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JDK7 和JDK8的ArrayList的区别对比
source link: https://blog.51cto.com/learningfish/5996699
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JDK7 和JDK8的ArrayList的区别对比
精选 原创public class ArrayListTest {
public static void main(String[] args) {
ArrayList<Object> jdk = new ArrayList<>();
jdk.add(123);
}
}
初始化操作
①调用无参构造器
jdk7从无参调用有参构造器，并初始化为10:
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this(10);
}
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
jdk8将其定义为长度为零的空数组，在之后的add方法中初始化大小(懒加载模式)
/**
* 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 = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
①调用add(E e)方法，jdk7源码与jdk8源码一致
/**
* 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!!
elementData[size++] = e;
return true;
}
②调用ensureCapacityInternal(int minCapacity)方法，jdk7源码与jdk8源码不同
jdk7如果加入元素后的个数大于数组长度则扩容（eg.在采用无参构造器初始化，添加到11个元素时会扩容）
private void ensureCapacityInternal(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
jdk8如果采用时默认初始化则数组elementData为DEFAULTCAPACITY_EMPTY_ELEMENTDATA，而第一次调用add时，minCapacity为1，DEFAULT_CAPACITY为10， minCapacity取两个中的最大值10，然后调用ensureExplicitCapacity会将数组elementData会在添加第一个元素时初始化大小为10
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
③扩容机制，一般情况下（不走if）扩容为1.5倍，特殊情况1（第一个if）当newCapacity<minCapacity时采用minCapacity的容量(eg.jdk8就是采用了该规则，传入minCapacity为10，默认elementData.length为0优化代码)，特殊情况2（第二个if）如果扩容后的newCapacity 大于了ArrayList规定的最大值MAX_ARRAY_SIZE 则会传入miniCapacity值采用hugeCapacity方法获取数组大小（尽量不让其出现OutOfMemoryError），jdk7源码与jdk8源码一致
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
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);
}
④特殊情况源码
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
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