blocks|key|4110298|text|不幸的是，由于Java处理原始类型、装箱、数组和泛型的本质，我不认为有更好的方法来实现这一点。特别是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4110299|4110300|List<T>.toArray不能工作，因为没有从int|unordered-list-item|offset|length|style|CODE|4110301|You到Integer的转换不能使用int作为泛型的类型参数，所以它必须是int-specific方法(或者使用反射来做讨厌的把戏的方法)。|4110302|4110303|我相信有一些库为所有的原始类型自动生成了这种方法的版本(也就是说，每个类型都有一个复制的模板)。很难看，但恐怕就是这么回事：|4110304|即使Arrays类在泛型出现之前就出现了，但如果今天引入它，它仍然必须包括所有可怕的重载(假设您想要使用原始数组)。|4110305|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/util/Arrays.html^0|0|0|0|F|P|3|0|4|7|I|3|11|3|0|0|0|2|6|2|6|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|10|8|@$F|11|G|12|H|I]|$F|13|G|14|H|I]]|9|@]|A|$]]|$1|J|3|K|5|E|7|15|8|@$F|16|G|17|H|I]|$F|18|G|19|H|I]|$F|1A|G|1B|H|I]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1D|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1E|8|@$F|1F|G|1G|H|I]]|9|@$F|1H|G|1I|1|1J]]|A|$]]|$1|Q|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Unfortunately, I don't believe there really is a better way of doing this due to the nature of Java's handling of primitive types, boxing, arrays and generics. In particular:

<ul>
<li><code>List&lt;T&gt;.toArray</code> won't work because there's no conversion from <code>Integer</code> to <code>int</code></li>
<li>You can't use <code>int</code> as a type argument for generics, so it would have to be an <code>int</code>-specific method (or one which used reflection to do nasty trickery).</li>
</ul>

I believe there are libraries which have autogenerated versions of this kind of method for all the primitive types (i.e. there's a template which is copied for each type). It's ugly, but that's the way it is I'm afraid :(

Even though the <a href="http://java.sun.com/javase/6/docs/api/java/util/Arrays.html" rel="noreferrer"><code>Arrays</code></a> class came out before generics arrived in Java, it would still have to include all the horrible overloads if it were introduced today (assuming you want to use primitive arrays).

blocks|key|892197|text|因为toArray返回一个Object[]，并且您不能从Object[]转换为int[]或从Integer[]转换为int[]，所以确实没有办法对您正在尝试做的事情进行“单行处理”|type|unstyled|depth|inlineStyleRanges|entityRanges|data|892198|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

There is really no way of "one-lining" what you are trying to do because toArray returns an Object[] and you cannot cast from Object[] to int[] or Integer[] to int[]

blocks|key|9276|text|最简单的方法是使用Apache+Commons+Lang。它有一个方便的ArrayUtils类，可以做你想做的事情。对Integer数组使用带有重载的toPrimitive方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|9277|List<Integer>+myList;
+...+assign+and+fill+the+list
int[]+intArray+=+ArrayUtils.toPrimitive(myList.toArray(new+Integer[myList.size()]));|code-block|syntax|javascript|9278|这样你就不会重复发明轮子了。Commons+Lang有很多Java遗漏的有用的东西。在上面，我选择创建一个大小合适的Integer列表。您还可以使用长度为0的静态整数数组，并让Java分配一个大小合适的数组：|9279|static+final+Integer[]+NO_INTS+=+new+Integer[0];
+++....
int[]+intArray2+=+ArrayUtils.toPrimitive(myList.toArray(NO_INTS));|9280|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/lang|1|http://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/ArrayUtils.html#toPrimitive%2528java.lang.Integer%255b%255d%2529^0|1N|7|23|B|9|J|0|23|B|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]|$9|16|A|17|1|18]]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

The easiest way to do this is to make use of <a href="http://commons.apache.org/lang" rel="noreferrer">Apache Commons Lang</a>. It has a handy ArrayUtils class that can do what you want. Use the <a href="http://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/ArrayUtils.html#toPrimitive%28java.lang.Integer%5b%5d%29" rel="noreferrer"><code>toPrimitive</code></a> method with the overload for an array of <code>Integer</code>s.

