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  • ArrayList

    ArrayList维护的数据结构是c++数组。创建方式是Arrays.copyOf

        public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
            @SuppressWarnings("unchecked")
            T[] copy = ((Object)newType == (Object)Object[].class)
                ? (T[]) new Object[newLength]
                : (T[]) Array.newInstance(newType.getComponentType(), newLength);
            System.arraycopy(original, 0, copy, 0,
                             Math.min(original.length, newLength));
            return copy;
        }
    

      

    ArrayList扩容会重新分配空间,建议在使用前估计好需要的容量大小。你可以通过构造函数传参,或者使用ensureCapacity

        /**
         * Increases the capacity of this <tt>ArrayList</tt> instance, if
         * necessary, to ensure that it can hold at least the number of elements
         * specified by the minimum capacity argument.
         *
         * @param   minCapacity   the desired minimum capacity
         */
        public void ensureCapacity(int minCapacity) {
            int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                // any size if not default element table
                ? 0
                // larger than default for default empty table. It's already
                // supposed to be at default size.
                : DEFAULT_CAPACITY;
    
            if (minCapacity > minExpand) {
                ensureExplicitCapacity(minCapacity);
            }
        }
    

      

    ArrayList扩容的默认方式是,增加1/2大小。           int newCapacity = oldCapacity + (oldCapacity >> 1);

        /**
         * Increases the capacity to ensure that it can hold at least the
         * number of elements specified by the minimum capacity argument.
         *
         * @param minCapacity the desired minimum capacity
         */
        private void grow(int minCapacity) {
            // overflow-conscious code
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);
            if (newCapacity - minCapacity < 0)
                newCapacity = minCapacity;
            if (newCapacity - MAX_ARRAY_SIZE > 0)
                newCapacity = hugeCapacity(minCapacity);
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
        }
    

      

    ArrayList随机访问效率高,因为他使用的是数组。

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  • 原文地址:https://www.cnblogs.com/afraidToForget/p/10116297.html
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