java.util.ArrayList源码解析

ArrayList

1. 介绍

(基于jdk11)
size、isEmpty、get、set、iterator、listIterator 常数时间

add O(n)

其他的操作也在一个线性时间

2. 属性

2.1 DEFAULT_CAPACITY

默认容量10

    private static final int DEFAULT_CAPACITY = 10;

2.2 EMPTY_ELEMENTDATA

用在空实例上，共享空数组实例

    private static final Object[] EMPTY_ELEMENTDATA = {};

2.3 DEFAULTCAPACITY_EMPTY_ELEMENTDATA

用于默认尺寸大小的空实例，共享空数组实例，和2.2不同的地方是怎样取扩容（主要看下面代码）

	private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

2.4 elementData

ArrayList存储元素的数组，集合的容量就是数组的length，当添加第一个元素的时候任何empty ArrayList，elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA将会被扩容至DEFAULT_CAPACITY大小

transient Object[] elementData; 

2.5 size

ArrayList的长度，也就是包含元素的数量

private int size;

2.6 MAX_ARRAY_SIZE

 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

3. 方法

3.1 构造方法

    //根据指定容量创建集合
	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);
        }
    }
	//创建一个空集合
	public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
	
	//创建一个包含指定元素的集合
	public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // defend against c.toArray (incorrectly) not returning Object[]
            // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

3.2 trimToSize

使ArrayList的内部数组elementData大小与size(真实存储的元素数量)相等

    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

3.3 ensureCapacity

判断是否扩容

    public void ensureCapacity(int minCapacity) {
        if (minCapacity > elementData.length
            && !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
                 && minCapacity <= DEFAULT_CAPACITY)) {
            modCount++;
            grow(minCapacity);
        }
    }

3.4 grow

扩容，每次扩容MAX(oldCapacity + (oldCapacity >> 1)，minCapacity)

	private Object[] grow(int minCapacity) {
        return elementData = Arrays.copyOf(elementData,
                                           newCapacity(minCapacity));
    }
 	private Object[] grow() {
        return grow(size + 1);
    }
    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity <= 0) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                return Math.max(DEFAULT_CAPACITY, minCapacity);
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            return minCapacity;
        }
        return (newCapacity - MAX_ARRAY_SIZE <= 0)
            ? newCapacity
            : hugeCapacity(minCapacity);
    }

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

3.5 size

ArrayList元素数量

 	public int size() {
        return size;
    }

3.6 isEmpty

集合是否为空

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

3.7 contains

​ 是否包含某个元素，里面调用的indexOf，判断索引值是否>=0

	public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

3.8 indexOf

​ 从位置0扫描整个集合，查询是否包含某个元素，找不到返回-1

	public int indexOf(Object o) {
        return indexOfRange(o, 0, size);
    }
    int indexOfRange(Object o, int start, int end) {
        Object[] es = elementData;
        if (o == null) {
            for (int i = start; i < end; i++) {
                if (es[i] == null) {
                    return i;
                }
            }
        } else {
            for (int i = start; i < end; i++) {
                if (o.equals(es[i])) {
                    return i;
                }
            }
        }
        return -1;
    }

3.9 lastIndexOf

​ 倒序查找某个元素位置，找不到返回-1

	public int lastIndexOf(Object o) {
        return lastIndexOfRange(o, 0, size);
    }

    int lastIndexOfRange(Object o, int start, int end) {
        Object[] es = elementData;
        if (o == null) {
            for (int i = end - 1; i >= start; i--) {
                if (es[i] == null) {
                    return i;
                }
            }
        } else {
            for (int i = end - 1; i >= start; i--) {
                if (o.equals(es[i])) {
                    return i;
                }
            }
        }
        return -1;
    }

3.10 clone

浅克隆

	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);
        }
    }

3.11 toArray

​ 返回一个数组，虽然创造了一个新的数组，但是数组里面的对象是引用传递，不要被官方这句话迷惑The returned array will be "safe" in that no references to it aremaintained by this list

	public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    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;
    }

3.12 get

​ 根据索引，返回元素

    public E get(int index) {
        Objects.checkIndex(index, size);
        return elementData(index);
    }
    E elementData(int index) {
        return (E) elementData[index];
    }

