一、Arrays類是java.util包下的對數組進行操作的一個工具類,Arrays中的方法大體可以分爲以下8類:
sort(對數組排序)
binarySearch(二分查找數組中的元素)
equals(比較兩個數組是否相等)
copyOf(數組的拷貝)
hashCode(計算數組的哈希值)
toString(以特定形式輸出數組)
fill(對數組中的指定位置填充相同的內容)
asList(將數組轉換爲List對象)
注:Arrays只有一個空的構造方法:private Arrays() {}
二、上述各個方法的具體實現
1.Sort 排序(會根據具體的對象採用不同的排序方式,主要是快排(Dual-Pivot Quicksort),JDK1.7專門把排序功能封裝成一個類DualPivotQuicksort)
1.1 對int型數組的排序
public static void sort(int[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
}
public static void sort(int[] a, int fromIndex, int toIndex) {//知道起始位置和終止位置
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
}
具體排序實現sort內部:i使用了快速排序(Quicksort)和歸併排序(mergeSort),臨界爲286,數據量大的用後者
1.2 對long類型數組的排序
public static void sort(long[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
}
public static void sort(long[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
}
注:內部使用了快速排序算法。1.3 對short類型的排序
public static void sort(short[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
}
public static void sort(short[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
}
1.4 對char類型的排序
public static void sort(char[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
}
public static void sort(char[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
}
1.5 對byte類型的排序(使用了計數排序和插入排序,n小的時候插入較快)
public static void sort(byte[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1);
}
public static void sort(byte[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1);
}
1.6 對float類型的排序
public static void sort(float[] a) {
DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
}
public static void sort(float[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
}
1.7 對double類型的排序(與上面套路一樣,再次不再贅述)1.8 對對象類型進行排序
public static void sort(Object[] a) {
Object[] aux = (Object[])a.clone();
mergeSort(aux, a, 0, a.length, 0);
}
public static void sort(Object[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
Object[] aux = copyOfRange(a, fromIndex, toIndex);
mergeSort(aux, a, fromIndex, toIndex, -fromIndex);
}
注:在mergeSort方法中使用的是經過修改的歸併排序算法(modified mergesort)
1.9 parallelSort
java8中新出並行排序,Arrays.parallelSort使用了Java7的Fork/Join框架使排序任務可以在線程池中的多個線程中進行,Fork/Join實現了一種任務竊取算法,一個閒置的線程可以竊取其他線程的閒置任務進行處理。代碼如下:
public static void parallelSort(byte[] a) {
int n = a.length, p, g;
if (n <= MIN_ARRAY_SORT_GRAN ||
(p = ForkJoinPool.getCommonPoolParallelism()) == 1)
DualPivotQuicksort.sort(a, 0, n - 1);
else
new ArraysParallelSortHelpers.FJByte.Sorter
(null, a, new byte[n], 0, n, 0,
((g = n / (p << 2)) <= MIN_ARRAY_SORT_GRAN) ?
MIN_ARRAY_SORT_GRAN : g).invoke();
}
這個方法使用了一個臨界值,如果是小容量的數組或者分塊爲1,其實和普通的sort方法一樣。否則就用的是Fork/Join。
性能:發現數據量越大,parallelSort的優勢就越明顯。
parallelSort也有對各種類型的排序
2.binarySearch 二分查找,前提是原數組有序,不然查找無意義
binarySearch 對各種類型的查找都有兩種方式
public static int binarySearch(long[]
a, int fromIndex, int toIndex, long key) //告訴數組和關鍵字和查找的範圍
public static int binarySearch(long[]
a, long key) //告訴數組和關鍵字進行查找
每個方法都調用了binarySearch0(),內部實現是一樣的
private static int binarySearch0(long[] a, int fromIndex, int toIndex,
long key) {
int low = fromIndex;
int high = toIndex - 1;
while (low <= high) {
int mid = (low + high) >>> 1;
long midVal = a[mid];
if (midVal < key)
low = mid + 1;
else if (midVal > key)
high = mid - 1;
else
return mid; // key found
}
return -(low + 1); // key not found.
