如何在Java-8中正确查找流特性？ [英] How to correctly find the stream characteristics in Java-8?

问题描述

在进行流操作时，在中间/ pleleline操作期间，将创建具有不同特征的流（例如SORTED / SIZED / DISTINCT / ORDERED） - 掌握Lambdas（第6章）

While doing stream operations, during the intermediate/pipleline operations the streams would be created with different characteristics(e.g SORTED/SIZED/DISTINCT/ORDERED) - Mastering Lambdas(Ch 6)

Stream.of(8,3,5,6,7,4)//ORDERED, SIZED
.filer(i->i%2==0) // ORDERED
.sorted() // ORDERED, SORTED
.distinct() // DISTINCT, ORDERED, SORTED
.map(i->i+1) // ORDERED
.unordered(); //none

我们如何找出上述代码段中提到的流的不同特征？

How do we find out the different characteristics of the stream as mentioned in the above snippet?

推荐答案

我想稍微扩展一下assylias所说的内容（绝对正确）。

I would like to slightly extend what assylias said (which is absolutely correct).

首先，这些特性实现为普通的 int ，它是二进制表示。首先它是全零，但当你添加某个特征时，它的位被设置为一个通过 OR 操作，删除通过 AND 操作。

First, these characteristics are implemented as plain int, it's binary representation. First it's all zeroes, but when you add a certain characteristic it's bit is set to one via the OR operation, removed via the AND operation.

您可以通过以下方式查看某个Spliterator属性设置其一个的位置：

You can see where a certain Spliterator property sets its one simply by doing this for example:

System.out.println(Integer.toBinaryString(Spliterator.SIZED)); // 1000000

它将第7位设置为右边的一位。因此，当您检查：

It's setting the 7-th bit into one from the right. So when you check:

Spliterator<Integer> spliterator = Stream.of(8, 3, 5, 6, 7, 4).spliterator();
System.out.println((spliterator.characteristics() & Spliterator.SIZED) == Spliterator.SIZED);

您实际上正在检查此特定位是否已设置。

You are actually checking if this particular bit is set.

第二

4 流特征设置为第一个流创建的结果（而不是两个）。这本书有点过时或你没有向我们展示整个例子：

There are 4 stream characteristics that are set as the result of your first stream creation(and not two). Either the book is a bit outdated or you have not showed us the entire example:

Spliterator<Integer> spliterator = Stream.of(8, 3, 5, 6, 7, 4).spliterator();

System.out.println(Integer.bitCount(spliterator.characteristics())); // 4
System.out.println(Integer.toBinaryString(spliterator.characteristics()));// 100010001010000

这些设置位（等于一个）对应 SIZED ， ORDERED ， IMMUTABLE ， SUBSIZED 。

These set bits (that are equal to one) correspond to SIZED, ORDERED, IMMUTABLE, SUBSIZED.

您展示的其他人显然也略显偏差 - 您可以自行查看。

The others that you have shown are obviously slightly off too - you can check those yourself.

第三次

这些特性在流处理中非常重要。一些例子：

These characteristics are extremely important in stream processing. A few examples:

long howMany = Stream.of(1, 2, 3).map(x -> {
        System.out.println("mapping");
        return x * 2;
    }).count();
    System.out.println(howMany); // 3

在java-9中你不会看到的映射打印，因为你没有更改流（你还没有清除 SIZED 特征）;因此根本不需要甚至评估映射。

In java-9 you will not see the mapping printed, because you have not changed the stream (you have not cleared the SIZED characteristic); thus no need to even evaluate the mapping at all.

Stream<Integer> unlimited = Stream.iterate(0, x -> x + 1); 
System.out.println(unlimited.spliterator().hasCharacteristics(Spliterator.SIZED));
Stream<Integer> limited = unlimited.limit(3);          
System.out.println(limited.spliterator().hasCharacteristics(Spliterator.SIZED));

你会认为输出应该是 false true - 毕竟我们正在添加限制，但是没有;结果是 false false ：没有进行这样的优化，即使没有太多阻止它。

You would think that the output should be false true - we are adding a limit after all, but no; the result is false false: no such optimization is done, even if there is not much preventing it.

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