3.13 elementAt

    static <E> E elementAt(Object[] es, int index) {
        return (E) es[index];
    }

3.14 set

​ 替换某个索引位置的元素

    public E set(int index, E element) {
        Objects.checkIndex(index, size);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

3.15 add

​ 添加元素 /在指定位置添加元素

    public boolean add(E e) {
        modCount++;
        add(e, elementData, size);
        return true;
    }
    private void add(E e, Object[] elementData, int s) {
        if (s == elementData.length)
            elementData = grow();
        elementData[s] = e;
        size = s + 1;
    }

    public void add(int index, E element) {
        rangeCheckForAdd(index);
        modCount++;
        final int s;
        Object[] elementData;
        if ((s = size) == (elementData = this.elementData).length)
            elementData = grow();
        System.arraycopy(elementData, index,
                         elementData, index + 1,
                         s - index);
        elementData[index] = element;
        size = s + 1;
    }
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

3.16 remove

​ 移除指定索引元素

	public E remove(int index) {
        Objects.checkIndex(index, size);
        final Object[] es = elementData;

        @SuppressWarnings("unchecked") E oldValue = (E) es[index];
        fastRemove(es, index);

        return oldValue;
    }
	private void fastRemove(Object[] es, int i) {
        modCount++;
        final int newSize;
        if ((newSize = size - 1) > i)
            System.arraycopy(es, i + 1, es, i, newSize - i);
        es[size = newSize] = null;
    }

​ 移除指定元素

    public boolean remove(Object o) {
        final Object[] es = elementData;
        final int size = this.size;
        int i = 0;
        found: {
            if (o == null) {
                for (; i < size; i++)
                    if (es[i] == null)
                        break found;
            } else {
                for (; i < size; i++)
                    if (o.equals(es[i]))
                        break found;
            }
            return false;
        }
        fastRemove(es, i);
        return true;
    }

3.17 equals

根据是否是ArrayList，一个用迭代器，一个用下标

	public boolean equals(Object o) {
        if (o == this) {
            return true;
        }

        if (!(o instanceof List)) {
            return false;
        }

        final int expectedModCount = modCount;
        // ArrayList can be subclassed and given arbitrary behavior, but we can
        // still deal with the common case where o is ArrayList precisely
        boolean equal = (o.getClass() == ArrayList.class)
            ? equalsArrayList((ArrayList<?>) o)
            : equalsRange((List<?>) o, 0, size);

        checkForComodification(expectedModCount);
        return equal;
    }

    boolean equalsRange(List<?> other, int from, int to) {
        final Object[] es = elementData;
        if (to > es.length) {
            throw new ConcurrentModificationException();
        }
        var oit = other.iterator();
        for (; from < to; from++) {
            if (!oit.hasNext() || !Objects.equals(es[from], oit.next())) {
                return false;
            }
        }
        return !oit.hasNext();
    }

    private boolean equalsArrayList(ArrayList<?> other) {
        final int otherModCount = other.modCount;
        final int s = size;
        boolean equal;
        if (equal = (s == other.size)) {
            final Object[] otherEs = other.elementData;
            final Object[] es = elementData;
            if (s > es.length || s > otherEs.length) {
                throw new ConcurrentModificationException();
            }
            for (int i = 0; i < s; i++) {
                if (!Objects.equals(es[i], otherEs[i])) {
                    equal = false;
                    break;
                }
            }
        }
        other.checkForComodification(otherModCount);
        return equal;
    }

    private void checkForComodification(final int expectedModCount) {
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

3.18 hashCode

​ 计算hashCode，也是31的因子

	public int hashCode() {
        int expectedModCount = modCount;
        int hash = hashCodeRange(0, size);
        checkForComodification(expectedModCount);
        return hash;
    }

    int hashCodeRange(int from, int to) {
        final Object[] es = elementData;
        if (to > es.length) {
            throw new ConcurrentModificationException();
        }
        int hashCode = 1;
        for (int i = from; i < to; i++) {
            Object e = es[i];
            hashCode = 31 * hashCode + (e == null ? 0 : e.hashCode());
        }
        return hashCode;
    }

