@kiraSally
2018-03-12T18:48:28.000000Z
字数 11451
阅读 1898
JAVA
COLLECTIONS
源码
1.8版
- 笔者个人博客 kiraSally的掘金个人博客 感谢支持
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
public interface Collection<E> extends Iterable<E> {
.....
//1.8新增方法:提供了接口默认实现,返回分片迭代器
@Override
default Spliterator<E> spliterator() {
return Spliterators.spliterator(this, 0);
}
//1.8新增方法:提供了接口默认实现,返回串行流对象
default Stream<E> stream() {
return StreamSupport.stream(spliterator(), false);
}
//1.8新增方法:提供了接口默认实现,返回并行流对象
default Stream<E> parallelStream() {
return StreamSupport.stream(spliterator(), true);
}
/**
* Removes all of the elements of this collection that satisfy the given
* predicate. Errors or runtime exceptions thrown during iteration or by
* the predicate are relayed to the caller.
* 1.8新增方法:提供了接口默认实现,移除集合内所有匹配规则的元素,支持Lambda表达式
*/
default boolean removeIf(Predicate<? super E> filter) {
//空指针校验
Objects.requireNonNull(filter);
//注意:JDK官方推荐的遍历方式还是Iterator,虽然forEach是直接用for循环
boolean removed = false;
final Iterator<E> each = iterator();
while (each.hasNext()) {
if (filter.test(each.next())) {
each.remove();//移除元素必须选用Iterator.remove()方法
removed = true;//一旦有一个移除成功,就返回true
}
}
//这里补充一下:由于一旦出现移除失败将抛出异常,因此返回false指的仅仅是没有匹配到任何元素而不是运行异常
return removed;
}
}
public interface Iterable<T>{
.....
//1.8新增方法:提供了接口默认实现,用于遍历集合
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
//1.8新增方法:提供了接口默认实现,返回分片迭代器
default Spliterator<T> spliterator() {
return Spliterators.spliteratorUnknownSize(iterator(), 0);
}
}
public interface List<E> extends Collection<E> {
//1.8新增方法:提供了接口默认实现,用于对集合进行排序,主要是方便Lambda表达式
default void sort(Comparator<? super E> c) {
Object[] a = this.toArray();
Arrays.sort(a, (Comparator) c);
ListIterator<E> i = this.listIterator();
for (Object e : a) {
i.next();
i.set((E) e);
}
}
//1.8新增方法:提供了接口默认实现,支持批量删除,主要是方便Lambda表达式
default void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
final ListIterator<E> li = this.listIterator();
while (li.hasNext()) {
li.set(operator.apply(li.next()));
}
}
/**
* 1.8新增方法:返回ListIterator实例对象
* 1.8专门为List提供了专门的ListIterator,相比于Iterator主要有以下增强:
* 1.ListIterator新增hasPrevious()和previous()方法,从而可以实现逆向遍历
* 2.ListIterator新增nextIndex()和previousIndex()方法,增强了其索引定位的能力
* 3.ListIterator新增set()方法,可以实现对象的修改
* 4.ListIterator新增add()方法,可以向List中添加对象
*/
ListIterator<E> listIterator();
}
- 1.8的
ArrayList
只是在1.7的基础上做了很少的改动,主要集中于初始化以及实现接口新增方法方面- 1.7版本请参见笔者的 集合番@ArrayList一文通(1.7版)
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer.
* Any empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
* 数组缓存,跟1.7版本相比,主要有两个变化:
* 1.去掉private属性,使用默认的friendly作用域,开放给同包类使用
* 2.一个空数组的elementData将设置为EMPTY_ELEMENTDATA直到第一个元素新增时
* 使用DEFAULT_CAPACITY(10)完成有容量的初始化 -- 优化:这里选择将内存分配后置,而从尽可能节省空间
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* Shared empty array instance used for empty instances.
* 当时用空构造时,给予数组(elementData)默认值
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Constructs an empty list with an initial capacity of ten.
* 1.8版的默认构造器,只会初始化一个空数组
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;//初始化一个空数组
}
/**
* Constructs an empty list with an initial capacity of ten.
* 1.7版的默认构造器,会直接初始化一个10容量的数组
*/
public ArrayList() {
this(10);
}
/**
* 1.8版的trimToSize,跟1.7版相比:
* 可以明显的看到去掉了oldCapacity这一临时变量
* 笔者认为这进一步强调了HashMap是非线程安全的,因此直接用length即可
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
/**
* 1.7版的trimToSize
*/
public void trimToSize() {
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
}
/**
* Performs the given action for each element of the {@code Iterable}
* until all elements have been processed or the action throws an
* exception. Unless otherwise specified by the implementing class,
* actions are performed in the order of iteration (if an iteration order
* is specified). Exceptions thrown by the action are relayed to the
* caller.
