https://openjdk.java.net/ 
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 *
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package jdk.internal.util;

import jdk.internal.HotSpotIntrinsicCandidate;
import jdk.internal.misc.Unsafe;


Utility methods to work with arrays.  This includes a set of methods
to find a mismatch between two primitive arrays.  Also included is
a method to calculate the new length of an array to be reallocated.

Array equality and lexicographical comparison can be built on top of
array mismatch functionality.

The mismatch method implementation, vectorizedMismatch, leverages vector-based techniques to access and compare the contents of two arrays. The Java implementation uses Unsafe.getLongUnaligned to access the content of an array, thus access is supported on platforms that do not support unaligned access. For a byte[] array, 8 bytes (64 bits) can be accessed and compared as a unit rather than individually, which increases the performance when the method is compiled by the HotSpot VM. On supported platforms the mismatch implementation is intrinsified to leverage SIMD instructions. So for a byte[] array, 16 bytes (128 bits), 32 bytes (256 bits), and perhaps in the future even 64 bytes (512 bits), platform permitting, can be accessed and compared as a unit, which further increases the performance over the Java implementation. 
None of the mismatch methods perform array bounds checks.  It is the
responsibility of the caller (direct or otherwise) to perform such checks
before calling this method.

/**
 * Utility methods to work with arrays.  This includes a set of methods
 * to find a mismatch between two primitive arrays.  Also included is
 * a method to calculate the new length of an array to be reallocated.
 *
 * <p>Array equality and lexicographical comparison can be built on top of
 * array mismatch functionality.
 *
 * <p>The mismatch method implementation, {@link #vectorizedMismatch}, leverages
 * vector-based techniques to access and compare the contents of two arrays.
 * The Java implementation uses {@code Unsafe.getLongUnaligned} to access the
 * content of an array, thus access is supported on platforms that do not
 * support unaligned access.  For a byte[] array, 8 bytes (64 bits) can be
 * accessed and compared as a unit rather than individually, which increases
 * the performance when the method is compiled by the HotSpot VM.  On supported
 * platforms the mismatch implementation is intrinsified to leverage SIMD
 * instructions.  So for a byte[] array, 16 bytes (128 bits), 32 bytes
 * (256 bits), and perhaps in the future even 64 bytes (512 bits), platform
 * permitting, can be accessed and compared as a unit, which further increases
 * the performance over the Java implementation.
 *
 * <p>None of the mismatch methods perform array bounds checks.  It is the
 * responsibility of the caller (direct or otherwise) to perform such checks
 * before calling this method.
 */
public class ArraysSupport {
    static final Unsafe U = Unsafe.getUnsafe();

    private static final boolean BIG_ENDIAN = U.isBigEndian();

    public static final int LOG2_ARRAY_BOOLEAN_INDEX_SCALE = exactLog2(Unsafe.ARRAY_BOOLEAN_INDEX_SCALE);
    public static final int LOG2_ARRAY_BYTE_INDEX_SCALE = exactLog2(Unsafe.ARRAY_BYTE_INDEX_SCALE);
    public static final int LOG2_ARRAY_CHAR_INDEX_SCALE = exactLog2(Unsafe.ARRAY_CHAR_INDEX_SCALE);
    public static final int LOG2_ARRAY_SHORT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_SHORT_INDEX_SCALE);
    public static final int LOG2_ARRAY_INT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_INT_INDEX_SCALE);
    public static final int LOG2_ARRAY_LONG_INDEX_SCALE = exactLog2(Unsafe.ARRAY_LONG_INDEX_SCALE);
    public static final int LOG2_ARRAY_FLOAT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_FLOAT_INDEX_SCALE);
    public static final int LOG2_ARRAY_DOUBLE_INDEX_SCALE = exactLog2(Unsafe.ARRAY_DOUBLE_INDEX_SCALE);

    private static final int LOG2_BYTE_BIT_SIZE = exactLog2(Byte.SIZE);

    private static int exactLog2(int scale) {
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        return Integer.numberOfTrailingZeros(scale);
    }

    private ArraysSupport() {}

    
Find the relative index of the first mismatching pair of elements in two
primitive arrays of the same component type.  Pairs of elements will be
tested in order relative to given offsets into both arrays.

This method does not perform type checks or bounds checks.  It is the
responsibility of the caller to perform such checks before calling this
method.

