ArrayList 源码解析

// 无参构造方法，初始容量10public ArrayList() {    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}
// 初始容量为设定值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);    }}
// 通过集合初始化一个ArrayList// c不能为null，否则报 NPEpublic ArrayList(Collection<? extends E> c) {    Object[] a = c.toArray();    if ((size = a.length) != 0) {        if (c.getClass() == ArrayList.class) {            elementData = a;        } else {            elementData = Arrays.copyOf(a, size, Object[].class);        }    } else {        // replace with empty array.        elementData = EMPTY_ELEMENTDATA;    }}

// java.util.Arrayspublic static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {    @SuppressWarnings("unchecked")    T[] copy = ((Object)newType == (Object)Object[].class)        ? (T[]) new Object[newLength]        : (T[]) Array.newInstance(newType.getComponentType(), newLength);    //     System.arraycopy(original, 0, copy, 0,                     Math.min(original.length, newLength));    return copy;}

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 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;}

private Object[] grow() {    return grow(size + 1);}
private Object[] grow(int minCapacity) {    // 拷贝元素到新的数组中，底层调用的是 System.arraycopy方法    return elementData = Arrays.copyOf(elementData,                                       newCapacity(minCapacity));}

// 计算新的容量： 1.5倍的上一次的容量private int newCapacity(int minCapacity) {    // overflow-conscious code    int oldCapacity = elementData.length;    int newCapacity = oldCapacity + (oldCapacity >> 1);    if (newCapacity - minCapacity <= 0) {        // 无参构造器初始化        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);}

@Testvoid debugArrayList() {    List<String> arrayList =  new ArrayList<>();    // 第一次扩容 size=10    arrayList.add("1");
    for (int i = 2; i <= 10; i++) {        arrayList.add(""+i);    }    // 第二次扩容 size=15    arrayList.add("11");
    for (int i = 12; i <= 15; i++) {        arrayList.add(""+i);    }    // 第二次扩容 size=22    arrayList.add("16");}

@Testvoid debugArrayListWithInitialCap() {    List<String> arrayList =  new ArrayList<>(20);    for (int i = 1; i <= 20; i++) {        arrayList.add("" + i);    }    // 第一次扩容， int newCapacity = oldCapacity + (oldCapacity >> 1);    // newCapacity =20+10=30    arrayList.add("21");
    for (int i = 22; i <= 30; i++) {        arrayList.add(""+i);    }    // 第二次扩容 size=30+15=45    arrayList.add("31");}

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