}
public static int binarySearch(long[] a, long key) {
return binarySearch0(a, 0, a.length, key);
}
public static int binarySearch(long[] a, int fromIndex, int toIndex,
long key) {
rangeCheck(a.length, fromIndex, toIndex);
return binarySearch0(a, fromIndex, toIndex, key);
}
3.equals 比較兩個數組
3.1 對基本類型進行比較
public static boolean equals(int[] a, int[] a2) {
if (a==a2)
return true;
if (a==null || a2==null)
return false;
int length = a.length;
if (a2.length != length)
return false;
for (int i=0; i<length; i++)
if (a[i] != a2[i])
return false;
return true;
}
3.2比較特別的是 double和float
public static boolean equals(double[] a, double[] a2) {
if (a==a2)
return true;
if (a==null || a2==null)
return false;
int length = a.length;
if (a2.length != length)
return false;
for (int i=0; i<length; i++)
if (Double.doubleToLongBits(a[i])!=Double.doubleToLongBits(a2[i]))
return false;
return true;
}
public static boolean equals(float[] a, float[] a2) {
if (a==a2)
return true;
if (a==null || a2==null)
return false;
int length = a.length;
if (a2.length != length)
return false;
for (int i=0; i<length; i++)
if (Float.floatToIntBits(a[i])!=Float.floatToIntBits(a2[i]))
return false;
return true;
}
注:if (Float.floatToIntBits(a[i])!=Float.floatToIntBits(a2[i])) 爲了精確比較
3.3 對Object類型進行比較
public static boolean equals(Object[] a, Object[] a2) {
if (a==a2)
return true;
if (a==null || a2==null)
return false;
int length = a.length;
if (a2.length != length)
return false;
for (int i=0; i<length; i++) {
Object o1 = a[i];
Object o2 = a2[i];
if (!(o1==null ? o2==null : o1.equals(o2)))
return false;
}
return true;
}
3.4 對Object類型對象的深度比較
static boolean deepEquals0(Object e1, Object e2) {
assert e1 != null;
boolean eq;
if (e1 instanceof Object[] && e2 instanceof Object[])
eq = deepEquals ((Object[]) e1, (Object[]) e2);
else if (e1 instanceof byte[] && e2 instanceof byte[])
eq = equals((byte[]) e1, (byte[]) e2);
else if (e1 instanceof short[] && e2 instanceof short[])
eq = equals((short[]) e1, (short[]) e2);
else if (e1 instanceof int[] && e2 instanceof int[])
eq = equals((int[]) e1, (int[]) e2);
else if (e1 instanceof long[] && e2 instanceof long[])
eq = equals((long[]) e1, (long[]) e2);
else if (e1 instanceof char[] && e2 instanceof char[])
eq = equals((char[]) e1, (char[]) e2);
else if (e1 instanceof float[] && e2 instanceof float[])
eq = equals((float[]) e1, (float[]) e2);
else if (e1 instanceof double[] && e2 instanceof double[])
eq = equals((double[]) e1, (double[]) e2);
else if (e1 instanceof boolean[] && e2 instanceof boolean[])
eq = equals((boolean[]) e1, (boolean[]) e2);
else
eq = e1.equals(e2);
return eq;
}
3.5 對Object數組的深度比較
public static boolean deepEquals(Object[] a1, Object[] a2) {
if (a1 == a2)
return true;
if (a1 == null || a2==null)
return false;
int length = a1.length;
if (a2.length != length)
return false;
for (int i = 0; i < length; i++) {
Object e1 = a1[i];
Object e2 = a2[i];
if (e1 == e2)
continue;
if (e1 == null)
return false;
// Figure out whether the two elements are equal
boolean eq = deepEquals0(e1, e2);
if (!eq)
return false;
}
return true;
}
deepEquals的具體實現:
static boolean deepEquals0(Object e1, Object e2) {
assert e1 != null;
boolean eq;
if (e1 instanceof Object[] && e2 instanceof Object[])
eq = deepEquals ((Object[]) e1, (Object[]) e2);
else if (e1 instanceof byte[] && e2 instanceof byte[])
eq = equals((byte[]) e1, (byte[]) e2);
else if (e1 instanceof short[] && e2 instanceof short[])
eq = equals((short[]) e1, (short[]) e2);
else if (e1 instanceof int[] && e2 instanceof int[])
eq = equals((int[]) e1, (int[]) e2);
else if (e1 instanceof long[] && e2 instanceof long[])
eq = equals((long[]) e1, (long[]) e2);
else if (e1 instanceof char[] && e2 instanceof char[])
eq = equals((char[]) e1, (char[]) e2);
else if (e1 instanceof float[] && e2 instanceof float[])
eq = equals((float[]) e1, (float[]) e2);
else if (e1 instanceof double[] && e2 instanceof double[])
