解析字节算法面试真题，深入探究ArrayList应用原理

ArrayList是Java的链表类，作为Java三大容器组成之一的List的组成部分，如下图所示

解析字节算法面试真题，深入探究ArrayList应用原理 在项目开发中常用，虽然他有一些不足的地方，但是这不足以替代他帮助程序员解决大量问题的光辉，正是由于他被经常使用，所以在面试的时候也被经常问到，比方说字节、腾讯这一类对于算法实现以及源码考察比较重视的公司更是如此，今天我就通过几道面试题，以面试的身份对于ArrayList进行讲解

题目：【java源码】ArrayList ArrayList 常用功能：构造函数、增、批量增、删、批量删、批量保留 ArrayList 属性：

// 默认数组长度（数组，而不是数据个数）
private static final int DEFAULT_CAPACITY = 10;
// 空数据
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 实际用于存放数据的地方
transient Object[] elementData;
// 数据个数
private int size;

ArrayList 常用功能：构造函数、增、批量增、删、批量删、批量保留 ArrayList 属性：

// 默认数组长度（数组，而不是数据个数）
private static final int DEFAULT_CAPACITY = 10;
// 空数据
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 实际用于存放数据的地方
transient Object[] elementData;
// 数据个数
private int size;

1、构造函数 ①public ArrayList(); 只干了一件事：this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; ②public ArrayList(int initialCapacity); 指定初始化elementData数组的初始大小。this.elementData = new Object[initialCapacity];

   ③public ArrayList(Collection<? extends E> c) {
       elementData = c.toArray();
       if ((size = elementData.length) != 0) {
           // c.toArray might (incorrectly) not return Object[] (see 6260652)
           if (elementData.getClass() != Object[].class)
               elementData = Arrays.copyOf(elementData, size, Object[].class);
       } else {
           // replace with empty array.
           this.elementData = EMPTY_ELEMENTDATA;
       }
   }

2、增：就是数组中插一个元素操作思路 ①public boolean add(E e); 1、确保elementData数组能够装下 首先判断原来数组长度是否为0，如果为零，那么新数组长度为Math.max(DEFAULT_CAPACITY, 加入后数组最小应该长度)； 判断原来数组是否还装的下，如果需要扩容那么：

           private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度
               // 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);
           }

2、elementData[size++] = e;return true;

②public void add(int index, E element);
    1、rangeCheckForAdd(index); // 检测index是否越接 if (index > size || index < 0)
    2、确保elementData数组能够装下
    3、System.arraycopy(elementData, index, elementData, index + 1, size - index); // 数组挪位
    4、elementData[index] = element;
    5、size++;

3、批量增：就是数组中插多个元素操作思路

①public boolean addAll(Collection<? extends E> c);
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // 同上：确保elementData数组能够装下
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }
②public boolean addAll(int index, Collection<? extends E> c)
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // // 同上：确保elementData数组能够装下

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

4、删：三点注意①按内容删，只删除第一个；②删的是equals为真的；③注意看fastRemove(index)源码

①public boolean remove(Object o);
    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;
    }
②public E remove(int index); // 同理
③private void fastRemove(int index);
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

5、批量删、批量保留：一点注意：batchRemove方法。

①public boolean removeAll(Collection<?> c);
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }
②public boolean retainAll(Collection<?> c);
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }
③private boolean batchRemove(Collection<?> c, boolean complement);
    // 思想：维护两个指针。一个读指针(r)，一个写指针(w)。读指针从0遍历到数组尾，在遍历中，如果符合条件就elementData[w++] = elementData[r];
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }
   

总结：

1、ArrayList通过内置一个Object数组实现顺序表功能。通过grow函数实现动态增长，最大长度可在源码中找答案。 2、删除功能通过fastRemove函数实现对一个删除，通过batchRemove函数实现对多个删除。通过elementData[i] = null；或elementData[--size] = null; 的方式，让JVM垃圾回收，自动回收。

private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度
    // 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 void fastRemove(int index) {
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work
}
    
private boolean batchRemove(Collection<?> c, boolean complement) {
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
        for (; r < size; r++)
            if (c.contains(elementData[r]) == complement)
                elementData[w++] = elementData[r];
    } finally {
        // Preserve behavioral compatibility with AbstractCollection,
        // even if c.contains() throws.
        if (r != size) {
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        if (w != size) {
            // clear to let GC do its work
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}