<pre><code>List&lt;Integer&gt; myList;
 ... assign and fill the list
int[] intArray = ArrayUtils.toPrimitive(myList.toArray(new Integer[myList.size()]));
</code></pre>

This way you don't reinvent the wheel. Commons Lang has a great many useful things that Java left out. Above, I chose to create an Integer list of the right size. You can also use a 0-length static Integer array and let Java allocate an array of the right size:

<pre><code>static final Integer[] NO_INTS = new Integer[0];
 ....
int[] intArray2 = ArrayUtils.toPrimitive(myList.toArray(NO_INTS));
</code></pre>

blocks|key|4110412|text|int[]+toIntArray(List<Integer>+list)++{
++++int[]+ret+=+new+int[list.size()];
++++int+i+=+0;
++++for+(Integer+e+:+list)++
++++++++ret[i%2B%2B]+=+e;
++++return+ret;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4110413|稍微修改代码以避免昂贵的列表索引(因为列表不一定是ArrayList，但可以是链表，对它的随机访问代价很高)|unstyled|4110414|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>int[] toIntArray(List&lt;Integer&gt; list) {
 int[] ret = new int[list.size()];
 int i = 0;
 for (Integer e : list) 
 ret[i++] = e;
 return ret;
}
</code></pre>

Slight change to your code to avoid expensive list indexing (since a List is not necessarily an ArrayList, but could be a linked list, for which random access is expensive)

blocks|key|892343|text|除了Commons+Lang之外，您还可以使用Guava的方法Ints.toArray(Collection+collection)来实现这一点|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|892344|List<Integer>+list+=+...
int[]+ints+=+Ints.toArray(list);|code-block|syntax|javascript|892345|这使您不必执行Commons+Lang等效项要求您自己进行的中间数组转换。|892346|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/google/guava|1|http://google.github.io/guava/releases/snapshot/api/docs/com/google/common/primitives/Ints.html#toArray(java.util.Collection))^0|V|Z|N|5|0|V|Z|1|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]|$9|12|A|13|1|14]]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]]]

In addition to Commons Lang, you can do this with <a href="https://github.com/google/guava" rel="noreferrer">Guava</a>'s method <a href="http://google.github.io/guava/releases/snapshot/api/docs/com/google/common/primitives/Ints.html#toArray(java.util.Collection)" rel="noreferrer"><code>Ints.toArray(Collection&lt;Integer&gt; collection)</code></a>:

<pre><code>List&lt;Integer&gt; list = ...
int[] ints = Ints.toArray(list);
</code></pre>

This saves you having to do the intermediate array conversion that the Commons Lang equivalent requires yourself.

blocks|key|9523|text|也可以尝试Dollar+(check+this+revision)：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|9524|import+static+com.humaorie.dollar.Dollar.*
...

List<Integer>+source+=+...;
int[]+ints+=+$(source).convert().toIntArray();|code-block|syntax|javascript|9525|entityMap|0|LINK|mutability|MUTABLE|url|http://bitbucket.org/dfa/dollar/|1|http://bitbucket.org/dfa/dollar/changeset/cea42757d873^0|5|6|0|D|J|1|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|F|7|Z|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

try also <a href="http://bitbucket.org/dfa/dollar/" rel="nofollow noreferrer">Dollar</a> (<a href="http://bitbucket.org/dfa/dollar/changeset/cea42757d873" rel="nofollow noreferrer">check this revision</a>):

<pre><code>import static com.humaorie.dollar.Dollar.*
...

List&lt;Integer&gt; source = ...;
int[] ints = $(source).convert().toIntArray();
</code></pre>

blocks|key|4110499|text|我会推荐你使用java+collection+API中的List<?>框架实现，它在这种特殊情况下似乎很有帮助：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4110500|package+mypackage;

import+java.util.AbstractList;
import+java.util.Arrays;
import+java.util.Collections;
import+java.util.List;

public+class+Test+{

//Helper+method+to+convert+int+arrays+into+Lists
static+List<Integer>+intArrayAsList(final+int[]+a)+{
++++if(a+==+null)
++++++++throw+new+NullPointerException();
++++return+new+AbstractList<Integer>()+{

++++++++@Override
++++++++public+Integer+get(int+i)+{
++++++++++++return+a[i];//autoboxing
++++++++}
++++++++@Override
++++++++public+Integer+set(int+i,+Integer+val)+{
++++++++++++final+int+old+=+a[i];
++++++++++++a[i]+=+val;//auto-unboxing
++++++++++++return+old;//autoboxing
++++++++}
++++++++@Override
++++++++public+int+size()+{
++++++++++++return+a.length;
++++++++}
++++};
}

public+static+void+main(final+String[]+args)+{
++++int[]+a+=+{1,+2,+3,+4,+5};
++++Collections.reverse(intArrayAsList(a));
++++System.out.println(Arrays.toString(a));
}
}|code-block|syntax|javascript|4110501|注意装箱/拆箱的缺点|4110502|entityMap^0|S|7|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

I would recommend you to use <code>List&lt;?&gt;</code> skeletal implementation from the java collections API, it appears to be quite helpful in this particular case:

