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JDK8之ArrayList源码
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ArrayList三个构造器/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; //初始化容量大小 /**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {}; /**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access //保存数据的数组 /**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;//ArrayList实际数据的数量
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; //初始化容量大小 /**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {}; /**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access //保存数据的数组 /**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;//ArrayList实际数据的数量
三个构造器主要目的是初始化Object[] 的长度
1、ArrayList()，默认构造数组长度为10空列表，但是DEFAULTCAPACITY_EMPTY_ELEMENTDATA长度我们并不知道，真正设置数组长度是在add()方法扩容时候设置
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
2、new Array()，指定数组长度的列表，这个构造器的优点是如果我们知道数据的大小，初始化时指定长度则会减少数组扩容的次数，大大提高ArrayList的效率
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);
}
}
3、new ArrayList(Collection c)，这个构造器是可以把其它实现Collection接口的类当做参数，构造Object[]
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
ArrayList-add
1、add(E e)
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!! //容量扩容
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);//DEFAULT_CAPACITY默认是10，Math.max 取最大值
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;//modCount 数组修改次数 ，无论add remove 都会++
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
