1 public class ArrayList<E> extends AbstractList<E>  2         implements List<E>, RandomAccess, Cloneable, java.io.Serializable  3 {  4     private static final long serialVersionUID = 8683452581122892189L;  5   6     //设置arrayList默认容量  7     private static final int DEFAULT_CAPACITY = 10;  8   9     //空数组，当调用无参数构造函数的时候默认给个空数组 10     private static final Object[] EMPTY_ELEMENTDATA =http://www.mamicode.com/ {}; 11  12     //这才是真正保存数据的数组 13     private transient Object[] elementData; 14  15     //arrayList的实际元素数量 16     private int size; 17  18     //构造方法传入默认的capacity 设置默认数组大小 19     public ArrayList(int initialCapacity) { 20         super(); 21         if (initialCapacity < 0) 22             throw new IllegalArgumentException("Illegal Capacity: "+ 23                                                initialCapacity); 24         this.elementData = http://www.mamicode.com/new Object[initialCapacity]; 25     } 26  27     //无参数构造方法默认为空数组 28     public ArrayList() { 29         super(); 30         this.elementData =http://www.mamicode.com/ EMPTY_ELEMENTDATA; 31     } 32  33     //构造方法传入一个Collection， 则将Collection里面的值copy到arrayList 34     public ArrayList(Collection<? extends E> c) { 35         elementData =http://www.mamicode.com/ c.toArray(); 36         size = elementData.length; 37         // c.toArray might (incorrectly) not return Object[] (see 6260652) 38         if (elementData.getClass() != Object[].class) 39             elementData = http://www.mamicode.com/Arrays.copyOf(elementData, size, Object[].class); 40     } 41      42     //下面主要看看ArrayList 是如何将数组进行动态扩充实现add 和 remove 43      44  45     public boolean add(E e) { 46         ensureCapacityInternal(size + 1);  // Increments modCount!! 47         elementData[size++] = e; 48         return true; 49     } 50  51      52     public void add(int index, E element) { 53         rangeCheckForAdd(index); 54  55         ensureCapacityInternal(size + 1);  // Increments modCount!! 56         System.arraycopy(elementData, index, elementData, index + 1, 57                          size - index); 58         elementData[index] = element; 59         size++; 60     } 61      62     private void ensureCapacityInternal(int minCapacity) { 63         if (elementData =http://www.mamicode.com/= EMPTY_ELEMENTDATA) { 64             minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); 65         } 66  67         ensureExplicitCapacity(minCapacity); 68     } 69      70     private void ensureExplicitCapacity(int minCapacity) { 71         modCount++; 72  73         //超出了数组可容纳的长度，需要进行动态扩展 74         if (minCapacity - elementData.length > 0) 75             grow(minCapacity); 76     } 77      78     //这才是动态扩展的精髓，看到这个方法，ArrayList瞬间被打回原形 79     private void grow(int minCapacity) { 80         int oldCapacity = elementData.length; 81         //设置新数组的容量扩展为原来数组的1.5倍 82         int newCapacity = oldCapacity + (oldCapacity >> 1); 83         //再判断一下新数组的容量够不够，够了就直接使用这个长度创建新数组，  84         //不够就将数组长度设置为需要的长度 85         if (newCapacity - minCapacity < 0) 86             newCapacity = minCapacity; 87         //判断有没超过最大限制 88         if (newCapacity - MAX_ARRAY_SIZE > 0) 89             newCapacity = hugeCapacity(minCapacity); 90         //将原来数组的值copy新数组中去， ArrayList的引用指向新数组 91         //这儿会新创建数组，如果数据量很大，重复的创建的数组，那么还是会影响效率， 92         //因此鼓励在合适的时候通过构造方法指定默认的capaticy大小 93         elementData =http://www.mamicode.com/ Arrays.copyOf(elementData, newCapacity); 94     } 95      96     private static int hugeCapacity(int minCapacity) { 97         if (minCapacity < 0) // overflow 98             throw new OutOfMemoryError(); 99         return (minCapacity > MAX_ARRAY_SIZE) ?100             Integer.MAX_VALUE :101             MAX_ARRAY_SIZE;102     }103     104 105 }