* 1.8新增方法,重写Iterable接口的forEach方法
* 提供对数组的遍历操作,由于支持Consumer因此在遍历时将执行传入的方法
*/
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int expectedModCount = modCount;
@SuppressWarnings("unchecked")
final E[] elementData = (E[]) this.elementData;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
action.accept(elementData[i]);//执行传入的自定义方法
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
--------------
List<String> list = new ArrayList<>();
list.add("有村架纯");
list.add("桥本环奈");
list.add("斋藤飞鸟");
list.forEach(s -> System.out.print(s + "!!")); //输出:有村架纯!!桥本环奈!!斋藤飞鸟!!
/**
* Removes all of the elements of this collection that satisfy the given
* predicate. Errors or runtime exceptions thrown during iteration or by
* the predicate are relayed to the caller.
* 1.8新增方法,重写Collection接口的removeIf方法
* 移除集合内所有复合匹配条件的元素,迭代时报错会抛出异常 或 把断言传递给调用者(即断言中断)
* 该方法主要干了两件事情:
* 1.根据匹配规则找到所有符合要求的元素
* 2.移除元素并转移剩余元素位置
* 补充:为了安全和快速,removeIf分成两步走,而不是直接找到就执行删除和转移操作,写法值得借鉴
*/
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
// figure out which elements are to be removed any exception thrown from
// the filter predicate at this stage will leave the collection unmodified
int removeCount = 0;
//BitSet用于按位存储,这里用作存储待移除元素(即符合匹配规则的元素)
//BitSet能够通过位图算法大幅减少数据占用存储空间和内存,尤其适合在海量数据方面,这里是个很明显的优化
//有机会会在基础番中解析一下BitSet的奇妙之处
final BitSet removeSet = new BitSet(size);
final int expectedModCount = modCount;
final int size = this.size;
//每次循环都要判断modCount == expectedModCount!
for (int i=0; modCount == expectedModCount && i < size; i++) {
@SuppressWarnings("unchecked")
final E element = (E) elementData[i];
if (filter.test(element)) {
removeSet.set(i);
removeCount++;
}
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
// shift surviving elements left over the spaces left by removed elements
// 当有元素被移除,需要对剩余元素进行位移
final boolean anyToRemove = removeCount > 0;
if (anyToRemove) {
final int newSize = size - removeCount;
for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
for (int k=newSize; k < size; k++) {
elementData[k] = null; // Let gc do its work
}
this.size = newSize;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
//正常情况下,一旦匹配到元素,应该删除成功,否则将抛出异常,当没有匹配到任何元素时,返回false
return anyToRemove;
}
--------------
List<String> list = new ArrayList<>();
list.add("有村架纯");
list.add("桥本环奈");
list.add("斋藤飞鸟");
list.forEach(s -> System.out.print(s + "!!")); //输出:有村架纯!!桥本环奈!!斋藤飞鸟!!
System.out.println(list.removeIf(s -> s.startsWith("斋藤")));//输出:true
list.forEach(s -> System.out.print(s + ",")); //输出:有村架纯!!桥本环奈!!
--------------
//这里补充一点,使用Arrays.asList()生成的ArrayList是Arrays自己的私有静态内部类
//强行使用removeIf的话会抛出java.lang.UnsupportedOperationException的异常(因为它没实现这个方法)
/**
* Replaces each element of this list with the result of applying the operator to that element.
* Errors or runtime exceptions thrown by the operator are relayed to the caller.
* 1.8新增方法,重写List接口的replaceAll方法
* 提供支持一元操作的批量替换功能
*/
@Override
@SuppressWarnings("unchecked")
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
final int expectedModCount = modCount;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
elementData[i] = operator.apply((E) elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
--------------
List<String> list = new ArrayList<>();
list.add("有村架纯");
list.add("桥本环奈");
list.add("斋藤飞鸟");
list.forEach(s -> System.out.print(s + "!!")); //输出:有村架纯!!桥本环奈!!斋藤飞鸟!!
list.replaceAll(t -> {
if(t.equals("桥本环奈")) t = "逢泽莉娜";//这里我们将"桥本环奈"替换成"逢泽莉娜"
return t;//注意如果是语句块的话一定要返回
});
list.forEach(s -> System.out.print(s + "!!")); //输出:有村架纯!!逢泽莉娜!!斋藤飞鸟!!
/**
* Sorts this list according to the order induced by the specified
* 1.8新增方法,重写List接口的sort方法
* 支持对数组进行排序,主要方便于Lambda表达式
*/
@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
final int expectedModCount = modCount;
//Arrays.sort底层是结合归并排序和插入排序的混合排序算法,有不错的性能
//有机会在基础番对Timsort(1.8版)和ComparableTimSort(1.7版)进行解析
Arrays.sort((E[]) elementData, 0, size, c);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
--------------
List<String> list = new ArrayList<>();
list.add("有村架纯");
list.add("桥本环奈");
list.add("斋藤飞鸟");
list.forEach(s -> System.out.print(s + "!!")); //输出:有村架纯!!桥本环奈!!斋藤飞鸟!!
list.sort((prev, next) -> prev.compareTo(next));//这里我们选用自然排序
list.forEach(s -> System.out.print(s + "!!"));//输出:斋藤飞鸟!!有村架纯!!桥本环奈!!