<pre><code>package mypackage;

import java.util.AbstractList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Test {

//Helper method to convert int arrays into Lists
static List&lt;Integer&gt; intArrayAsList(final int[] a) {
 if(a == null)
 throw new NullPointerException();
 return new AbstractList&lt;Integer&gt;() {

 @Override
 public Integer get(int i) {
 return a[i];//autoboxing
 }
 @Override
 public Integer set(int i, Integer val) {
 final int old = a[i];
 a[i] = val;//auto-unboxing
 return old;//autoboxing
 }
 @Override
 public int size() {
 return a.length;
 }
 };
}

public static void main(final String[] args) {
 int[] a = {1, 2, 3, 4, 5};
 Collections.reverse(intArrayAsList(a));
 System.out.println(Arrays.toString(a));
}
}
</code></pre>

Beware of boxing/unboxing drawbacks

blocks|key|9347|text|int[]+ret+=+new+int[list.size()];+++++++
Iterator<Integer>+iter+=+list.iterator();
for+(int+i=0;+iter.hasNext();+i%2B%2B)+{+++++++
++++ret[i]+=+iter.next();++++++++++++++++
}++++++++++++++++++++++++++++++++++++++++
return+ret;++++++++++++++++++++++++++++++|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|9348|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>int[] ret = new int[list.size()]; 
Iterator&lt;Integer&gt; iter = list.iterator();
for (int i=0; iter.hasNext(); i++) { 
 ret[i] = iter.next(); 
} 
return ret; 
</code></pre>

blocks|key|4110531|text|使用lambda可以做到这一点(在jdk+lambda中编译)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4110532|public+static+void+main(String+ars[])+{
++++++++TransformService+transformService+=+(inputs)+->+{
++++++++++++int[]+ints+=+new+int[inputs.size()];
++++++++++++int+i+=+0;
++++++++++++for+(Integer+element+:+inputs)+{
++++++++++++++++ints[+i%2B%2B+]+=+element;
++++++++++++}
++++++++++++return+ints;
++++++++};

++++++++List<Integer>+inputs+=+new+ArrayList<Integer>(5)+{+{add(10);+add(10);}+};

++++++++int[]+results+=+transformService.transform(inputs);
++++}

++++public+interface+TransformService+{
++++++++int[]+transform(List<Integer>+inputs);
++++}|code-block|syntax|javascript|4110533|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Using a lambda you could do this (compiles in jdk lambda):

<pre><code>public static void main(String ars[]) {
 TransformService transformService = (inputs) -&gt; {
 int[] ints = new int[inputs.size()];
 int i = 0;
 for (Integer element : inputs) {
 ints[ i++ ] = element;
 }
 return ints;
 };

 List&lt;Integer&gt; inputs = new ArrayList&lt;Integer&gt;(5) { {add(10); add(10);} };

 int[] results = transformService.transform(inputs);
 }

 public interface TransformService {
 int[] transform(List&lt;Integer&gt; inputs);
 }
</code></pre>

blocks|key|9572|text|我再扔一个进去。我注意到了for循环的几种用法，但您甚至不需要在循环中使用任何东西。我之所以提到这一点，只是因为最初的问题是试图找到不那么冗长的代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|9573|int[]+toArray(List<Integer>+list)+{
++++int[]+ret+=+new+int[+list.size()+];
++++int+i+=+0;
++++for(+Iterator<Integer>+it+=+list.iterator();+
+++++++++it.hasNext();+
+++++++++ret[i%2B%2B]+=+it.next()+);
++++return+ret;
}|code-block|syntax|javascript|9574|如果Java允许像C%2B%2B那样在for循环中使用多个声明，我们可以更进一步，使用for(int+=+0，Iterator+it...|9575|最后(这部分只是我的意见)，如果你打算有一个帮助函数或方法来为你做一些事情，那就把它设置好，然后忘记它。它可以是一行，也可以是10；如果你再也不看它，你就不会知道其中的区别。|9576|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I'll throw one more in here. I've noticed several uses of for loops, but you don't even need anything inside the loop. I mention this only because the original question was trying to find less verbose code.

<pre><code>int[] toArray(List&lt;Integer&gt; list) {
 int[] ret = new int[ list.size() ];
 int i = 0;
 for( Iterator&lt;Integer&gt; it = list.iterator(); 
 it.hasNext(); 
 ret[i++] = it.next() );
 return ret;
}
</code></pre>

If Java allowed multiple declarations in a for loop the way C++ does, we could go a step further and do for(int i = 0, Iterator it...