3.19 clear

​ 清空元素

    public void clear() {
        modCount++;
        final Object[] es = elementData;
        for (int to = size, i = size = 0; i < to; i++)
            es[i] = null;
    }

3.20 addAll

​ 添加集合内所有元素

    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        modCount++;
        int numNew = a.length;
        if (numNew == 0)
            return false;
        Object[] elementData;
        final int s;
        if (numNew > (elementData = this.elementData).length - (s = size))
            elementData = grow(s + numNew);
        System.arraycopy(a, 0, elementData, s, numNew);
        size = s + numNew;
        return true;
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        modCount++;
        int numNew = a.length;
        if (numNew == 0)
            return false;
        Object[] elementData;
        final int s;
        if (numNew > (elementData = this.elementData).length - (s = size))
            elementData = grow(s + numNew);

        int numMoved = s - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index,
                             elementData, index + numNew,
                             numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        size = s + numNew;
        return true;
    }

3.21 removeRange

​ 移除某个范围内元素

  protected void removeRange(int fromIndex, int toIndex) {
        if (fromIndex > toIndex) {
            throw new IndexOutOfBoundsException(
                    outOfBoundsMsg(fromIndex, toIndex));
        }
        modCount++;
        shiftTailOverGap(elementData, fromIndex, toIndex);
    }

    /** Erases the gap from lo to hi, by sliding down following elements. */
    private void shiftTailOverGap(Object[] es, int lo, int hi) {
        System.arraycopy(es, hi, es, lo, size - hi);
        for (int to = size, i = (size -= hi - lo); i < to; i++)
            es[i] = null;
    }

3.22 outOfBoundsMsg

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * A version used in checking (fromIndex > toIndex) condition
     */
    private static String outOfBoundsMsg(int fromIndex, int toIndex) {
        return "From Index: " + fromIndex + " > To Index: " + toIndex;
    }

3.23 removeAll

​ 移除所有指定集合内元素

 	public boolean removeAll(Collection<?> c) {
        return batchRemove(c, false, 0, size);
    }

3.24 retainAll

​ 保留所有指定集合内元素

	public boolean retainAll(Collection<?> c) {
        return batchRemove(c, true, 0, size);
    }

3.25 batchRemove

boolean batchRemove(Collection c, boolean complement,
                        final int from, final int end) {
        Objects.requireNonNull(c);
        final Object[] es = elementData;
        int r;
        // Optimize for initial run of survivors
        for (r = from;; r++) {
            if (r == end)
                return false;
            if (c.contains(es[r]) != complement)
                break;
        }
        int w = r++;
        try {
            for (Object e; r < end; r++)
                if (c.contains(e = es[r]) == complement)
                    es[w++] = e;
        } catch (Throwable ex) {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            System.arraycopy(es, r, es, w, end - r);
            w += end - r;
            throw ex;
        } finally {
            modCount += end - w;
            shiftTailOverGap(es, w, end);
        }
        return true;
    }

3.26 writeObject

​ 把当前ArrayList写入到流中，序列化

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioral compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

3.27 readObject

​ 反序列化

	private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // like clone(), allocate array based upon size not capacity
            SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size);
            Object[] elements = new Object[size];

            // Read in all elements in the proper order.
            for (int i = 0; i < size; i++) {
                elements[i] = s.readObject();
            }

            elementData = elements;
        } else if (size == 0) {
            elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new java.io.InvalidObjectException("Invalid size: " + size);
        }
    }

3.28 listIterator

​ 返回listIterator迭代器

    public ListIterator<E> listIterator(int index) {
        rangeCheckForAdd(index);
        return new ListItr(index);
    }
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor - 1;
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

3.29 iterator

​ 返回iterator迭代器

	public Iterator<E> iterator() {
        return new Itr();
    }
     private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        // prevent creating a synthetic constructor
        Itr() {}

        public boolean hasNext() {
            return cursor != size;
        }

        @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];
        }

        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();
            }
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i < size) {
                final Object[] es = elementData;
                if (i >= es.length)
                    throw new ConcurrentModificationException();
                for (; i < size && modCount == expectedModCount; i++)
                    action.accept(elementAt(es, i));
                // update once at end to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

