arraylist实现原理与使用_arraylist原理面试必问知识

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ArrayList使用非常广泛，无论是在数据库表中查询，还是网络信息爬取都需要使用，所以了解ArrayList的原理就十分重要了（此处中若无特地说明，ArrayList版本基于JDK 1.8）。

//默认创建一个ArrayList集合
ArrayList<String> a1 = new ArrayList<>();
//创建一个初始长度为12的ArrayList集合
ArrayList<String> a2 = new ArrayList<>(12);
//将其他类型的集合转为ArrayList
ArrayList<String> a3 = new ArrayList<>(new HashSet());

/**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

/**
     * Shared empty array instance used for empty instances.
     此共享空数组实例用于空实例
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /*
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     此共享空数组用于默认大小的空实例。我们将此与EMPTY_ELEMENTDATA区分开，以了解填充第一个元素时需要多少（空间）
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

public class Test<E> extends ArrayList {
    @Override
    public Integer[] toArray() {
        return new Integer[]{0, 23};
    }

    public static void main(String[] args){
        Object[] elementData = new Test<Integer>().toArray();
        System.out.println(elementData.getClass());
        System.out.println(Object[].class);
        System.out.println(elementData.getClass() == Object[].class);
    }
}

class [Ljava.lang.Integer;
class [Ljava.lang.Object;
false

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;
    }

public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

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);
        }
    }

public ArrayList() {
  this(10);
}

ArrayList<String> list = new ArrayList<>();
list.add("双皮奶");
list.add("云吞面");
list.add("煲仔饭");
list.add("叉烧包");
list.add("艇仔粥");
list.add("糯米鸡");

public boolean add(E e) {
    ensureCapacityInternal(size + 1);
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
      minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

protected transient int modCount = 0;

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);
    }

private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0)
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

public E get(int index) {
    rangeCheck(index);
    return elementData(index);
}

E elementData(int index) {
  return (E) elementData[index];
}

public E remove(int index) {
        rangeCheck(index);
        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null;
        return oldValue;
    }

public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }