简单了解ArrayList源码

ArrayList
1.创建ArrayList list1 = new ArrayList();
默认创建了一个空的数组

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }

2.执行 list1.add(1);

public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // 确保数组容量，如果是空数组，那么就设置默认长度或是设置设置为数据的长度。如果非空数组，长度不够就进行扩容。
        elementData[size++] = e; //新添加的对象进行赋值
        return true;
}
 private static final int DEFAULT_CAPACITY = 10; //默认长度为10

private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

    ensureExplicitCapacity(minCapacity);//进行分配数组长度
 }  

  private void ensureExplicitCapacity(int minCapacity) {
    modCount++;  
    if (minCapacity - elementData.length > 0) //需要的长度大于当前数组的长度，就进行扩容
        grow(minCapacity);
    }

private void grow(int minCapacity) { 
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);  //先扩容为当前数组的一半
    if (newCapacity - minCapacity < 0) //如果还小于需要的数组长度，那么新数组的长度等于需要的长度
        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);//进行数组的复制，创建一个新数组将旧数组的值复制到新数组中。
}

3.执行list1.addAll(list2);

public boolean addAll(Collection<? extends E> c) {
    Object[] a = c.toArray();  //上来马上将类型转化成数组，然后其他的步骤跟add()基本相同
    int numNew = a.length;
    ensureCapacityInternal(size + numNew);  // Increments modCount
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
}

4.list1.add(index,element) 指定的位置上，添加一个元素，这时候集合的长度会添加

public void add(int index, E element) {
    if (index > size || index < 0)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    //下面进行数组的指定位置的复制，将elementData中的index位置开始的数据复制到当前数据的
       index+1位置开始逐条复制。并最后将element赋值到index位置上
    System.arraycopy(elementData, index, elementData, index + 1,
            size - index);    
    elementData[index] = element;
    size++;
}

5. list1.set(2,10); 这个方法是将索引为2的位置上的数据进行赋值，并返回被替换的数据。

     public E set(int index, E element) {
        if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

    E oldValue = (E) elementData[index];
    elementData[index] = element;
    return oldValue;
}

6. list1.remove((Integer) 11);

//无非是进行遍历，然后删除数据
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;
}

private void fastRemove(int index) {
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
//还是将数组中从index+1处的数据开始复制到索引在index处。
        System.arraycopy(elementData, index + 1, elementData, index,
                numMoved); 
    elementData[--size] = null; //将最后一条数据清空
}

7.list1.indexOf(1) //进行遍历，找到元素，并返回索引

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;

8.list1.contains(-1)

public boolean contains(Object o) {
    return indexOf(o) >= 0;  //如果不为-1，证明找到数据，返回true
}

9.list1.remove(0)和直接remove对象的方法的区别，就只不用先找到数据的索引。

public E remove(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

    modCount++;
    E oldValue = (E) elementData[index];

    int numMoved = size - index - 1;
    if (numMoved > 0)
//删除某个索引的元素，仍然用的数组复制的方法
        System.arraycopy(elementData, index + 1, elementData, index,
                numMoved);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

10.list1.get(0)

public E get(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
//直接取数组中的数据
    return (E) elementData[index];

11.list1.toArray(); 其实就是数组的复制

  public Object[] toArray() {
    return Arrays.copyOf(elementData, size);
}
  public static <T> T[] copyOf(T[] original, int newLength) {
    return (T[]) copyOf(original, newLength, original.getClass());
}

   public 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;
}

执行代码Iterator iterator = list1.iterator();

    while (iterator.hasNext()){
    Log.e("TAG", "iterator==="+iterator.next().toString());
    iterator.remove();
}
在ArrayList中发现源码，iteraor就是一个对象，里面封装了索引。
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;

public boolean hasNext() {
    return cursor < limit;   //判断当前索引是否小于集合长度
}

@SuppressWarnings("unchecked")
public E next() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
    int i = cursor;
    if (i >= limit)
        throw new NoSuchElementException();
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
        throw new ConcurrentModificationException();
    cursor = i + 1;
    return (E) elementData[lastRet = i];//在这取出数组中的数据，并且为lastRet赋值
}

public void remove() {
    if (lastRet < 0)
        throw new IllegalStateException();
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();

    try {
        ArrayList.this.remove(lastRet);  //根据在next()中的被赋值的lastRet,调用ArrayList中的根据索引删除的remove()方法，进行删除。因此如果调用remove() 必须先调用next()
        cursor = lastRet;
        lastRet = -1;
        expectedModCount = modCount;
        limit--;
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}

@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
    Objects.requireNonNull(consumer);
    final int size = ArrayList.this.size;
    int i = cursor;
    if (i >= size) {
        return;
    }
    final Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length) {
        throw new ConcurrentModificationException();
    }
    while (i != size && modCount == expectedModCount) {
        consumer.accept((E) elementData[i++]);
    }
    // update once at end of iteration to reduce heap write traffic
    cursor = i;
    lastRet = i - 1;

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

13.List integers = list1.subList(1, 3);

    for(Integer i:integers) {
        Log.e("TAG", "integers==="+i);
    }

 public List<E> subList(int fromIndex, int toIndex) {
    //检查是否越界
    subListRangeCheck(fromIndex, toIndex, size);
     //竟然创建了一个新的对象SubList,这个SubList相当于一个和ArrayList相同的类，里面有和ArrayList相同的方法  
   return new SubList(this, 0, fromIndex, toIndex);
}

//虽然我用了增强for循环，但实质还是调用了get（）方法，但是发现，和ArrayList的区别就是，增加了偏移量，仍然维护了由于的数组
这里写图片描述

简单了解ArrayList源码

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