In the end though (this part is just my opinion), if you are going to have a helping function or method to do something for you, just set it up and forget about it. It can be a one-liner or ten; if you'll never look at it again you won't know the difference.

blocks|key|9403|text|还没有人提到Java+8中添加的streams，所以它是这样的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|9404|int[]+array+=+list.stream().mapToInt(i->i).toArray();
//+OR
//+int[]+array+=+list.stream().mapToInt(Integer::intValue).toArray();|code-block|syntax|javascript|9405|9406|思考过程：|9407|9408|简单的Stream#toArray返回一个Object[]数组，所以它不是我们想要的。而且，因为泛型类型int|unordered-list-item|offset|length|style|CODE|9409|不能表示基本类型Stream#toArray(IntFunction<A[]>+generator)，所以A不能做我们想要的事情|9410|9411|所以有一些流可以处理原始类型int而不是包装器Integer会很好，因为它的toArray方法很可能也会返回一个int[]数组(在这里返回其他东西，如Object[]或甚至装箱的Integer[]将是不自然的)。幸运的是，Java8有这样一个流，就是IntStream|9412|9413|，|9414|，所以现在我们唯一需要弄清楚的就是如何将我们的Stream<Integer>+(将从list.stream()返回)转换成那个闪亮的IntStream。这里，Stream#mapToInt(ToIntFunction+mapper)方法应运而生。我们所需要做的就是向它传递一个从Integer到int的映射。|9415|9416|我们可以使用类似Integer#intValue的代码，它返回一个int，如下所示：|9417|mapToInt(+(Integer+i)+->+i.intValue()+)(或者一些人可能更喜欢：mapToInt(Integer::intValue)。)|9418|但是可以使用拆箱生成类似的代码，因为编译器知道这个lambda的结果必须是int类型的(+mapToInt中使用的lambda是ToIntFunction接口的一个实现，该接口期望一个类型为：int+applyAsInt(T+value)的方法作为主体，该方法期望返回一个int)。|9419|所以我们可以简单地写：|9420|mapToInt((整数i)->i)|9421|此外，由于(Integer+i)中的Integer类型可以由编译器推断，因为List<Integer>#stream()返回一个Stream<Integer>，所以我们也可以跳过它，这样我们就可以|9422|mapToInt(i+->+i)|9423|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html|1|https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#mapToInt-java.util.function.ToIntFunction-|2|https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#intValue--^0|0|0|0|0|0|3|E|L|8|1G|3|0|8|16|1H|1|0|0|E|3|N|7|12|7|1K|5|23|8|2H|9|3H|9|3H|9|0|0|0|0|N|F|16|D|1U|9|27|11|3V|7|43|3|27|11|1|0|0|8|G|X|3|8|G|2|0|1F|R|0|11|3|19|8|1S|D|2O|N|3S|3|0|0|0|5|B|I|7|12|M|1S|F|0|0^^$0|@$1|2|3|4|5|6|7|1S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1T|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|1U|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|1V|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|1W|8|@]|9|@]|A|$]]|$1|K|3|L|5|M|7|1X|8|@$N|1Y|O|1Z|P|Q]|$N|20|O|21|P|Q]|$N|22|O|23|P|Q]]|9|@]|A|$]]|$1|R|3|S|5|6|7|24|8|@$N|25|O|26|P|Q]|$N|27|O|28|P|Q]]|9|@]|A|$]]|$1|T|3|-4|5|6|7|29|8|@]|9|@]|A|$]]|$1|U|3|V|5|M|7|2A|8|@$N|2B|O|2C|P|Q]|$N|2D|O|2E|P|Q]|$N|2F|O|2G|P|Q]|$N|2H|O|2I|P|Q]|$N|2J|O|2K|P|Q]|$N|2L|O|2M|P|Q]|$N|2N|O|2O|P|Q]]|9|@$N|2P|O|2Q|1|2R]]|A|$]]|$1|W|3|-4|5|6|7|2S|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|2T|8|@]|9|@]|A|$]]|$1|Z|3|10|5|M|7|2U|8|@$N|2V|O|2W|P|Q]|$N|2X|O|2Y|P|Q]|$N|2Z|O|30|P|Q]|$N|31|O|32|P|Q]|$N|33|O|34|P|Q]|$N|35|O|36|P|Q]]|9|@$N|37|O|38|1|39]]|A|$]]|$1|11|3|-4|5|6|7|3A|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|3B|8|@$N|3C|O|3D|P|Q]|$N|3E|O|3F|P|Q]]|9|@$N|3G|O|3H|1|3I]]|A|$]]|$1|14|3|15|5|6|7|3J|8|@$N|3K|O|3L|P|Q]]|9|@]|A|$]]|$1|16|3|17|5|6|7|3M|8|@$N|3N|O|3O|P|Q]|$N|3P|O|3Q|P|Q]|$N|3R|O|3S|P|Q]|$N|3T|O|3U|P|Q]|$N|3V|O|3W|P|Q]]|9|@]|A|$]]|$1|18|3|19|5|6|7|3X|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|6|7|3Y|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|6|7|3Z|8|@$N|40|O|41|P|Q]|$N|42|O|43|P|Q]|$N|44|O|45|P|Q]|$N|46|O|47|P|Q]]|9|@]|A|$]]|$1|1E|3|1F|5|6|7|48|8|@]|9|@]|A|$]]|$1|1G|3|-4|5|6|7|49|8|@]|9|@]|A|$]]]|1H|$1I|$5|1J|1K|1L|A|$1M|1N]]|1O|$5|1J|1K|1L|A|$1M|1P]]|1Q|$5|1J|1K|1L|A|$1M|1R]]]]

