ArrayList方法详解

Bertha 。 2022-06-01 06:19 230阅读 0赞

首先咱们看看ArrayList继承的类，实现的接口

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

> 1、继承AbstractList抽象类

AbstractList类继承了AbstractCollection抽象类，AbstractCollection类现实了Collection接口，Collection类继承Iterable接口

Iterable接口：迭代器，提供了一个数据迭代循环的接口forEach  
Collection接口：所有集合都会现实这个接口，由于继承Iterable，所有包含迭代器功能，还包含了add/remove/get等方法  
AbstractCollection抽象类:主要是对Collection接口方法进行了重写，包含toString  
AbstractList抽象类:主要是对AbstractCollection类方法重写，新增了RandomAccessSubList、SubList两个内部类。SubList:切分。RandomAccess是一个空的接口，它用来标识某个类是否支持 随机访问（随机访问，相对比“按顺序访问”）。一个支持随机访问的类明显可以使用更加高效的算法。

> 2、实现的接口

List接口：List接口继承了Collection接口，主要是对Collection接口方法重写，  
RandomAccess接口：标识类支持随机访问，随机访问的效率更快  
Cloneable接口：说明这个类可以被克隆的。Object类提供了clone方法  
Serializable接口：序列化后，可以实现list被io传输

> 3、ArrayList详解

ArrayList源码如下：

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { 
        private static final long serialVersionUID = 8683452581122892189L;
    
        // list默认初始化容量
        private static final int DEFAULT_CAPACITY = 10;
    
        // 用于空实例的共享空数组实例。
        private static final Object[] EMPTY_ELEMENTDATA = {};
    
        //用于默认大小的空实例的共享空数组实例。
        //我们将其与EMPTY_ELEMENTDATA区分开来，以了解在什么时候膨胀添加第一个元素。
        private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    
        /** * ArrayList的数据结构（数组） * * ArrayList里面的数据保存在这个 数组缓冲区。 * ArrayList的容量是这个数组缓冲区的长度。 * 任何带有elementData==DEFAULTCAPACITY_EMPTY_ELEMENTDATA的空ArrayList * 当添加第一个元素时，将被扩展到DEFAULT_CAPACITY。 */
        transient Object[] elementData; // non-private to simplify nested class access
    
        // ArrayList的大小(它包含的元素的数量)。
        private int size;
    
        // 带初始化容量的构造器
        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);
            }
        }
    
        // 无参数构造器，数组容量默认10
        public ArrayList() {
            this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
        }
    
        // 带初始化数据的构造器
        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;
            }
        }
    
        /** *去掉多余的null容量空间 * 而是ArrayList每次扩容时，容量大小为原来的1.5倍 * 这样就会出现当数据只有1000个的时候（size=1000），ArrayList却申请了1200空间容量 * trimToSize 的作用只是去掉预留元素位置，就是删除多余的200，改为只申请1000,内存紧张的时候会用到. */
        public void trimToSize() {
            modCount++;
            if (size < elementData.length) {
                elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size);
            }
        }
    
        // 增加了这个数组的容量，如果是。这是必要的，确保它至少能容纳元素的数量由最小容量参数指定。
        public void ensureCapacity(int minCapacity) {
            int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;
            if (minCapacity > minExpand) {
                // 扩容
                ensureExplicitCapacity(minCapacity);
            }
        }
    
        // 设置最小容量空间
        private void ensureCapacityInternal(int minCapacity) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
                minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
            }
            ensureExplicitCapacity(minCapacity);
        }
    
        // 扩容，扩展容量空间
        private void ensureExplicitCapacity(int minCapacity) {
            modCount++;
            if (minCapacity - elementData.length > 0)
                grow(minCapacity);
        }
    
        // 分配的数组的最大大小,一些vm在数组中保留一些头字,防止OutOfMemoryError
        private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
        // 增加容量以确保它至少能容纳最小容量参数指定的元素个数
        // 多线程做list.add的时候，容易导致数据丢失
        private void grow(int minCapacity) {
    
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);
            if (newCapacity - minCapacity < 0)
                newCapacity = minCapacity;
            if (newCapacity - MAX_ARRAY_SIZE > 0)
                newCapacity = hugeCapacity(minCapacity);
            // 返回一个新的数组对象
            elementData = Arrays.copyOf(elementData, newCapacity);
        }
    
        // list最大空间设置Integer.MAX_VALUE
        private static int hugeCapacity(int minCapacity) {
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
        }
    
        public int size() {
            return size;
        }
    
        public boolean isEmpty() {
            return size == 0;
        }
    
        public boolean contains(Object o) {
            return indexOf(o) >= 0;
        }
    
        // 返回第一个和o相等的对象的位置
        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;
        }
    
        // 返回最后一个和o相等的对象的位置
        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;
        }
    
        // 返回一个新对象（实例）
        public Object clone() {
            try {
                ArrayList<?> v = (ArrayList<?>) super.clone();
                // 只做了素组copy
                v.elementData = Arrays.copyOf(elementData, size); 
                v.modCount = 0;
                return v;
            } catch (CloneNotSupportedException e) {
                throw new InternalError(e);
            }
        }
    
        // 返回一个新的数组对象（实例）。将list转换为数组
        public Object[] toArray() {
            return Arrays.copyOf(elementData, size);
        }
    
