1.ArrayList
添加元素的方式和移除元素
add(E e)
add(int index,E element)
addAll(Collection<? extends E> c)
addAll(int index, Collection<? extends E> c)
remove(int index)
remove(Object o)
public boolean add(E e) { //确保添加元素后的容器（数组.length）是否放得下，size为数组中元素个数,size+1是添加后元素个数 ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private static final int DEFAULT_CAPACITY = 10; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private static int calculateCapacity(Object[] elementData, int minCapacity) { //如果数组elementData为空,说明现在数组还没被初始化，仅仅是有个引用 //那么minCapacity=size+1=1,空的数组没有办法存放元素，那么就将默认数组容量设为10并返回 //如果数组elementData不为空直接返回minCapacity if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; } private void ensureExplicitCapacity(int minCapacity) { //List,map这些集合结构中常能看到modCount属性，在对集合进行新增或移除操作时会使modCount+1,这 //是一个记录集合变化次数的变量。作用是在使用迭代器Iterator对集合进行遍历时，用modCount来判断集 //合内数据没有发生变化，否则会抛出异常。 链接：https://www.jianshu.com/p/d01e9cf207f9 //判断是否需要扩容 // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } private void grow(int minCapacity) { // overflow-conscious code //将当前数组长度赋值为老数组容量 int oldCapacity = elementData.length; //将老数组容量扩容1.5倍后赋值给新数组容量 int newCapacity = oldCapacity + (oldCapacity >> 1); //这种情况出现的原因为如果返回的minCapacity==10，此时elementDate还为{}，没有被初始化， //newCapacity==oldCapacity==0<minCapacity==10 if (newCapacity - minCapacity < 0) newCapacity = minCapacity; //在这里如果扩容后的newCapacity移除为负数也符合条件，负数-正数=正数>0,int范围为-2^31~2^31-1 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); } private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }@SuppressWarnings("unchecked")
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
//调用了本地方法arraycopy
/**
* @param src the source array.
* @param srcPos starting position in the source array.
* @param dest the destination array.
* @param destPos starting position in the destination data.
* @param length the number of array elements to be copied.
*\
把src数组从srcPos位置其长度为length复制到dest数组的destPos位置后
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
int[] arr = new int[100];
int[] arr2 = {11,12,23,23};
for (int i = 0; i <10 ; i++) {
arr[i]=i;
}
System.arraycopy(arr, 0, arr, 4, 10);
System.out.println(Arrays.toString(arr));
System.arraycopy(arr2, 0, arr, 0, 4);
System.out.println(Arrays.toString(arr));
[0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] [11, 12, 23, 23, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
add(int index,E element)
public void add(int index, E element) {
//判断添加位置合法性
rangeCheckForAdd(index);
//上面分析过了
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
addAll(Collection c)
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;
}
addAll(int index, Collection c)
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
remove(int index)
public E remove(int index) { rangeCheck(index); modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; //表示移动元素的个数，也是元素移动的步数 if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; }remove(Object o)
public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } private void fastRemove(int index) { modCount++; 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 }