Android源码之ArrayList，androidarraylist

Android源码之ArrayList，androidarraylist

今天是上班后的第一天，由于项目正在测试阶段，比较空闲，所以研究了一下Android中的ArrayList源码，先说明本篇文章是基于android-23进行的分析，因为我看到网上很多的arrayList源码和我看到的android源码不一样。

首先我们看下Java集合框架中的继承图。

由于今天主要讲的是ArrayList，以后的类我会在以后一一分析。那现在就开始我们今天的源码阅读。

1：我们都知道arraylist是基于数组实现的，是一个动态的数组，其容量能够自动的增长，
2：ArrayList不是线程安全的，只能用在单线程环境下，多线程环境下可以考Collections.synchronizedList(List l)函数返回一个线程安全的ArrayList类，也可以使用concurrent并发包下的CopyOnWriteArrayList类。
3：ArrayList实现了Serializable接口，因此它支持序列化，能够通过序列化传送，实现了RandowAccess接口，支持快速随机访问，实现了Cloneable接口，能被克隆。

public class ArrayList<E> extends AbstractList<E> implements Cloneable, Serializable, RandomAccess {
   //默认的最小的增加数量
    private static final int MIN_CAPACITY_INCREMENT = 12;

   //ArrayList中的个数
    int size;

    //当前的数组
    transient Object[] array;

    //初始化array,EmptyArray.OBJECT是长度为0的数组
    public ArrayList(int capacity) {
        if (capacity < 0) {
            throw new IllegalArgumentException("capacity < 0: " + capacity);
        }
        array = (capacity == 0 ? EmptyArray.OBJECT : new Object[capacity]);
    }

    //EmptyArray.OBJECT是长度为0的数组
    public ArrayList() {
        array = EmptyArray.OBJECT;
    }

    //初始化传进来的collection长度的数组
    public ArrayList(Collection<? extends E> collection) {
        if (collection == null) {
            throw new NullPointerException("collection == null");
        }

        Object[] a = collection.toArray();
        if (a.getClass() != Object[].class) {
            Object[] newArray = new Object[a.length];
            System.arraycopy(a, 0, newArray, 0, a.length);
            a = newArray;
        }
        array = a;
        size = a.length;
    }


    @Override public boolean add(E object) {
        Object[] a = array;
        int s = size;
        if (s == a.length) {
        //s小于6的话，设置newArray的长度为s+12,不然的话newArray的长度是1.5倍的s
            Object[] newArray = new Object[s +
                    (s < (MIN_CAPACITY_INCREMENT / 2) ?
                     MIN_CAPACITY_INCREMENT : s >> 1)];
            System.arraycopy(a, 0, newArray, 0, s);
            array = a = newArray;
        }
        a[s] = object;
        size = s + 1;
        modCount++;
        return true;
    }

    //添加对象到指定的位置
    @Override public void add(int index, E object) {
        Object[] a = array;
        int s = size;
        if (index > s || index < 0) {
            throwIndexOutOfBoundsException(index, s);
        }
        //长度小于a的话，直接添加
        if (s < a.length) {
            System.arraycopy(a, index, a, index + 1, s - index);
        } else {
            //长度大于a的话，又是编程原来的1.5倍
            Object[] newArray = new Object[newCapacity(s)];
            //index位置前面的copy
            System.arraycopy(a, 0, newArray, 0, index);
            //index位置后面的copy
            System.arraycopy(a, index, newArray, index + 1, s - index);
            array = a = newArray;
        }
        //赋值当前的index值为object
        a[index] = object;
        size = s + 1;
        modCount++;
    }

    //判断当前的数组长度，若小于6.则增长值为12，不然的话增长值为把数组的长度*2
    private static int newCapacity(int currentCapacity) {
        int increment = (currentCapacity < (MIN_CAPACITY_INCREMENT / 2) ?
                MIN_CAPACITY_INCREMENT : currentCapacity >> 1);
        return currentCapacity + increment;
    }

    //添加一个集合到当前的集合中
    @Override public boolean addAll(Collection<? extends E> collection) {
        Object[] newPart = collection.toArray();
        int newPartSize = newPart.length;
        if (newPartSize == 0) {
            return false;
        }
        Object[] a = array;
        int s = size;
        int newSize = s + newPartSize; // If add overflows, arraycopy will fail
        if (newSize > a.length) {
            int newCapacity = newCapacity(newSize - 1);  // ~33% growth room
            Object[] newArray = new Object[newCapacity];
            System.arraycopy(a, 0, newArray, 0, s);
            array = a = newArray;
        }
        System.arraycopy(newPart, 0, a, s, newPartSize);
        size = newSize;
        modCount++;
        return true;
    }

