Java面试题之List(一)

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

/**
 * 默认构造函数
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * capacity是ArrayList的默认容量大小。当由于增加数据导致容量不足时，容量会添加上一次容量大小的一半。
 *
 * @param  initialCapacity  the initial capacity of the list
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
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);
    }
}

/**
 * 创建一个包含collection的ArrayList
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public 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;
    }
}

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    // 序列版本号
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * 默认初始化容量
     */
    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 = {};

    /**
     *  保存ArrayList中数据的数组
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * ArrayList中实际数据的数量
     */
    private int size;

    /**
     * 带容量的构造函数
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            // 新建一个数组
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * 无参构造
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 使用一个Collection的创建ArrayList
     */
    public 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;
        }
    }

    /**
     * 将当前容量值设为 = 实际元素个数
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * 若ArrayList的容量不足以容纳当前的全部元素，设置 新的容量=“(原始容量x3)/2 + 1”
     */
    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);
        }
    }

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

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

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

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 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);
    }

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

    /**
     *  返回ArrayList的实际大小
     */
    public int size() {
        return size;
    }

    /**
     * 是否为空
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 返回ArrayList是否包含指定元素
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 正向查找，返回元素的索引值
     */
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * 反向查找(从数组末尾向开始查找)，返回元素(o)的索引值
     */
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * 拷贝
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    /**
     * 返回ArrayList的Object数组
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 返回ArrayList的Object数组
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 获取index位置的元素值
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 设置index位置的值为element
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /**
     * 将e添加到ArrayList的指定位置
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 将e添加到ArrayList的指定位置
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * 删除ArrayList指定位置的元素
     */
    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; // clear to let GC do its work

        return oldValue;
    }

    /**
     * 删除ArrayList的指定元素
     */
    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;
    }

    /*
     * 快速删除第index个元素
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
       // 从"index+1"开始，用后面的元素替换前面的元素。
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

    /**
     * 清空ArrayList，将全部的元素设为null
     */
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    /**
     * 将集合c追加到ArrayList中
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * 从index位置开始，将集合c添加到ArrayList
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount

        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;
    }

    /**
     * 删除fromIndex到toIndex之间的全部元素
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * 删除所有的元素
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * 删除当前集合中C中含有的元素
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    /**
     * 迭代器方法
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * 内部类：迭代器
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一个元素的索引
        int lastRet = -1; // 上一个返回的元素的位置
        int expectedModCount = modCount;

        Itr() {}
        /**
         * 内部类：迭代器
         */
        public boolean hasNext() {
            return cursor != size;
        }
        /**
         * 内部类：迭代器下一个元素
         */
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
/**
         * 内部类：迭代器删除
         */
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        /**
         * 内部类：校验修改
         */
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    

public class ArrayListDemo {

    public static void main(String[] args) {
        //测试单线程情况下集合的操作
        testSingleThread();

        //测试多线程情况下集合的操作
        testMultiThread();
    }

    /**
     * 测试单线程环境下ArrayList操作
     *
     * 1. 这个程序输出的结果是什么？
     * 2. 为什么是："李四".equals(element) 而不是 element.equals("李四")？
     * 3. 有没有优化方案？
     */
    public static void testSingleThread() {
        List<String> list = new ArrayList<>();
        list.add("张三");
        list.add("李四");
        list.add("王五");
        list.add("赵六");
        list.add("孙七");
        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            //迭代每个元素
            String element = iterator.next();
            //删除李四这个元素
            if ("李四".equals(element)) {
                list.remove(element);
//                iterator.remove();
            }
        }
        System.out.println(list.size());
     }

    /**
     * 测试多线程环境下ArrayList操作
     * 1.多线程下使用迭代器，是否线程安全
     * 2.有没有优化方案？
     */
    public static void testMultiThread() {
        List<String> list = new ArrayList<>();
        list.add("张三");
        list.add("李四");
        list.add("王五");
        list.add("赵六");
        list.add("孙七");

        //线程1
        Thread thread1 = new Thread(() -> {
            Iterator<String> iterator = list.iterator();
            while (iterator.hasNext()) {
                String element = iterator.next();
                if ("李四".equals(element)) {
                    list.remove(element);
                    System.out.println("线程1删除李四");
                } else {
                    System.out.println("线程1遍历的元素：" + element);
                }
            }
        });

        //线程2
        Thread thread2 = new Thread(() -> {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            Iterator<String> iterator = list.iterator();
            while (iterator.hasNext()) {
                String element = iterator.next();
                System.out.println("线程2遍历的元素：" + element);
            }
        });

        thread1.start();
        thread2.start();
    }
}

线程1遍历的元素：张三
线程1删除李四
Exception in thread "Thread-0" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
    at java.util.ArrayList$Itr.next(ArrayList.java:859)
    at com.yunxi.concurrent.concurrentinterview.test.single.ArrayListDemo.lambda$testMultiThread$0(ArrayListDemo.java:71)
    at java.lang.Thread.run(Thread.java:748)
线程2遍历的元素：张三
线程2遍历的元素：王五
线程2遍历的元素：赵六
线程2遍历的元素：孙七