The given offsets, in bytes, need not be aligned according to the
given log2 size the array elements.  More specifically, an
offset modulus the size need not be zero.

Params:
  • a – the first array to be tested for mismatch, or null for direct memory access
  • aOffset – the relative offset, in bytes, from the base address of the first array to test from, otherwise if the first array is null, an absolute address pointing to the first element to test.
  • b – the second array to be tested for mismatch, or null for direct memory access
  • bOffset – the relative offset, in bytes, from the base address of the second array to test from, otherwise if the second array is null, an absolute address pointing to the first element to test.
  • length – the number of array elements to test
  • log2ArrayIndexScale – log2 of the array index scale, that
corresponds to the size, in bytes, of an array element.
Returns:if a mismatch is found a relative index, between 0 (inclusive) and length (exclusive), of the first mismatching pair of elements in the two arrays. Otherwise, if a mismatch is not found the bitwise compliment of the number of remaining pairs of elements to be checked in the tail of the two arrays.
/**
     * Find the relative index of the first mismatching pair of elements in two
     * primitive arrays of the same component type.  Pairs of elements will be
     * tested in order relative to given offsets into both arrays.
     *
     * <p>This method does not perform type checks or bounds checks.  It is the
     * responsibility of the caller to perform such checks before calling this
     * method.
     *
     * <p>The given offsets, in bytes, need not be aligned according to the
     * given log<sub>2</sub> size the array elements.  More specifically, an
     * offset modulus the size need not be zero.
     *
     * @param a the first array to be tested for mismatch, or {@code null} for
     * direct memory access
     * @param aOffset the relative offset, in bytes, from the base address of
     * the first array to test from, otherwise if the first array is
     * {@code null}, an absolute address pointing to the first element to test.
     * @param b the second array to be tested for mismatch, or {@code null} for
     * direct memory access
     * @param bOffset the relative offset, in bytes, from the base address of
     * the second array to test from, otherwise if the second array is
     * {@code null}, an absolute address pointing to the first element to test.
     * @param length the number of array elements to test
     * @param log2ArrayIndexScale log<sub>2</sub> of the array index scale, that
     * corresponds to the size, in bytes, of an array element.
     * @return if a mismatch is found a relative index, between 0 (inclusive)
     * and {@code length} (exclusive), of the first mismatching pair of elements
     * in the two arrays.  Otherwise, if a mismatch is not found the bitwise
     * compliment of the number of remaining pairs of elements to be checked in
     * the tail of the two arrays.
     */
    @HotSpotIntrinsicCandidate
    public static int vectorizedMismatch(Object a, long aOffset,
                                         Object b, long bOffset,
                                         int length,
                                         int log2ArrayIndexScale) {
        // assert a.getClass().isArray();
        // assert b.getClass().isArray();
        // assert 0 <= length <= sizeOf(a)
        // assert 0 <= length <= sizeOf(b)
        // assert 0 <= log2ArrayIndexScale <= 3

        int log2ValuesPerWidth = LOG2_ARRAY_LONG_INDEX_SCALE - log2ArrayIndexScale;
        int wi = 0;
        for (; wi < length >> log2ValuesPerWidth; wi++) {
            long bi = ((long) wi) << LOG2_ARRAY_LONG_INDEX_SCALE;
            long av = U.getLongUnaligned(a, aOffset + bi);
            long bv = U.getLongUnaligned(b, bOffset + bi);
            if (av != bv) {
                long x = av ^ bv;
                int o = BIG_ENDIAN
                        ? Long.numberOfLeadingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale)
                        : Long.numberOfTrailingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale);
                return (wi << log2ValuesPerWidth) + o;
            }
        }

        // Calculate the tail of remaining elements to check
        int tail = length - (wi << log2ValuesPerWidth);

        if (log2ArrayIndexScale < LOG2_ARRAY_INT_INDEX_SCALE) {
            int wordTail = 1 << (LOG2_ARRAY_INT_INDEX_SCALE - log2ArrayIndexScale);
            // Handle 4 bytes or 2 chars in the tail using int width
            if (tail >= wordTail) {
                long bi = ((long) wi) << LOG2_ARRAY_LONG_INDEX_SCALE;
                int av = U.getIntUnaligned(a, aOffset + bi);
                int bv = U.getIntUnaligned(b, bOffset + bi);
                if (av != bv) {
                    int x = av ^ bv;
                    int o = BIG_ENDIAN
                            ? Integer.numberOfLeadingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale)
                            : Integer.numberOfTrailingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale);
                    return (wi << log2ValuesPerWidth) + o;
                }
                tail -= wordTail;
            }
            return ~tail;
        }
        else {
            return ~tail;
        }
    }

