在Java8中，为什么ArrayList的默认容量现在为零？-InJava8,whyisthedefaultcapacityofArrayListnowzero?

87  

Technically, it's 10, not zero, if you admit for a lazy initialisation of the backing array. See:

从技术上讲，如果您承认后备阵列的延迟初始化，则为10，而不是零。看到：

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

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}

where

哪里

/**
 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

What you're referring to is just the zero-sized initial array object that is shared among all initially empty ArrayList objects. I.e. the capacity of 10 is guaranteed lazily, an optimisation that is present also in Java 7.

你所指的只是在所有最初空的ArrayList对象之间共享的零大小的初始数组对象。即懒惰保证了10的容量，Java 7中也存在一个优化。

Admittedly, the constructor contract is not entirely accurate. Perhaps this is the source of confusion here.

不可否认，构造函数合同并不完全准确。也许这就是混乱的根源。

Background

Here's an E-Mail by Mike Duigou

这是Mike Duigou的电子邮件

I have posted an updated version of the empty ArrayList and HashMap patch.

我发布了空ArrayList和HashMap补丁的更新版本。

http://cr.openjdk.java.net/~mduigou/JDK-7143928/1/webrev/

http://cr.openjdk.java.net/~mduigou/JDK-7143928/1/webrev/

This revised implementation introduces no new fields to either class. For ArrayList the lazy allocation of the backing array occurs only if the list is created at default size. According to our performance analysis team, approximately 85% of ArrayList instances are created at default size so this optimization will be valid for an overwhelming majority of cases.

此修订的实现不会为任何一个类引入新字段。对于ArrayList，仅当列表以默认大小创建时，才会发生后备阵列的延迟分配。根据我们的性能分析团队，大约85％的ArrayList实例是以默认大小创建的，因此这种优化对于绝大多数情况都是有效的。

For HashMap, creative use is made of the threshold field to track the requested initial size until the bucket array is needed. On the read side the empty map case is tested with isEmpty(). On the write size a comparison of (table == EMPTY_TABLE) is used to detect the need to inflate the bucket array. In readObject there's a little more work to try to choose an efficient initial capacity.

对于HashMap，创建使用阈值字段来跟踪请求的初始大小，直到需要存储桶阵列。在读取端，使用isEmpty（）测试空映射大小写。在写入大小上，使用（table == EMPTY_TABLE）的比较来检测是否需要给桶阵列充气。在readObject中，尝试选择有效的初始容量还有一些工作要做。

From: http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-April/015585.html

来自：http：//mail.openjdk.java.net/pipermail/core-libs-dev/2013-April/015585.html

8  

In java 8 default capacity of ArrayList is 0 until we add at least one object into the ArrayList object (You can call it lazy initialization). Please see below code for help.

在java 8中，ArrayList的默认容量为0，直到我们将至少一个对象添加到ArrayList对象中（您可以将其称为延迟初始化）。请参阅以下代码以获取帮助。

ArrayList al = new ArrayList();          //Size:  0, Capacity:  0
ArrayList al = new ArrayList(5);         //Size:  0, Capacity:  5
ArrayList al = new ArrayList(new ArrayList(5)); //Size:  0, Capacity:  0
al.add( "shailesh" );                    //Size:  1, Capacity: 10

public static void main( String[] args )
        throws Exception
    {
        ArrayList al = new ArrayList();
        getCapacity( al );
        al.add( "shailesh" );
        getCapacity( al );
    }

    static void getCapacity( ArrayList l )
        throws Exception
    {
        Field dataField = ArrayList.class.getDeclaredField( "elementData" );
        dataField.setAccessible( true );
        System.out.format( "Size: %2d, Capacity: %2d%n", l.size(), ( (Object[]) dataField.get( l ) ).length );
}

Response: - 
Size:  0, Capacity:  0
Size:  1, Capacity: 10

Now question is why this change has been done in JAVA 8?

现在问题是为什么在JAVA 8中完成了这个改变？

Answer is to save memory consumption. Millions of array list objects are created in real time java applications. Default size of 10 objects means that we allocate 10 pointers (40 or 80 bytes) for underlying array at creation and fill them in with nulls. An empty array (filled with nulls) occupy lot of memory .

答案是节省内存消耗。在实时Java应用程序中创建了数百万个数组列表对象。默认大小为10个对象意味着我们在创建时为底层数组分配10个指针（40或80个字节），并用空值填充它们。空数组（填充空值）占用大量内存。

Lazy initialization postpones this memory consumption till moment you will actually use the array list.

延迟初始化会推迟此内存消耗，直到您实际使用数组列表为止。

Article Default capacity of ArrayList in Java 8 explains it in details.

