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Java 8 中 ArrayList 的变化源码分析

1.ArrayList上级接口的变化

先回顾一下ArrayList的类定义

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

Java 8 中 ArrayList 的变化源码分析

接口/类新增方法

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();
}

2.ArrayList的变化

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)

3.ArrayList的属性变化

全局变量的变更

/**
  * 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 = {};

构造器的变更

/**
  * 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);
}

4.ArrayList的方法变化

trimToSize方法变更

/**
  * 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);
    }
}

5.ArrayList的新增方法

5.1 forEach方法

/**
  * 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!!

5.2 removeIf方法

/**
  * 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)

5.3 replaceAll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!!

5.4 sort方法

/**
  * 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!!

6.ArrayList的新增并行分片迭代器

什么是并行分片迭代器?

  • 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

ArrayListSpliterator类解析

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
原文  https://juejin.im/post/5abc9d826fb9a028dd4e4e77
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