    // Booleans
    // Each boolean element takes up one byte

    public static int mismatch(boolean[] a,
                               boolean[] b,
                               int length) {
        int i = 0;
        if (length > 7) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_BOOLEAN_BASE_OFFSET,
                    b, Unsafe.ARRAY_BOOLEAN_BASE_OFFSET,
                    length, LOG2_ARRAY_BOOLEAN_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(boolean[] a, int aFromIndex,
                               boolean[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 7) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_BOOLEAN_BASE_OFFSET + aFromIndex;
            int bOffset = Unsafe.ARRAY_BOOLEAN_BASE_OFFSET + bFromIndex;
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_BOOLEAN_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Bytes

    
Find the index of a mismatch between two arrays.

This method does not perform bounds checks. It is the responsibility
of the caller to perform such bounds checks before calling this method.

Params:
  • a – the first array to be tested for a mismatch
  • b – the second array to be tested for a mismatch
  • length – the number of bytes from each array to check
Returns:the index of a mismatch between the two arrays, otherwise -1 if
no mismatch.  The index will be within the range of (inclusive) 0 to
(exclusive) the smaller of the two array lengths.
/**
     * Find the index of a mismatch between two arrays.
     *
     * <p>This method does not perform bounds checks. It is the responsibility
     * of the caller to perform such bounds checks before calling this method.
     *
     * @param a the first array to be tested for a mismatch
     * @param b the second array to be tested for a mismatch
     * @param length the number of bytes from each array to check
     * @return the index of a mismatch between the two arrays, otherwise -1 if
     * no mismatch.  The index will be within the range of (inclusive) 0 to
     * (exclusive) the smaller of the two array lengths.
     */
    public static int mismatch(byte[] a,
                               byte[] b,
                               int length) {
        // ISSUE: defer to index receiving methods if performance is good
        // assert length <= a.length
        // assert length <= b.length

        int i = 0;
        if (length > 7) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_BYTE_BASE_OFFSET,
                    b, Unsafe.ARRAY_BYTE_BASE_OFFSET,
                    length, LOG2_ARRAY_BYTE_INDEX_SCALE);
            if (i >= 0)
                return i;
            // Align to tail
            i = length - ~i;
//            assert i >= 0 && i <= 7;
        }
        // Tail < 8 bytes
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    
Find the relative index of a mismatch between two arrays starting from
given indexes.

This method does not perform bounds checks. It is the responsibility
of the caller to perform such bounds checks before calling this method.

Params:
  • a – the first array to be tested for a mismatch
  • aFromIndex – the index of the first element (inclusive) in the first
array to be compared
  • b – the second array to be tested for a mismatch
  • bFromIndex – the index of the first element (inclusive) in the
second array to be compared
  • length – the number of bytes from each array to check
Returns:the relative index of a mismatch between the two arrays,
otherwise -1 if no mismatch.  The index will be within the range of
(inclusive) 0 to (exclusive) the smaller of the two array bounds.
/**
     * Find the relative index of a mismatch between two arrays starting from
     * given indexes.
     *
     * <p>This method does not perform bounds checks. It is the responsibility
     * of the caller to perform such bounds checks before calling this method.
     *
     * @param a the first array to be tested for a mismatch
     * @param aFromIndex the index of the first element (inclusive) in the first
     * array to be compared
     * @param b the second array to be tested for a mismatch
     * @param bFromIndex the index of the first element (inclusive) in the
     * second array to be compared
     * @param length the number of bytes from each array to check
     * @return the relative index of a mismatch between the two arrays,
     * otherwise -1 if no mismatch.  The index will be within the range of
     * (inclusive) 0 to (exclusive) the smaller of the two array bounds.
     */
    public static int mismatch(byte[] a, int aFromIndex,
                               byte[] b, int bFromIndex,
                               int length) {
        // assert 0 <= aFromIndex < a.length
        // assert 0 <= aFromIndex + length <= a.length
        // assert 0 <= bFromIndex < b.length
        // assert 0 <= bFromIndex + length <= b.length
        // assert length >= 0