文章Java 8中ArrayList的默认容量详细解释了它。

6  

If the very first operation that is done with an ArrayList is to pass addAll a collection which has more than ten elements, then any effort put into creating an initial ten-element array to hold the ArrayList's contents would be thrown out the window. Whenever something is added to an ArrayList it's necessary to test whether the size of the resulting list will exceed the size of the backing store; allowing the initial backing store to have size zero rather than ten will cause this test to fail one extra time in the lifetime of a list whose first operation is an "add" which would require creating the initial ten-item array, but that cost is less than the cost of creating a ten-item array that never ends up getting used.

如果使用ArrayList完成的第一个操作是向addAll传递一个包含十个以上元素的集合，那么创建一个初始十元素数组以保存ArrayList内容的任何工作都将被抛出窗口。每当有东西被添加到ArrayList时，都需要测试结果列表的大小是否超过后备存储的大小;允许初始后备存储的大小为零而不是10将导致此测试在列表的生命周期中失去一个额外的时间，该列表的第一个操作是“添加”，这将需要创建初始的十项数组，但该成本是低于创建一个永远不会被使用的十项数组的成本。

That having been said, it might have been possible to improve performance further in some contexts if there were a overload of "addAll" which specified how many items (if any) would likely be added to the list after the present one, and which could use that to influence its allocation behavior. In some cases code which adds the last few items to a list will have a pretty good idea that the list is never going to need any space beyond that. There are many situations where a list will get populated once and never modified after that. If at the point code knows that the ultimate size of a list will be 170 elements, it has 150 elements and a backing store of size 160, growing the backing store to size 320 will be unhelpful and leaving it at size 320 or trimming it to 170 will be less efficient than simply having the next allocation grow it to 170.

话虽如此，如果有一个“addAll”超载指定在当前列表之后可能会将多少项（如果有的话）添加到列表中，那么在某些情况下可能会进一步提高性能。用它来影响它的分配行为。在某些情况下，将最后几个项添加到列表中的代码将非常清楚该列表永远不需要除此之外的任何空间。在许多情况下，列表将填充一次，之后从未修改过。如果在点代码知道列表的最终大小将是170个元素，它有150个元素和大小为160的后备存储，将后备存储增加到320大小将是无益的，并将其保留为320或修改为170的效率低于简单地将下一个分配增加到170。

3  

The question is 'why?'.

问题是“为什么？”。

Memory profiling inspections (for example (https://www.yourkit.com/docs/java/help/inspections_mem.jsp#sparse_arrays) shows that empty (filled with nulls) arrays occupy tons of memory .

内存分析检查（例如（https://www.yourkit.com/docs/java/help/inspections_mem.jsp#sparse_arrays）显示空（填充空值）数组占用大量内存。

Default size of 10 objects means that we allocate 10 pointers (40 or 80 bytes) for underlying array at creation and fill them in with nulls. Real java applications create millions of array lists.

默认大小为10个对象意味着我们在创建时为底层数组分配10个指针（40或80个字节），并用空值填充它们。真正的Java应用程序创建数百万个数组列表

The introduced modification removes^W postpone this memory consumption till moment you will actually use the array list.

引入的修改删除了^ W推迟了这个内存消耗，直到您实际使用数组列表为止。

0  

ArrayList default size in JAVA 8 is stil 10. The only change made in JAVA 8 is that if a coder adds elements less than 10 then the remaining arraylist blank places are not specified to null. Saying so because I have myself gone through this situation and eclipse made me look into this change of JAVA 8.

JAVA 8中的ArrayList默认大小是stil 10.在JAVA 8中唯一的变化是，如果编码器添加小于10的元素，则剩余的arraylist空白位置不指定为null。这么说是因为我自己经历了这种情况并且日食使我看到了JAVA 8的这种变化。

You can justify this change by looking at below screenshot. In it you can see that ArrayList size is specified as 10 in Object[10] but the number of elements displayed are only 7. Rest null value elements are not displayed here. In JAVA 7 below screenshot is same with just a single change which is that the null value elements are also displayed for which the coder needs to write code for handling null values if he is iterating complete array list while in JAVA 8 this burden is removed from the head of coder/developer.

您可以通过查看下面的屏幕截图来证明这一变化。在其中，您可以看到在Object [10]中将ArrayList大小指定为10，但显示的元素数量仅为7.此处不显示其余的null值元素。在下面的JAVA 7中，屏幕截图与单个更改相同，即显示空值元素，如果编码器在JAVA 8中迭代完整数组列表时需要编写用于处理空值的代码，则此负担将从编码/开发人员。

Screen shot link.

屏幕截图链接。

0  

After above question I gone through ArrayList Document of Java 8. I found the default size is still 10 only.

在上述问题之后，我浏览了Java 8的ArrayList Document。我发现默认大小仍然是10。