        // 返回一个指定类型的新数组对象
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            if (a.length < size)
                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];
        }
    
        public E get(int index) {
            rangeCheck(index);
    
            return elementData(index);
        }
    
        public E set(int index, E element) {
            rangeCheck(index);
    
            E oldValue = elementData(index);
            elementData[index] = element;
            return oldValue;
        }
    
        public boolean add(E e) {
            ensureCapacityInternal(size + 1);  // Increments modCount!!
            elementData[size++] = e;
            return true;
        }
    
        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++;
        }
    
        // 移除第index个元素
        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;
        }
    
        // 移除第一个对象等于o的元素
        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;
        }
    
        // 快速清除数据
        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
        }
    
        // 从这个列表中删除所有元素。在这个调用返回之后，列表将是空的
        public void clear() {
            modCount++;
    
            for (int i = 0; i < size; i++)
                elementData[i] = null;
    
            size = 0;
        }
    
        // 添加一个集合
        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;
        }
    
        // 在某个位置添加一个集合
        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;
        }
    
        // 范围删除
        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));
        }
    
    
        private void rangeCheckForAdd(int index) {
            if (index > size || index < 0)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
    
        private String outOfBoundsMsg(int index) {
            return "Index: "+index+", Size: "+size;
        }
    
        // 移除所有c包含的元素
        public boolean removeAll(Collection<?> c) {
            Objects.requireNonNull(c);
            return batchRemove(c, false);
        }
    
        // 保留所有c包含的元素
        public boolean retainAll(Collection<?> c) {
            Objects.requireNonNull(c);
            return batchRemove(c, true);
        }
    
        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;
        }
    
        // 将数组列表的状态保存到一个流中(即序列化)。
        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 behavioural 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();
            }
        }
    
        // 从一个流中重新组合了ArrayList的作用(也就是说，反序列化)。
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            elementData = EMPTY_ELEMENTDATA;
    
            // Read in size, and any hidden stuff
            s.defaultReadObject();
    
            // Read in capacity
            s.readInt(); // ignored
    
            if (size > 0) {
                // be like clone(), allocate array based upon size not capacity
                ensureCapacityInternal(size);
    
                Object[] a = elementData;
                // Read in all elements in the proper order.
                for (int i=0; i<size; i++) {
                    a[i] = s.readObject();
                }
            }
        }
    
        // 返回一个迭代器
        public ListIterator<E> listIterator(int index) {
            if (index < 0 || index > size)
                throw new IndexOutOfBoundsException("Index: "+index);
            return new ListItr(index);
        }
    
        public ListIterator<E> listIterator() {
            return new ListItr(0);
        }
    
        public Iterator<E> iterator() {
            return new Itr();
        }
        public ListIterator<E> listIterator(final int index) {}
    
        // 分割list，返回返回内的list
        public List<E> subList(int fromIndex, int toIndex) {
             subListRangeCheck(fromIndex, toIndex, size);
             return new SubList(this, offset, fromIndex, toIndex);
        }
       // for循环
       public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final int expectedModCount = modCount;
            @SuppressWarnings("unchecked")
            final E[] elementData = (E[]) this.elementData;
            final int size = this.size;
            for (int i=0; modCount == expectedModCount && i < size; i++) {
                action.accept(elementData[i]);
            }
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    
        // 返回一个迭代器
        @Override
        public Spliterator<E> spliterator() {
            return new ArrayListSpliterator<>(this, 0, -1, 0);
        }
    
        // 删除满足给定的集合的所有元素
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            Objects.requireNonNull(filter);
            int removeCount = 0;
            final BitSet removeSet = new BitSet(size);
            final int expectedModCount = modCount;
            final int size = this.size;
            for (int i=0; modCount == expectedModCount && i < size; i++) {
                @SuppressWarnings("unchecked")
                final E element = (E) elementData[i];
                if (filter.test(element)) {
                    removeSet.set(i);
                    removeCount++;
                }
            }
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
    
            // shift surviving elements left over the spaces left by removed elements
            final boolean anyToRemove = removeCount > 0;
            if (anyToRemove) {
                final int newSize = size - removeCount;
                for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                    i = removeSet.nextClearBit(i);
                    elementData[j] = elementData[i];
                }
                for (int k=newSize; k < size; k++) {
                    elementData[k] = null;  // Let gc do its work
                }
                this.size = newSize;
                if (modCount != expectedModCount) {
                    throw new ConcurrentModificationException();
                }
                modCount++;
            }
    
            return anyToRemove;
        }
    
        // 根据规则替换所有元素
        @Override
        @SuppressWarnings("unchecked")
        public void replaceAll(UnaryOperator<E> operator) {
            Objects.requireNonNull(operator);
            final int expectedModCount = modCount;
            final int size = this.size;
            for (int i=0; modCount == expectedModCount && i < size; i++) {
                elementData[i] = operator.apply((E) elementData[i]);
            }
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
        }
    
        // 根据指定规则排序
        @Override
        @SuppressWarnings("unchecked")
        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++;
        }
    
    }

> 总结  
> ArrayList的方法主要是对单个元素，或者对个元素进行增删查操作，内部实现主要是通过Arrays.copyOf返回一个新的数组  
> ArrayList默认容量初始化容量10，每次扩容为原来的1.5倍（原来10，扩容后就是15）  
> ArrayList数据结构是数组，因此可以转换为数据结构  
> ArrayList线程不安全，为什么线程不安全，[请点我][Link 1]

[Link 1]: https://blog.csdn.net/zc375039901/article/details/79109930