Java Arrays.sort源代碼解析

  1 package com.util;
  2 
  3 public class ArraysPrimitive {
  4     private ArraysPrimitive() {}
  5 
  6     /**
  7      * 對指定的 int 型數組按數字升序進行排序。
  8      */
  9     public static void sort(int[] a) {
 10         sort1(a, 0, a.length);
 11     }
 12     
 13     /**
 14      * 對指定 int 型數組的指定范圍按數字升序進行排序。
 15      */
 16     public static void sort(int[] a, int fromIndex, int toIndex) {
 17         rangeCheck(a.length, fromIndex, toIndex);
 18         sort1(a, fromIndex, toIndex - fromIndex);
 19     }
 20 
 21     private static void sort1(int x[], int off, int len) {
 22         /*
 23          * 當待排序的數組中的元素個數小于 7 時，采用插入排序 。
 24          * 
 25          * 盡管插入排序的時間復雜度為O(n^2),但是當數組元素較少時， 插入排序優于快速排序，因為這時快速排序的遞歸操作影響性能。
 26          */
 27         if (len < 7) {
 28             for (int i = off; i < len + off; i++)
 29                 for (int j = i; j > off && x[j - 1] > x[j]; j--)
 30                     swap(x, j, j - 1);
 31             return;
 32         }
 33         /*
 34          * 當待排序的數組中的元素個數大于 或等于7 時，采用快速排序 。
 35          * 
 36          * Choose a partition element, v
 37          * 選取一個劃分元，V
 38          * 
 39          * 較好的選擇了劃分元(基準元素)。能夠將數組分成大致兩個相等的部分，避免出現最壞的情況。例如當數組有序的的情況下，
 40          * 選擇第一個元素作為劃分元，將使得算法的時間復雜度達到O(n^2).
 41          */
 42         // 當數組大小為size=7時 ，取數組中間元素作為劃分元。
 43         int m = off + (len >> 1);
 44         // 當數組大小 7<size<=40時，取首、中、末 三個元素中間大小的元素作為劃分元。
 45         if (len > 7) {
 46             int l = off;
 47             int n = off + len - 1;
 48             /*
 49              * 當數組大小  size>40 時 ，從待排數組中較均勻的選擇9個元素，
 50              * 選出一個偽中數做為劃分元。
 51              */
 52             if (len > 40) {
 53                 int s = len / 8;
 54                 l = med3(x, l, l + s, l + 2 * s);
 55                 m = med3(x, m - s, m, m + s);
 56                 n = med3(x, n - 2 * s, n - s, n);
 57             }
 58             // 取出中間大小的元素的位置。
 59             m = med3(x, l, m, n); // Mid-size, med of 3
 60         }
 61         
 62         //得到劃分元V
 63         int v = x[m];
 64         
 65         // Establish Invariant: v* (<v)* (>v)* v*
 66         int a = off, b = a, c = off + len - 1, d = c;
 67         while (true) {
 68             while (b <= c && x[b] <= v) {
 69                 if (x[b] == v)
 70                     swap(x, a++, b);
 71                 b++;
 72             }
 73             while (c >= b && x[c] >= v) {
 74                 if (x[c] == v)
 75                     swap(x, c, d--);
 76                 c--;
 77             }
 78             if (b > c)
 79                 break;
 80             swap(x, b++, c--);
 81         }
 82         // Swap partition elements back to middle
 83         int s, n = off + len;
 84         s = Math.min(a - off, b - a);
 85         vecswap(x, off, b - s, s);
 86         s = Math.min(d - c, n - d - 1);
 87         vecswap(x, b, n - s, s);
 88         // Recursively sort non-partition-elements
 89         if ((s = b - a) > 1)
 90             sort1(x, off, s);
 91         if ((s = d - c) > 1)
 92             sort1(x, n - s, s);
 93     }
 94     
 95     /**
 96      * Swaps x[a] with x[b].
 97      */
 98     private static void swap(int x[], int a, int b) {
 99         int t = x[a];
100         x[a] = x[b];
101         x[b] = t;
102     }
103     
104     /**
105      * Swaps x[a .. (a+n-1)] with x[b .. (b+n-1)].
106      */
107     private static void vecswap(int x[], int a, int b, int n) {
108     for (int i=0; i<n; i++, a++, b++)
109         swap(x, a, b);
110     }
111     
112     /**
113      * Returns the index of the median of the three indexed integers.
114      */
115     private static int med3(int x[], int a, int b, int c) {
116         return (x[a] < x[b] ? (x[b] < x[c] ? b : x[a] < x[c] ? c : a)
117                 : (x[b] > x[c] ? b : x[a] > x[c] ? c : a));
118     }
119 
120     /**
121      * Check that fromIndex and toIndex are in range, and throw an
122      * appropriate exception if they aren't.
123      */
124     private static void rangeCheck(int arrayLen, int fromIndex, int toIndex) {
125         if (fromIndex > toIndex)
126             throw new IllegalArgumentException("fromIndex(" + fromIndex
127                     + ") > toIndex(" + toIndex + ")");
128         if (fromIndex < 0)
129             throw new ArrayIndexOutOfBoundsException(fromIndex);
130         if (toIndex > arrayLen)
131             throw new ArrayIndexOutOfBoundsException(toIndex);
132     }
133 }

