java学习笔记（2）-ArrayList类

1.为了方便查看我把源码几个方法和变量复制出来了

package com.itao.study.study2;

public class ArrayList {

private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

transient Object[] elementData;

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);
}
}

public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

}


定义了默认的容量：DEFAULT_CAPACITY=10；
通常我们在代码中会这样使用     List list=new ArrayList();   其实就是复制对象数组elementData为空数组

public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); //Math.max()返回两个参数的最大值
}

ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
        modCount++;	//这里的modCount在AbstractList中定义的，值为0

        // 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);
}

在添加元素的时候，注意这里的size初始值是0，因为是类变量，在类初始化的时候jvm自动赋值为0
添加的时候数组长度增长，当对象数组长度大于默认的10时，数组会进行扩容，扩容的值为：原来数组的长度size  +  (size>>1)

比如原来List中已有10个数据，在此添加则扩容为10+ 10>>1 =15

从上面的源码也可以看出：size,elementData根本没进行同步，也就是说ArrayList是线程不安全的

public void add(int index, E element) {
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index); //复值数组的后半部分，从index开始移到index+1开始，index位置添加element
elementData[index] = element;
size++;
}

private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}

这几个方法也很简单，主要就是调用系统数组拷贝函数，关键点在复值原数组和目的数组的起始位置

public boolean contains(Object o) {
return indexOf(o) >= 0;
}

public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}

arrayList.contain(Object o) 其实就是遍历数组，来比较的

public E remove(int index) {
rangeCheck(index);

modCount++;
E oldValue = elementData(index);

int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work

return oldValue;
}

E elementData(int index) {
return (E) elementData[index];
}

remove(int index)方法就是把index位置的元素置为null而已