    //添加集合到指定的位置
    @Override
    public boolean addAll(int index, Collection<? extends E> collection) {
        int s = size;
        if (index > s || index < 0) {
            throwIndexOutOfBoundsException(index, s);
        }
        Object[] newPart = collection.toArray();
        int newPartSize = newPart.length;
        if (newPartSize == 0) {
            return false;
        }
        Object[] a = array;
        int newSize = s + newPartSize; // If add overflows, arraycopy will fail
        if (newSize <= a.length) {
             System.arraycopy(a, index, a, index + newPartSize, s - index);
        } else {
            int newCapacity = newCapacity(newSize - 1);  // ~33% growth room
            Object[] newArray = new Object[newCapacity];
            System.arraycopy(a, 0, newArray, 0, index);
            System.arraycopy(a, index, newArray, index + newPartSize, s-index);
            array = a = newArray;
        }
        System.arraycopy(newPart, 0, a, index, newPartSize);
        size = newSize;
        modCount++;
        return true;
    }


    static IndexOutOfBoundsException throwIndexOutOfBoundsException(int index, int size) {
        throw new IndexOutOfBoundsException("Invalid index " + index + ", size is " + size);
    }

    //移除当前数组的内容，每个字都会编程null，然后设置size=0
    @Override public void clear() {
        if (size != 0) {
            Arrays.fill(array, 0, size, null);
            size = 0;
            modCount++;
        }
    }


    @Override public Object clone() {
        try {
            ArrayList<?> result = (ArrayList<?>) super.clone();
            result.array = array.clone();
            return result;
        } catch (CloneNotSupportedException e) {
           throw new AssertionError();
        }
    }

    //保证当前数组能够容纳
    public void ensureCapacity(int minimumCapacity) {
        Object[] a = array;
        if (a.length < minimumCapacity) {
            Object[] newArray = new Object[minimumCapacity];
            System.arraycopy(a, 0, newArray, 0, size);
            array = newArray;
            modCount++;
        }
    }

    @Override 
    public E get(int index) {
        if (index >= size) {
            throwIndexOutOfBoundsException(index, size);
        }
        return (E) array[index];
    }


    @Override public int size() {
        return size;
    }

    @Override public boolean isEmpty() {
        return size == 0;
    }


    @Override public boolean contains(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return true;
                }
            }
        }
        return false;
    }

    @Override public int indexOf(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return i;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return i;
                }
            }
        }
        return -1;
    }

    @Override public int lastIndexOf(Object object) {
        Object[] a = array;
        if (object != null) {
            for (int i = size - 1; i >= 0; i--) {
                if (object.equals(a[i])) {
                    return i;
                }
            }
        } else {
            for (int i = size - 1; i >= 0; i--) {
                if (a[i] == null) {
                    return i;
                }
            }
        }
        return -1;
    }


    @Override public E remove(int index) {
        Object[] a = array;
        int s = size;
        if (index >= s) {
            throwIndexOutOfBoundsException(index, s);
        }
        @SuppressWarnings("unchecked") E result = (E) a[index];
        System.arraycopy(a, index + 1, a, index, --s - index);
        a[s] = null;  // Prevent memory leak
        size = s;
        modCount++;
        return result;
    }

    @Override public boolean remove(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;
                    return true;
                }
            }
        }
        return false;
    }

    @Override protected void removeRange(int fromIndex, int toIndex) {
        if (fromIndex == toIndex) {
            return;
        }
        Object[] a = array;
        int s = size;
        if (fromIndex >= s) {
            throw new IndexOutOfBoundsException("fromIndex " + fromIndex
                    + " >= size " + size);
        }
        if (toIndex > s) {
            throw new IndexOutOfBoundsException("toIndex " + toIndex
                    + " > size " + size);
        }
        if (fromIndex > toIndex) {
            throw new IndexOutOfBoundsException("fromIndex " + fromIndex
                    + " > toIndex " + toIndex);
        }

        System.arraycopy(a, toIndex, a, fromIndex, s - toIndex);
        int rangeSize = toIndex - fromIndex;
        Arrays.fill(a, s - rangeSize, s, null);
        size = s - rangeSize;
        modCount++;
    }


    @Override public E set(int index, E object) {
        Object[] a = array;
        if (index >= size) {
            throwIndexOutOfBoundsException(index, size);
        }
        @SuppressWarnings("unchecked") E result = (E) a[index];
        a[index] = object;
        return result;
    }


    @Override public Object[] toArray() {
        int s = size;
        Object[] result = new Object[s];
        System.arraycopy(array, 0, result, 0, s);
        return result;
    }

    @Override public <T> T[] toArray(T[] contents) {
        int s = size;
        if (contents.length < s) {
            @SuppressWarnings("unchecked") T[] newArray
                = (T[]) Array.newInstance(contents.getClass().getComponentType(), s);
            contents = newArray;
        }
        System.arraycopy(this.array, 0, contents, 0, s);
        if (contents.length > s) {
            contents[s] = null;
        }
        return contents;
    }


    public void trimToSize() {
        int s = size;
        if (s == array.length) {
            return;
        }
        if (s == 0) {
            array = EmptyArray.OBJECT;
        } else {
            Object[] newArray = new Object[s];
            System.arraycopy(array, 0, newArray, 0, s);
            array = newArray;
        }
        modCount++;
    }

    @Override public Iterator<E> iterator() {
        return new ArrayListIterator();
    }

    private class ArrayListIterator implements Iterator<E> {
        /** Number of elements remaining in this iteration */
        private int remaining = size;

