ArrayList其实就那么一回事儿之源码浅析

 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 = {};  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 = 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 = 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 = c.toArray();  36 size = elementData.length;  37 // c.toArray might (incorrectly) not return Object[] (see 6260652)  38 if (elementData.getClass() != Object[].class)  39 elementData = 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 == 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 = 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 }