ArrayList与LinkedList
目录
1. 关于ArrayList常见问题
1.1. ArrayList的扩容机制
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
// 默认容纳大小
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private transient Object[] elementData;
private int size;
...
// 继承自AbstractList, 记录size变化的次数
protected transient int modCount;
...
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " + initialCapacity);
this.elementData = new Object[initialCapacity];
}
// 当使用无参构造方法初始化ArrayList对象时，会在首次调用add方法时，生成一个长度为10的Object类型数组。
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
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;
// 1.5倍
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);
}
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;
}
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
}
}
1.2. ArrayList是实现了基于动态数组的数据结构，因为地址连续，一旦数据存储好了，查询操作效率会比较高，但插入和删除操作时，要移动数据效率比较低
1.3. 当传递ArrayList到某个方法中，或者某个方法返回ArrayList，什么时候要考虑安全隐患？如果修复呢？
如果Array在没有被复制的情况下直接被分配给成员变量，就会发生当原始的数组被改变时，传递到这个方法中的数组也会改变。
public void set(String[] array) {
this.array = array;
}
// 修复安全隐患
public void set(String[] array) {
if (array == null) {
this.array = new Stringp[0];
} else {
this.array = Arrays.copayOf(array, array.length);
}
}
1.4. 如何复制一个ArrayList到另一个ArrayList中去？
// 1. 使用`clone()`方法
ArrayList newArray = oldArray.clone();
// 2. 使用ArrayList构造方法
ArrayList newArray = new ArrayList(oldArray);
// 3. 使用Collections.copy()方法
List<String> src = new ArrayList<>();
src.add("1");
src.add("1");
src.add("1");
// 初始化一个`size = 3`的空数组，否则会报错
List<String> dest = new ArrayList<>(Arrays.asList(new String[src.size()]));
Collections.copy(dest, src);
1.5. 理解fail-fast原理
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
...
//
protected transient int modCount = 0;
private class Itr implements Iterator<E> {
int cursor = 0;
int lastRet = -1;
// 修改数的记录值
// 每次新建Itr()对象时，都会保存新建该对象时对应的modCount
// 以后每次遍历List中的元素时，都会比较expectedModCount与modCount是否相等
// 若不相等，则抛出ConcurrentModificationException异常，产生fail-fast事件
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size();
}
public E next() {
// 获取下一个元素之前，都会判断“新建Itr对象时保存的modCount”和“当前的modeCount”是否相等
checkForComodification();
try {
int i = cursor;
E next = get(i);
lastRet = i;
cursor = i + 1;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
}
List<Integer> ls = Lists.newArrayList(1, 1, 2, 2);
for (Integer i : ls) {
if(i.equals(1)){
ls.remove(i);
}
}
System.out.println(JSON.toJSONString(ls));
for语法糖，反编译
List<Integer> ls = Lists.newArrayList(new Integer[]{1, 1, 2, 2});
Iterator i$ = ls.iterator();
while(i$.hasNext()) {
// 再次执行迭代器的next方法时，checkForComodification会报错
Integer i = (Integer)i$.next();
if (i.equals(1)) {
// 调用的ArrayList的remove方法，可以删掉，modCount++
ls.remove(i);
}
}
System.out.println(JSON.toJSONString(ls));
1.6. 使用CopyOnWriteArrayList解决fail-fast问题
2. LinkedList
2.1. LinkedList基于链表的数据结构，地址是任意，所以在开辟内存空间的时候不需要等一个连续的地址，对于插入和删除操作效率会比较高，但查询操作时，要移动指针效率比较低
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0;
// Pointer to first node.
transient Node<E> first;
// Pointer to last node.
transient Node<E> last;
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;
Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}
/**
* Links e as first element.
*/
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element.
*/
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
}
多知道一点
public final class System {
/**
* 源数组中位置在 srcPos 到 srcPos+length-1 之间的元素被复制到目标数组中的 destPos 到 destPos+length-1 位置.
* @param src 源数组.
* @param srcPos 源数组中的起始位置, 若 srcPos+length 大于 src.length，则抛出 IndexOutOfBoundsException 异常
* @param dest 目标数组.
* @param destPos 目标数组中的起始位置, 若 destPos+length 大于 dest.length，则抛出 IndexOutOfBoundsException 异常
* @param length 要复制的数组元素的数量.
*/
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length){}
}
...
public static void main(String[] args) {
int index = 1;
int[] elementData = new int[]{1, 2, 3, 4, 5};
int size = elementData.length;
nums = Arrays.copyOf(elementData, size + 1);
System.out.println(JSON.toJSONString(elementData));
// [1,2,3,4,5,0]
System.arraycopy(elementData, index, elementData, index + 1, size - index);
System.out.println(JSON.toJSONString(elementData));
// [1,2,2,3,4,5]
elementData[index] = 6;
System.out.println(JSON.toJSONString(elementData));
// [1,6,2,3,4,5]
}