标签:1.5 容量 ++ ArrayList elementData int minCapacity 源代码 size
都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事
就用下面这段代码在jdk的三个版本运行看了下效果:
import java.lang.reflect.Field; import java.util.ArrayList; public class ArrayListDemo { public static void main(String[] args) { ArrayList<Integer> list1 = new ArrayList<Integer>(); Integer capacity = getCapacity(list1);// 获取容量 int size = list1.size(); System.out.println("list1的容量:" + capacity); System.out.println("list1的大小:" + size); System.out.println("----------------------------"); ArrayList<Integer> list2 = new ArrayList<Integer>(); list2.add(1); capacity = getCapacity(list2);// 获取容量,arraylist初始化容量是10 size = list2.size(); System.out.println("list2的容量:" + capacity); System.out.println("list2的大小:" + size); System.out.println("----------------------------"); ArrayList<Integer> list3 = new ArrayList<Integer>(); capacity = getCapacity(list3);// 获取容量,arraylist初始化容量是10 for (int i = 0; i < 10; i++) { list3.add(i); } capacity = getCapacity(list3); size = list3.size(); System.out.println("list3的容量:" + capacity); System.out.println("list3的大小:" + size); System.out.println("----------------------------"); ArrayList<Integer> list4 = new ArrayList<Integer>(); for (int i = 0; i < 11; i++) { list4.add(i); } capacity = getCapacity(list4);// 获取容量 size = list4.size(); System.out.println("list4的容量:" + capacity); System.out.println("list4的大小:" + size); } // 获取list容量 public static Integer getCapacity(ArrayList list) { Integer length = null; Class clazz = list.getClass(); Field field; try { field = clazz.getDeclaredField("elementData"); field.setAccessible(true); Object[] object = (Object[]) field.get(list); length = object.length; return length; } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } return length; } }
JDK1.6:运行结果:
list1的容量:10 list1的大小:0 ---------------------------- list2的容量:10 list2的大小:1 ---------------------------- list3的容量:10 list3的大小:10 ---------------------------- list4的容量:16 list4的大小:11
部分源码:
public void ensureCapacity(int var1) { ++this.modCount; int var2 = this.elementData.length; if (var1 > var2) { Object[] var3 = this.elementData; int var4 = var2 * 3 / 2 + 1; if (var4 < var1) { var4 = var1; } this.elementData = Arrays.copyOf(this.elementData, var4); } }
JDK1.7:运行结果:
list1的容量:0 list1的大小:0 ---------------------------- list2的容量:10 list2的大小:1 ---------------------------- list3的容量:10 list3的大小:10 ---------------------------- list4的容量:15 list4的大小:11
部分源码:
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { if (elementData == 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); } 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); }
JDK1.8:运行结果:
list1的容量:0 list1的大小:0 ---------------------------- list2的容量:10 list2的大小:1 ---------------------------- list3的容量:10 list3的大小:10 ---------------------------- list4的容量:15 list4的大小:11
部分源码:
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } 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); }
总结:
发现不同jdk是不一样的,关于(1.5倍+1)出现在jdk1.6,其他1.7和1.8都是(1.5倍扩容)。
关于 详细代码和原理可以参考 3。参考1和2也是不错的!
更多参考: 1: ArrayList扩容1.5倍 2 :ArrayList源码解析 3 :ArrayList初始默认容量(长度)
标签:1.5,容量,++,ArrayList,elementData,int,minCapacity,源代码,size 来源: https://www.cnblogs.com/codingmode/p/12716495.html
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