        int i = 0;
        if (length > 7) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_BYTE_BASE_OFFSET + aFromIndex;
            int bOffset = Unsafe.ARRAY_BYTE_BASE_OFFSET + bFromIndex;
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_BYTE_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Chars

    public static int mismatch(char[] a,
                               char[] b,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_CHAR_BASE_OFFSET,
                    b, Unsafe.ARRAY_CHAR_BASE_OFFSET,
                    length, LOG2_ARRAY_CHAR_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(char[] a, int aFromIndex,
                               char[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_CHAR_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_CHAR_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_CHAR_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_CHAR_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_CHAR_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Shorts

    public static int mismatch(short[] a,
                               short[] b,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_SHORT_BASE_OFFSET,
                    b, Unsafe.ARRAY_SHORT_BASE_OFFSET,
                    length, LOG2_ARRAY_SHORT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(short[] a, int aFromIndex,
                               short[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_SHORT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_SHORT_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_SHORT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_SHORT_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_SHORT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Ints

    public static int mismatch(int[] a,
                               int[] b,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_INT_BASE_OFFSET,
                    b, Unsafe.ARRAY_INT_BASE_OFFSET,
                    length, LOG2_ARRAY_INT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(int[] a, int aFromIndex,
                               int[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_INT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_INT_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_INT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_INT_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_INT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Floats

    public static int mismatch(float[] a,
                               float[] b,
                               int length) {
        return mismatch(a, 0, b, 0, length);
    }

    public static int mismatch(float[] a, int aFromIndex,
                               float[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (Float.floatToRawIntBits(a[aFromIndex]) == Float.floatToRawIntBits(b[bFromIndex])) {
                int aOffset = Unsafe.ARRAY_FLOAT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_FLOAT_INDEX_SCALE);
                int bOffset = Unsafe.ARRAY_FLOAT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_FLOAT_INDEX_SCALE);
                i = vectorizedMismatch(
                        a, aOffset,
                        b, bOffset,
                        length, LOG2_ARRAY_FLOAT_INDEX_SCALE);
            }
            // Mismatched
            if (i >= 0) {
                // Check if mismatch is not associated with two NaN values
                if (!Float.isNaN(a[aFromIndex + i]) || !Float.isNaN(b[bFromIndex + i]))
                    return i;

                // Mismatch on two different NaN values that are normalized to match
                // Fall back to slow mechanism
                // ISSUE: Consider looping over vectorizedMismatch adjusting ranges
                // However, requires that returned value be relative to input ranges
                i++;
            }
            // Matched
            else {
                i = length - ~i;
            }
        }
        for (; i < length; i++) {
            if (Float.floatToIntBits(a[aFromIndex + i]) != Float.floatToIntBits(b[bFromIndex + i]))
                return i;
        }
        return -1;
    }

    // 64 bit sizes

    // Long

    public static int mismatch(long[] a,
                               long[] b,
                               int length) {
        if (length == 0) {
            return -1;
        }
        if (a[0] != b[0])
            return 0;
        int i = vectorizedMismatch(
                a, Unsafe.ARRAY_LONG_BASE_OFFSET,
                b, Unsafe.ARRAY_LONG_BASE_OFFSET,
                length, LOG2_ARRAY_LONG_INDEX_SCALE);
        return i >= 0 ? i : -1;
    }

    public static int mismatch(long[] a, int aFromIndex,
                               long[] b, int bFromIndex,
                               int length) {
        if (length == 0) {
            return -1;
        }
        if (a[aFromIndex] != b[bFromIndex])
            return 0;
        int aOffset = Unsafe.ARRAY_LONG_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_LONG_INDEX_SCALE);
        int bOffset = Unsafe.ARRAY_LONG_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_LONG_INDEX_SCALE);
        int i = vectorizedMismatch(
                a, aOffset,
                b, bOffset,
                length, LOG2_ARRAY_LONG_INDEX_SCALE);
        return i >= 0 ? i : -1;
    }