- 并行分片迭代器是Java为了并行遍历数据源中的元素而专门设计的迭代器
- 并行分片迭代器借鉴了Fork/Join框架的核心思想:用递归的方式把并行的任务拆分成更小的子任务,然后把每个子任务的结果合并起来生成整体结果
- 并行分片迭代器主要是提供给Stream,准确说是提供给并行流使用,使用时推荐直接用Stream即可
default Stream<E> parallelStream() {//并行流
return StreamSupport.stream(spliterator(), true);//true表示使用并行处理
}
static final class ArrayListSpliterator<E> implements Spliterator<E> {
private final ArrayList<E> list;
//起始位置(包含),advance/split操作时会修改
private int index; // current index, modified on advance/split
//结束位置(不包含),-1 表示到最后一个元素
private int fence; // -1 until used; then one past last index
private int expectedModCount; // initialized when fence set
/** Create new spliterator covering the given range */
ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
int expectedModCount) {
this.list = list; // OK if null unless traversed
this.index = origin;
this.fence = fence;
this.expectedModCount = expectedModCount;
}
/**
* 获取结束位置,主要用于第一次使用时对fence的初始化赋值
*/
private int getFence() { // initialize fence to size on first use
int hi; // (a specialized variant appears in method forEach)
ArrayList<E> lst;
if ((hi = fence) < 0) {
//当list为空,fence=0
if ((lst = list) == null)
hi = fence = 0;
else {
//否则,fence = list的长度
expectedModCount = lst.modCount;
hi = fence = lst.size;
}
}
return hi;
}
/**
* 对任务(list)分割,返回一个新的Spliterator迭代器
*/
public ArrayListSpliterator<E> trySplit() {
//二分法
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null : // divide range in half unless too small 分成两部分,除非不够分
new ArrayListSpliterator<E>(list, lo, index = mid,expectedModCount);
}
/**
* 对单个元素执行给定的执行方法
* 若没有元素需要执行,返回false;若可能还有元素尚未执行,返回true
*/
public boolean tryAdvance(Consumer<? super E> action) {
if (action == null)
throw new NullPointerException();
int hi = getFence(), i = index;
if (i < hi) {//起始位置 < 终止位置 -> 说明还有元素尚未执行
index = i + 1; //起始位置后移一位
@SuppressWarnings("unchecked") E e = (E)list.elementData[i];
action.accept(e);//执行给定的方法
if (list.modCount != expectedModCount)
throw new ConcurrentModificationException();
return true;
}
return false;
}
/**
* 对每个元素执行给定的方法,依次处理,直到所有元素已被处理或被异常终止
* 默认方法调用tryAdvance方法
*/
public void forEachRemaining(Consumer<? super E> action) {
int i, hi, mc; // hoist accesses and checks from loop
ArrayList<E> lst; Object[] a;
if (action == null)
throw new NullPointerException();
if ((lst = list) != null && (a = lst.elementData) != null) {
if ((hi = fence) < 0) {
mc = lst.modCount;
hi = lst.size;
}
else
mc = expectedModCount;
if ((i = index) >= 0 && (index = hi) <= a.length) {
for (; i < hi; ++i) {
@SuppressWarnings("unchecked") E e = (E) a[i];
action.accept(e);
}
if (lst.modCount == mc)
return;
}
}
throw new ConcurrentModificationException();
}
/**
* 计算尚未执行的任务个数
*/
public long estimateSize() {
return (long) (getFence() - index);
}
/**
* 返回当前对象的特征量
*/
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
}
--------------
List<String> list = new ArrayList<>();
list.add("有村架纯");
list.add("桥本环奈");
list.add("斋藤飞鸟");
Spliterator<String> spliterator = list.spliterator();
spliterator.forEachRemaining(s -> System.out.print(s += "妹子!!"));
//输出:有村架纯妹子!!桥本环奈妹子!!斋藤飞鸟妹子!!
//因为这个类是提供给Stream使用的,因此可以直接用Stream,下面的代码作用等同上面,但进行了并发优化
Stream<String> parallelStream = list.parallelStream();
parallelStream.forEach(s -> System.out.print(s += "妹子!!"));
//输出:桥本环奈妹子!!有村架纯妹子!!斋藤飞鸟妹子!! --> 因为引入并发,所有执行顺序会有些不同
集合番@ArrayList一文通(1.8版) 由 黄志鹏kira 创作,采用 知识共享 署名-非商业性使用 4.0 国际 许可协议 进行许可。
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