Arrays源码学习

Arrays 概述

Arrays 是操作Array对象的工具类，包括二分查找、排序、拷贝、等式判断和填充等方法。

接口实现

元类型，未继承和实现任何类、抽象类和接口。

构造函数

采用私有构造函数，因此该类提供的全部是静态方法。

内部定义类

ArrayList

private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public Object[] toArray() {
            return a.clone();
        }

        @Override
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                return Arrays.copyOf(this.a, size,
                                     (Class<? extends T[]>) a.getClass());
            System.arraycopy(this.a, 0, a, 0, size);
            if (a.length > size)
                a[size] = null;
            return a;
        }

        @Override
        public E get(int index) {
            return a[index];
        }

        @Override
        public E set(int index, E element) {
            E oldValue = a[index];
            a[index] = element;
            return oldValue;
        }

        @Override
        public int indexOf(Object o) {
            E[] a = this.a;
            if (o == null) {
                for (int i = 0; i < a.length; i++)
                    if (a[i] == null)
                        return i;
            } else {
                for (int i = 0; i < a.length; i++)
                    if (o.equals(a[i]))
                        return i;
            }
            return -1;
        }

        @Override
        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }

        @Override
        public Spliterator<E> spliterator() {
            return Spliterators.spliterator(a, Spliterator.ORDERED);
        }

        @Override
        public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            for (E e : a) {
                action.accept(e);
            }
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            Objects.requireNonNull(operator);
            E[] a = this.a;
            for (int i = 0; i < a.length; i++) {
                a[i] = operator.apply(a[i]);
            }
        }

        @Override
        public void sort(Comparator<? super E> c) {
            Arrays.sort(a, c);
        }
    }

内部静态私有类 ArrayList，继承AbstractList类，用来实现asList()方法：

public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

NaturalOrder

static final class NaturalOrder implements Comparator<Object> {
        @SuppressWarnings("unchecked")
        public int compare(Object first, Object second) {
            return ((Comparable<Object>)first).compareTo(second);
        }
        static final NaturalOrder INSTANCE = new NaturalOrder();
    }

静态fnal类NaturalOrder类继承Comparator，用来做大小比较。

方法

二分查找

public static int binarySearch(byte[] a, byte key) {
        return binarySearch0(a, 0, a.length, key);
    }

要求待查找数组必须是有序数组，并通过下面方法实现：

private static int binarySearch0(byte[] a, int fromIndex, int toIndex,
                                     byte key) {
        int low = fromIndex;
        int high = toIndex - 1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            byte 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.
    }

其他类型的重构方法类似，此处不做说明，可参考源码学习。

拷贝数组

public static boolean[] copyOf(boolean[] original, int newLength) {
        boolean[] copy = new boolean[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

public static boolean[] copyOfRange(boolean[] original, int from, int to) {
        int newLength = to - from;
        if (newLength < 0)
            throw new IllegalArgumentException(from + " > " + to);
        boolean[] copy = new boolean[newLength];
        System.arraycopy(original, from, copy, 0,
                         Math.min(original.length - from, newLength));
        return copy;
    }

通过系统方法实现数组拷贝。
其他类型的重构方法类似，此处不做说明，可参考源码学习。

相等判断

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;
    }

    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;
    }

public static boolean equals(boolean[] a, boolean[] 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;
    }

其他类型的重构方法类似，此处不做说明，可参考源码学习。

填充

public static void fill(boolean[] a, boolean val) {
        for (int i = 0, len = a.length; i < len; i++)
            a[i] = val;
    }

其他类型的重构方法类似，此处不做说明，可参考源码学习。

排序

public static void sort(byte[] a) {
        DualPivotQuicksort.sort(a, 0, a.length - 1);
    }

static void sort(byte[] a, int left, int right) {
        // Use counting sort on large arrays
        if (right - left > COUNTING_SORT_THRESHOLD_FOR_BYTE) {
            int[] count = new int[NUM_BYTE_VALUES];

            for (int i = left - 1; ++i <= right;
                count[a[i] - Byte.MIN_VALUE]++
            );
            for (int i = NUM_BYTE_VALUES, k = right + 1; k > left; ) {
                while (count[--i] == 0);
                byte value = (byte) (i + Byte.MIN_VALUE);
                int s = count[i];

                do {
                    a[--k] = value;
                } while (--s > 0);
            }
        } else { // Use insertion sort on small arrays
            for (int i = left, j = i; i < right; j = ++i) {
                byte ai = a[i + 1];
                while (ai < a[j]) {
                    a[j + 1] = a[j];
                    if (j-- == left) {
                        break;
                    }
                }
                a[j + 1] = ai;
            }
        }
    }

其他类型的重构方法类似，此处不做说明，可参考源码学习。