    // Double

    public static int mismatch(double[] a,
                               double[] b,
                               int length) {
        return mismatch(a, 0, b, 0, length);
    }

    public static int mismatch(double[] a, int aFromIndex,
                               double[] b, int bFromIndex,
                               int length) {
        if (length == 0) {
            return -1;
        }
        int i = 0;
        if (Double.doubleToRawLongBits(a[aFromIndex]) == Double.doubleToRawLongBits(b[bFromIndex])) {
            int aOffset = Unsafe.ARRAY_DOUBLE_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_DOUBLE_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_DOUBLE_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_DOUBLE_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_DOUBLE_INDEX_SCALE);
        }
        if (i >= 0) {
            // Check if mismatch is not associated with two NaN values
            if (!Double.isNaN(a[aFromIndex + i]) || !Double.isNaN(b[bFromIndex + i]))
                return i;

            // Mismatch on two different NaN values that are normalized to match
            // Fall back to slow mechanism
            // ISSUE: Consider looping over vectorizedMismatch adjusting ranges
            // However, requires that returned value be relative to input ranges
            i++;
            for (; i < length; i++) {
                if (Double.doubleToLongBits(a[aFromIndex + i]) != Double.doubleToLongBits(b[bFromIndex + i]))
                    return i;
            }
        }

        return -1;
    }

    
The maximum length of array to allocate (unless necessary). Some VMs reserve some header words in an array. Attempts to allocate larger arrays may result in OutOfMemoryError: Requested array size exceeds VM limit 
/**
     * The maximum length of array to allocate (unless necessary).
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * {@code OutOfMemoryError: Requested array size exceeds VM limit}
     */
    public static final int MAX_ARRAY_LENGTH = Integer.MAX_VALUE - 8;

    
Calculates a new array length given an array's current length, a preferred growth value, and a minimum growth value. If the preferred growth value is less than the minimum growth value, the minimum growth value is used in its place. If the sum of the current length and the preferred growth value does not exceed MAX_ARRAY_LENGTH, that sum is returned. If the sum of the current length and the minimum growth value does not exceed MAX_ARRAY_LENGTH, then MAX_ARRAY_LENGTH is returned. If the sum does not overflow an int, then Integer.MAX_VALUE is returned. Otherwise, OutOfMemoryError is thrown. 
Params:
  • oldLength –   current length of the array (must be non negative)
  • minGrowth –   minimum required growth of the array length (must be
                   positive)
  • prefGrowth –  preferred growth of the array length (ignored, if less then minGrowth)
Throws:
Returns:the new length of the array
/**
     * Calculates a new array length given an array's current length, a preferred
     * growth value, and a minimum growth value.  If the preferred growth value
     * is less than the minimum growth value, the minimum growth value is used in
     * its place.  If the sum of the current length and the preferred growth
     * value does not exceed {@link #MAX_ARRAY_LENGTH}, that sum is returned.
     * If the sum of the current length and the minimum growth value does not
     * exceed {@code MAX_ARRAY_LENGTH}, then {@code MAX_ARRAY_LENGTH} is returned.
     * If the sum does not overflow an int, then {@code Integer.MAX_VALUE} is
     * returned.  Otherwise, {@code OutOfMemoryError} is thrown.
     *
     * @param oldLength   current length of the array (must be non negative)
     * @param minGrowth   minimum required growth of the array length (must be
     *                    positive)
     * @param prefGrowth  preferred growth of the array length (ignored, if less
     *                    then {@code minGrowth})
     * @return the new length of the array
     * @throws OutOfMemoryError if increasing {@code oldLength} by
     *                    {@code minGrowth} overflows.
     */
    public static int newLength(int oldLength, int minGrowth, int prefGrowth) {
        // assert oldLength >= 0
        // assert minGrowth > 0

        int newLength = Math.max(minGrowth, prefGrowth) + oldLength;
        if (newLength - MAX_ARRAY_LENGTH <= 0) {
            return newLength;
        }
        return hugeLength(oldLength, minGrowth);
    }

    private static int hugeLength(int oldLength, int minGrowth) {
        int minLength = oldLength + minGrowth;
        if (minLength < 0) { // overflow
            throw new OutOfMemoryError("Required array length too large");
        }
        if (minLength <= MAX_ARRAY_LENGTH) {
            return MAX_ARRAY_LENGTH;
        }
        return Integer.MAX_VALUE;
    }
}