        /** Index of element that remove() would remove, or -1 if no such elt */
        private int removalIndex = -1;

        /** The expected modCount value */
        private int expectedModCount = modCount;

        public boolean hasNext() {
            return remaining != 0;
        }

        @SuppressWarnings("unchecked") public E next() {
            ArrayList<E> ourList = ArrayList.this;
            int rem = remaining;
            if (ourList.modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            if (rem == 0) {
                throw new NoSuchElementException();
            }
            remaining = rem - 1;
            return (E) ourList.array[removalIndex = ourList.size - rem];
        }

        public void remove() {
            Object[] a = array;
            int removalIdx = removalIndex;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            if (removalIdx < 0) {
                throw new IllegalStateException();
            }
            System.arraycopy(a, removalIdx + 1, a, removalIdx, remaining);
            a[--size] = null;  // Prevent memory leak
            removalIndex = -1;
            expectedModCount = ++modCount;
        }
    }

    @Override public int hashCode() {
        Object[] a = array;
        int hashCode = 1;
        for (int i = 0, s = size; i < s; i++) {
            Object e = a[i];
            hashCode = 31 * hashCode + (e == null ? 0 : e.hashCode());
        }
        return hashCode;
    }

    @Override public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (!(o instanceof List)) {
            return false;
        }
        List<?> that = (List<?>) o;
        int s = size;
        if (that.size() != s) {
            return false;
        }
        Object[] a = array;
        if (that instanceof RandomAccess) {
            for (int i = 0; i < s; i++) {
                Object eThis = a[i];
                Object ethat = that.get(i);
                if (eThis == null ? ethat != null : !eThis.equals(ethat)) {
                    return false;
                }
            }
        } else {  // Argument list is not random access; use its iterator
            Iterator<?> it = that.iterator();
            for (int i = 0; i < s; i++) {
                Object eThis = a[i];
                Object eThat = it.next();
                if (eThis == null ? eThat != null : !eThis.equals(eThat)) {
                    return false;
                }
            }
        }
        return true;
    }

    private static final long serialVersionUID = 8683452581122892189L;

    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        stream.writeInt(array.length);
        for (int i = 0; i < size; i++) {
            stream.writeObject(array[i]);
        }
    }

    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        stream.defaultReadObject();
        int cap = stream.readInt();
        if (cap < size) {
            throw new InvalidObjectException(
                    "Capacity: " + cap + " < size: " + size);
        }
        array = (cap == 0 ? EmptyArray.OBJECT : new Object[cap]);
        for (int i = 0; i < size; i++) {
            array[i] = stream.readObject();
        }
    }
 }

1、注意其三个不同的构造方法。无参构造方法构造的ArrayList的容量默认为0，带有Collection参数的构造方法，将Collection转化为数组赋给ArrayList的实现。

2、注意扩充容量的方法ensureCapacity。ArrayList在每次增加元素（可能是1个，也可能是一组）时，都要调用该方法来确保足够的容量。当容量不足以容纳当前的元素个数时，就设置新的容量为旧的容量的1.5倍，如果设置后的新容量还不够，则直接新容量设置为传入的参数（也就是所需的容量），而后用Arrays.copyof()方法将元素拷贝到新的数组（详见下面的第3点）。从中可以看出，当容量不够时，每次增加元素，都要将原来的元素拷贝到一个新的数组中，非常之耗时，也因此建议在事先能确定元素数量的情况下，才使用ArrayList，否则建议使用LinkedList。

3、ArrayList的实现中大量地调用了Arrays.copyof()和System.arraycopy()方法。
看System.arraycopy()方法，该方法被标记了native，调用了系统的C/C++代码，在JDK中是看不到的，但在openJDK中可以看到其源码。该函数实际上最终调用了C语言的memmove()函数，因此它可以保证同一个数组内元素的正确复制和移动，比一般的复制方法的实现效率要高很多，很适合用来批量处理数组。Java强烈推荐在复制大量数组元素时用该方法，以取得更高的效率。

4、注意ArrayList的两个转化为静态数组的toArray方法。
第一个，Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常，如果直接用向下转型的方法，将整个ArrayList集合转变为指定类型的Array数组，便会抛出该异常，而如果转化为Array数组时不向下转型，而是将每个元素向下转型，则不会抛出该异常，显然对数组中的元素一个个进行向下转型，效率不高，且不太方便。
第二个， T[] toArray(T[] a)方法。该方法可以直接将ArrayList转换得到的Array进行整体向下转型（转型其实是在该方法的源码中实现的），且从该方法的源码中可以看出，参数a的大小不足时，内部会调用Arrays.copyOf方法，该方法内部创建一个新的数组返回。

5、ArrayList基于数组实现，可以通过下标索引直接查找到指定位置的元素，因此查找效率高，但每次插入或删除元素，就要大量地移动元素，插入删除元素的效率低，建议使用LinkedList。

6、在查找给定元素索引值等的方法中，源码都将该元素的值分为null和不为null两种情况处理，ArrayList中允许元素为null。