ArrayList LinkedList Vector

ArrayList与LinkedList

ArrayList 通过数组实现，LinkedList 通过双向链表实现

get/set：ArrayList直接通过下标在数组中取，LinkedList需要从头/尾遍历到index

add/remove：ArrayList需要将index之后的元素都复制一遍，如果需要扩容需要将原数组所有元素复制到新数组

　　　　　　（如果一个包含大量元素的ArrayList对象，数组扩容1.5倍，那么最终将有很大的空间会被浪费掉，调用trimToSize方法去掉浪费掉的空间）

　　　　　　LinkedList需要从头/尾遍历到该元素并操作前后节点

get

    /**
     * ArrayList get
     * 直接通过下标在数组中取
     */
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }
    E elementData(int index) {
        return (E) elementData[index];
    }
    
    /**
     * LinkedList get
     * 从头/尾遍历到index
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    Node<E> node(int index) {
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

set

    /**
     * ArrayList set
     * 同get
     */
    public E set(int index, E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    
    /**
     * LinkedList
     * 同get
     */
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

add

    /**
     * ArrayList数组扩容
     * 1.需求容量 <= elementData.length, 不需要扩容
     * 2.newCapacity = 1.5*oldCapacity 或 需求容量
     * 3.扩容之后, 原数组元素复制到新数组
     */
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    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);
    }
    
    /**
     * ArrayList add(e)
     * 1.数组扩容(需要的话)
     * 2.添加e
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }
    
    /**
     * ArrayList add(index, e)
     * 1.数组可能要扩容
     * 2.index之后的元素复制一遍到index+1之后
     * 3.elementData[index] = e;
     */
    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++;
    }
    
    
    /**
     * LinkedList add(e)
     * 直接与最后一个元素互相添加节点
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }    
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }
    
    /**
     * LinkedList add(index, e)
     * 1.从头/尾遍历到index
     * 2.互相添加节点
     */
    public void add(int index, E element) {
        checkPositionIndex(index);
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

remove

    /**
     * ArrayList remove(index)
     * 1.index+1之后的元素复制到index之后
     * 2.elementData[--size] = null
     */
    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;
    }
    
    /**
     * ArrayList remove(Object)
     * 1.遍历数组找到Object
     * 2.index+1之后的元素复制到index之后
     * 3.elementData[--size] = null
     */
    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;
    }
    
    /**
     * LinkedList remove(index)
     * 1.从头/尾遍历到index
     * 2.将index的前后节点相关联
     */
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }
    
    /**
     * LinkedList remove(Object)
     * 1.从头/尾遍历到object
     * 2.将object的前后节点相关联
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

ArrayList与Vector

实现原理一样

主要区别：

　　①线程安全：Vector增删改查的方法都有synchronized，线程安全，但性能较ArrayList差

　　②扩容：ArrayList每次扩1.5倍(int newCapacity = oldCapacity + (oldCapacity >> 1))

　　　　　　Vector默认扩成原来的2倍(int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity))

参考资料：

ArrayList vs LinkedList vs Vector 区别中进行了实际的性能测试

Java 集合系列08之 List总结(LinkedList, ArrayList等使用场景和性能分析)