3.30 subList

​ 返回指定索引内的集合，其实还是当前集合，只不过里面的索引都用了偏移量

    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList<>(this, fromIndex, toIndex);
    }

3.31 forEach

​ 对集合内每一个值做处理

    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        final Object[] es = elementData;
        final int size = this.size;
        for (int i = 0; modCount == expectedModCount && i < size; i++)
            action.accept(elementAt(es, i));
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }

3.32 spliterator

​ 返回可分割迭代器

	public Spliterator<E> spliterator() {
        return new ArrayListSpliterator(0, -1, 0);
    }

    /** Index-based split-by-two, lazily initialized Spliterator */
    final class ArrayListSpliterator implements Spliterator<E> {
    
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set

        /** Creates new spliterator covering the given range. */
        ArrayListSpliterator(int origin, int fence, int expectedModCount) {
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        private int getFence() { // initialize fence to size on first use
            int hi; // (a specialized variant appears in method forEach)
            if ((hi = fence) < 0) {
                expectedModCount = modCount;
                hi = fence = size;
            }
            return hi;
        }

        public ArrayListSpliterator trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator(lo, index = mid, expectedModCount);
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            if (action == null)
                throw new NullPointerException();
            int hi = getFence(), i = index;
            if (i < hi) {
                index = i + 1;
                @SuppressWarnings("unchecked") E e = (E)elementData[i];
                action.accept(e);
                if (modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi, mc; // hoist accesses and checks from loop
            Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((a = elementData) != null) {
                if ((hi = fence) < 0) {
                    mc = modCount;
                    hi = size;
                }
                else
                    mc = expectedModCount;
                if ((i = index) >= 0 && (index = hi) <= a.length) {
                    for (; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) a[i];
                        action.accept(e);
                    }
                    if (modCount == mc)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

        public long estimateSize() {
            return getFence() - index;
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

3.33 removeIf

​ 移除符合条件的元素

	public boolean removeIf(Predicate<? super E> filter) {
        return removeIf(filter, 0, size);
    }

    /**
     * Removes all elements satisfying the given predicate, from index
     * i (inclusive) to index end (exclusive).
     */
    boolean removeIf(Predicate<? super E> filter, int i, final int end) {
        Objects.requireNonNull(filter);
        int expectedModCount = modCount;
        final Object[] es = elementData;
        // Optimize for initial run of survivors
        for (; i < end && !filter.test(elementAt(es, i)); i++)
            ;
        // Tolerate predicates that reentrantly access the collection for
        // read (but writers still get CME), so traverse once to find
        // elements to delete, a second pass to physically expunge.
        if (i < end) {
            final int beg = i;
            final long[] deathRow = nBits(end - beg);
            deathRow[0] = 1L;   // set bit 0
            for (i = beg + 1; i < end; i++)
                if (filter.test(elementAt(es, i)))
                    setBit(deathRow, i - beg);
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            modCount++;
            int w = beg;
            for (i = beg; i < end; i++)
                if (isClear(deathRow, i - beg))
                    es[w++] = es[i];
            shiftTailOverGap(es, w, end);
            return true;
        } else {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return false;
        }
    }
	private static long[] nBits(int n) {
        return new long[((n - 1) >> 6) + 1];
    }
    private static void setBit(long[] bits, int i) {
        bits[i >> 6] |= 1L << i;
    }
    private static boolean isClear(long[] bits, int i) {
        return (bits[i >> 6] & (1L << i)) == 0;
    }

3.34 replaceAll

​ 替换所有元素

    public void replaceAll(UnaryOperator<E> operator) {
        replaceAllRange(operator, 0, size);
        modCount++;
    }

    private void replaceAllRange(UnaryOperator<E> operator, int i, int end) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final Object[] es = elementData;
        for (; modCount == expectedModCount && i < end; i++)
            es[i] = operator.apply(elementAt(es, i));
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }

3.35 sort

​ 排序

    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        modCount++;
    }

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