i < numberOfItems; i++) {
int value = random.nextInt(maxValue);
list[i] = value;
}
}
protected int[][] cloneBuffer() {
int[][] clonedBuffer = new int[numberOfTests][];
for(int i = 0; i < buffer.length; i++){
int[] clonedList = new int[buffer[i].length];
int[] list = buffer[i];
for (int j = 0; j < list.length; j++) {
int element = list[j];
clonedList[j] = element;
}
clonedBuffer[i] = clonedList;
}
return clonedBuffer;
}
public abstract void test();
}
public class PerformanceTest extends Test {
private final Timer timer = new Timer();
public void test() {
createBuffer();
timer.reset();
testSystem();
timeResoult("System");
timer.reset();
testMy();
timeResoult("My List");
}
public void test(int numberOfTests) {
long myTotalTime = 0;
long systemTotalTime = 0;
for (int i = 0; i < numberOfTests; i++) {
createBuffer();
timer.reset();
testSystem();
long systemTime = timeResoult();
systemTotalTime += systemTime;
timer.reset();
testMy();
long myTime = timeResoult();
myTotalTime += myTime;
System.out.println("My Time / System Time = " + myTime + " / " + systemTime + " = \t"
+ ((double) myTime / systemTime));
}
System.out.println("My Time / System Time = " + ((double) myTotalTime / systemTotalTime));
}
private long timeResoult() {
return timeResoult(null);
}
private long timeResoult(String source) {
long time = timer.check();
if (source != null) {
System.out.println(source + ">\tTime: " + time);
}
return time;
}
private void testMy() {
int[][] buffer = cloneBuffer();
for (int i = 0; i < numberOfTests; i++) {
int[] list = buffer[i];
QuickSort.sort(list);
}
}
private void testSystem() {
int[][] buffer = cloneBuffer();
for (int i = 0; i < numberOfTests; i++) {
int[] list = buffer[i];
Arrays.sort(list);
}
}
}
public static void main(String[] args) {
PerformanceTest testBasics = new PerformanceTest();
testBasics.numberOfTests = 1000;
testBasics.numberOfItems = 1000;
testBasics.maxValue = 1000000;
testBasics.test(100);
}
答案 0 :(得分：8)
他们为Java 7改变了Arrays.sort的排序算法。对于排序对象，Arrays.sort现在使用名为Timsort的算法和基础的双枢轴快速排序。有关此answer的更多信息。
如果您真的想使用旧版本，显然您可以设置系统属性java.util.Arrays.useLegacyMergeSort。
参见Java 7 compatibility documentation。