 1 package com.test;
 2 
 3 import com.util.ArraysPrimitive;
 4 
 5 public class ArraysTest {
 6     public static void main(String[] args) {
 7         int [] a={15,93,15,41,6,15,22,7,15,20};
 8         ArraysPrimitive.sort(a);
 9         for(int i=0;i<a.length;i++){
10             System.out.print(a[i]+",");
11         }
12         //結果：6,7,15,15,15,15,20,22,41,93,
13     }
14 }

  1 package com.util;
  2 
  3 import java.lang.reflect.Array;
  4 
  5 public class ArraysObject {
  6     private static final int INSERTIONSORT_THRESHOLD = 7;
  7 
  8     private ArraysObject() {}
  9 
 10     public static void sort(Object[] a) {
 11         //java.lang.Object.clone()，理解深表復制和淺表復制
 12         Object[] aux = (Object[]) a.clone();
 13         mergeSort(aux, a, 0, a.length, 0);
 14     }
 15 
 16     public static void sort(Object[] a, int fromIndex, int toIndex) {
 17         rangeCheck(a.length, fromIndex, toIndex);
 18         Object[] aux = copyOfRange(a, fromIndex, toIndex);
 19         mergeSort(aux, a, fromIndex, toIndex, -fromIndex);
 20     }
 21 
 22     /**
 23      * Src is the source array that starts at index 0 
 24      * Dest is the (possibly larger) array destination with a possible offset 
 25      * low is the index in dest to start sorting 
 26      * high is the end index in dest to end sorting 
 27      * off is the offset to generate corresponding low, high in src
 28      */
 29     private static void mergeSort(Object[] src, Object[] dest, int low,
 30             int high, int off) {
 31         int length = high - low;
 32 
 33         // Insertion sort on smallest arrays
 34         if (length < INSERTIONSORT_THRESHOLD) {
 35             for (int i = low; i < high; i++)
 36                 for (int j = i; j > low && 
 37                         ((Comparable) dest[j - 1]).compareTo(dest[j]) > 0; j--)
 38                     swap(dest, j, j - 1);
 39             return;
 40         }
 41 
 42         // Recursively sort halves of dest into src
 43         int destLow = low;
 44         int destHigh = high;
 45         low += off;
 46         high += off;
 47         /*
 48          *  >>>：無符號右移運算符
 49          *  expression1 >>> expresion2：expression1的各個位向右移expression2
 50          *  指定的位數。右移后左邊空出的位數用0來填充。移出右邊的位被丟棄。
 51          *  例如：-14>>>2；  結果為：1073741820
 52          */
 53         int mid = (low + high) >>> 1;
 54         mergeSort(dest, src, low, mid, -off);
 55         mergeSort(dest, src, mid, high, -off);
 56 
 57         // If list is already sorted, just copy from src to dest. This is an
 58         // optimization that results in faster sorts for nearly ordered lists.
 59         if (((Comparable) src[mid - 1]).compareTo(src[mid]) <= 0) {
 60             System.arraycopy(src, low, dest, destLow, length);
 61             return;
 62         }
 63 
 64         // Merge sorted halves (now in src) into dest
 65         for (int i = destLow, p = low, q = mid; i < destHigh; i++) {
 66             if (q >= high || p < mid
 67                     && ((Comparable) src[p]).compareTo(src[q]) <= 0)
 68                 dest[i] = src[p++];
 69             else
 70                 dest[i] = src[q++];
 71         }
 72     }
 73 
 74     /**
 75      * Check that fromIndex and toIndex are in range, and throw an appropriate
 76      * exception if they aren't.
 77      */
 78     private static void rangeCheck(int arrayLen, int fromIndex, int toIndex) {
 79         if (fromIndex > toIndex)
 80             throw new IllegalArgumentException("fromIndex(" + fromIndex
 81                     + ") > toIndex(" + toIndex + ")");
 82         if (fromIndex < 0)
 83             throw new ArrayIndexOutOfBoundsException(fromIndex);
 84         if (toIndex > arrayLen)
 85             throw new ArrayIndexOutOfBoundsException(toIndex);
 86     }
 87 
 88     public static <T> T[] copyOfRange(T[] original, int from, int to) {
 89         return copyOfRange(original, from, to, (Class<T[]>) original.getClass());
 90     }
 91 
 92     public static <T, U> T[] copyOfRange(U[] original, int from, int to,
 93             Class<? extends T[]> newType) {
 94         int newLength = to - from;
 95         if (newLength < 0)
 96             throw new IllegalArgumentException(from + " > " + to);
 97         T[] copy = ((Object) newType == (Object) Object[].class)
 98                 ? (T[]) new Object[newLength]
 99                 : (T[]) Array.newInstance(newType.getComponentType(), newLength);
100         System.arraycopy(original, from, copy, 0,
101                 Math.min(original.length - from, newLength));
102         return copy;
103     }
104 
105     /**
106      * Swaps x[a] with x[b].
107      */
108     private static void swap(Object[] x, int a, int b) {
109         Object t = x[a];
110         x[a] = x[b];
111         x[b] = t;
112     }
113 }