eq = equals((double[]) e1, (double[]) e2);
else if (e1 instanceof boolean[] && e2 instanceof boolean[])
eq = equals((boolean[]) e1, (boolean[]) e2);
else
eq = e1.equals(e2);
return eq;
}
4.fill 對各個類型的數組都有兩種方式
public static void fill(int[] a, int val) //使用val的值對a數組進行填充
public static void fill(int[] a, int val) {
for (int i = 0, len = a.length; i < len; i++)
a[i] = val;
}
public static void fill(int[] a, int fromIndex, int toIndex, int val) //使用val的值對a數組從fromIndex(包含)到toIndex(不包含)進行填充public static void fill(int[] a, int fromIndex, int toIndex, int val) {
rangeCheck(a.length, fromIndex, toIndex);
for (int i = fromIndex; i < toIndex; i++)
a[i] = val;
}
4.copyOf 拷貝數組 對各個類型的數組和對象也只有兩種不同的形式
System.arraycopy()是淺拷貝,如果是Object類型的話,值拷貝了對象的引用,而沒有重新new每一個對象
public static byte[] copyOf(byte[] original, int newLength) //從0下標開始拷貝newLength個
public static byte[] copyOf(byte[] original, int newLength) {
byte[] copy = new byte[newLength];
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
public static byte[] copyOfRange(byte[] original, int from, int to) //從from(包含)拷貝到to(不包含)位置
public static byte[] copyOfRange(byte[] original, int from, int to) {
int newLength = to - from;
if (newLength < 0)
throw new IllegalArgumentException(from + " > " + to);
byte[] copy = new byte[newLength];
System.arraycopy(original, from, copy, 0,
Math.min(original.length - from, newLength));
return copy;
}
5.asList 用了可變參數和泛型,無論輸入什麼類型,都會返回一個相對於類型的ArrayList
Arrays裏面自己定義了一個ArrayList
還用了@SafeVarargs註解,表示可變參數和泛型一個使用時不會出現類型上的問題
@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
}
6.hashCode 6.1 對基本類型的計算(對每個具體的類型,for裏面的計算室友區別的,在此不再贅述)
public static int hashCode(long a[]) {
if (a == null)
return 0;
int result = 1;
for (long element : a) {
int elementHash = (int)(element ^ (element >>> 32));
result = 31 * result + elementHash;
}
return result;
}
public static int hashCode(int a[]) {
if (a == null)
return 0;
int result = 1;
for (int element : a)
result = 31 * result + element;
return result;
}
6.2 對Object類型的計算
public static int hashCode(Object a[]) {
if (a == null)
return 0;
int result = 1;
for (Object element : a)
result = 31 * result + (element == null ? 0 : element.hashCode());
return result;
}
6.3 對數組的hashCode的深度計算
public static int deepHashCode(Object a[]) {
if (a == null)
return 0;
int result = 1;
for (Object element : a) {
int elementHash = 0;
if (element instanceof Object[])
elementHash = deepHashCode((Object[]) element);
else if (element instanceof byte[])
elementHash = hashCode((byte[]) element);
else if (element instanceof short[])
elementHash = hashCode((short[]) element);
else if (element instanceof int[])
elementHash = hashCode((int[]) element);
else if (element instanceof long[])
elementHash = hashCode((long[]) element);
else if (element instanceof char[])
elementHash = hashCode((char[]) element);
else if (element instanceof float[])
elementHash = hashCode((float[]) element);
else if (element instanceof double[])
elementHash = hashCode((double[]) element);
else if (element instanceof boolean[])
elementHash = hashCode((boolean[]) element);
else if (element != null)
elementHash = element.hashCode();
result = 31 * result + elementHash;
}
return result;
}
7.toString 將已有數據轉化爲字符串
7.1 基本類型的toString(各個類型的實現都一樣)
public static String toString(long[] a) {
if (a == null)
return "null";
int iMax = a.length - 1;
if (iMax == -1)
return "[]";
StringBuilder b = new StringBuilder();
b.append('[');
for (int i = 0; ; i++) {
b.append(a[i]);
if (i == iMax)
return b.append(']').toString();
b.append(", ");
}
}
7.2 Object類型(使用valueOf轉)
public static String toString(Object[] a) {
if (a == null)
return "null";
int iMax = a.length - 1;
if (iMax == -1)
return "[]";
StringBuilder b = new StringBuilder();
b.append('[');
for (int i = 0; ; i++) {
b.append(String.valueOf(a[i]));
if (i == iMax)
return b.append(']').toString();
b.append(", ");
}
}
注:valueOf是String類的方法,具體實現:
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}