 1 package com.test;
 2 
 3 import com.util.ArraysObject;
 4 
 5 public class ArraysObjectSortTest {
 6     public static void main(String[] args) {
 7         Student stu1=new Student(1001,100.0F);
 8         Student stu2=new Student(1002,90.0F);
 9         Student stu3=new Student(1003,90.0F);
10         Student stu4=new Student(1004,95.0F);
11         Student[] stus={stu1,stu2,stu3,stu4};
12         //Arrays.sort(stus);
13         ArraysObject.sort(stus);
14         for(int i=0;i<stus.length;i++){
15             System.out.println(stus[i].getId()+" : "+stus[i].getScore());
16         }
17         /* 1002 : 90.0
18          * 1003 : 90.0
19          * 1004 : 95.0
20          * 1001 : 100.0
21          */
22     }
23 }
24 class Student implements Comparable<Student>{
25     private int id;  //學號
26     private float score;  //成績
27     public Student(){}
28     public Student(int id,float score){
29         this.id=id;
30         this.score=score;
31     }
32     @Override
33     public int compareTo(Student s) {
34         return (int)(this.score-s.getScore());
35     }
36     public int getId() {
37         return id;
38     }
39     public void setId(int id) {
40         this.id = id;
41     }
42     public float getScore() {
43         return score;
44     }
45     public void setScore(float score) {
46         this.score = score;
47     }
48 }

 1 package com.lang;
 2 
 3 public final class System {
 4     //System 類不能被實例化。 
 5     private System() {}
 6     //在 System 類提供的設施中，有標準輸入、標準輸出和錯誤輸出流；對外部定義的屬性
 7     //和環境變量的訪問；加載文件和庫的方法；還有快速復制數組的一部分的實用方法。
 8     /**
 9      * src and dest都必須是同類型或者可以進行轉換類型的數組．
10      * @param      src      the source array.
11      * @param      srcPos   starting position in the source array.
12      * @param      dest     the destination array.
13      * @param      destPos  starting position in the destination data.
14      * @param      length   the number of array elements to be copied.
15      */
16     public static native void arraycopy(Object src, int srcPos, Object dest,
17             int destPos, int length);
18 }

 1 package com.lang.reflect;
 2 
 3 public final class Array {
 4     private Array() {}
 5     
 6     //創建一個具有指定的組件類型和維度的新數組。
 7     public static Object newInstance(Class<?> componentType, int length)
 8             throws NegativeArraySizeException {
 9         return newArray(componentType, length);
10     }
11 
12     private static native Object newArray(Class componentType, int length)
13             throws NegativeArraySizeException;
14 }