No one mentioned yet streams added in Java 8 so here it goes:
<pre><code>int[] array = list.stream().mapToInt(i-&gt;i).toArray();
// OR
// int[] array = list.stream().mapToInt(Integer::intValue).toArray();
</code></pre>
<hr />
Thought process:
<ul>
<li>The simple <code>Stream#toArray</code> returns an <code>Object[]</code> array, so it is not what we want. Also, <code>Stream#toArray(IntFunction&lt;A[]&gt; generator)</code> doesn't do what we want because the generic type <code>A</code> can't represent the primitive type <code>int</code>
</li>
<li>So it would be nice to have some stream which could handle the primitive type <code>int</code> instead of the wrapper <code>Integer</code>, because its <code>toArray</code> method will most likely also return an <code>int[]</code> array (returning something else like <code>Object[]</code> or even boxed <code>Integer[]</code> would be unnatural here). And fortunately Java 8 has such a stream which is <a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html" rel="noreferrer"><code>IntStream</code></a>
</li>
<li>So now the only thing we need to figure out is how to convert our <code>Stream&lt;Integer&gt;</code> (which will be returned from <code>list.stream()</code>) to that shiny <code>IntStream</code>. Here the <a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#mapToInt-java.util.function.ToIntFunction-" rel="noreferrer"><code>Stream#mapToInt(ToIntFunction&lt;? super T&gt; mapper)</code></a> method comes to the rescue. All we need to do is pass it a mapping from <code>Integer</code> to <code>int</code>.
We could use something like <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#intValue--" rel="noreferrer"><code>Integer#intValue</code></a> which returns an <code>int</code> as shown below:
<pre><code>mapToInt( (Integer i) -&gt; i.intValue() ) 
</code></pre>
(or some may prefer: <code>mapToInt(Integer::intValue)</code>.)
But similar code can be generated using unboxing, since the compiler knows that the result of this lambda must be of type <code>int</code> (the lambda used in <code>mapToInt</code> is an implementation of the <code>ToIntFunction</code> interface which expects as body a method of type: <code>int applyAsInt(T value)</code> which is expected to return an <code>int</code>).
So we can simply write:
<pre><code>mapToInt((Integer i)-&gt;i)
</code></pre>
Also, since the <code>Integer</code> type in <code>(Integer i)</code> can be inferred by the compiler because <code>List&lt;Integer&gt;#stream()</code> returns a <code>Stream&lt;Integer&gt;</code>, we can also skip it which leaves us with
<pre><code>mapToInt(i -&gt; i)
</code></pre>
</li>
</ul>

blocks|key|4110671|text|如果您只是将Integer映射到int，那么您应该考虑using+parallelism，因为您的映射逻辑不依赖于其作用域之外的任何变量。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4110672|int[]+arr+=+list.parallelStream().mapToInt(Integer::intValue).toArray();|code-block|syntax|javascript|4110673|请注意这一点|4110674|4110675|请注意，并行并不会自动比串行执行操作更快，尽管如果您有足够的数据和处理器内核，就可以做到这一点。虽然聚合操作使您能够更轻松地实现并行性，但您仍有责任确定您的应用程序是否适合并行性。|blockquote|4110676|4110677|4110678|4110679|有两种方法可以将整数映射到它们的原始形式：|4110680|通过ToIntFunction的|4110681|。|ordered-list-item|4110682|4110683|带有λ表达式的mapToInt(Integer::intValue)|4110684|Via显式unboxing。|unordered-list-item|4110685|4110686|mapToInt(i+->+lambda+expression隐式(自动)拆箱。|4110687|mapToInt(i+->+i)|4110688|4110689|给定一个具有null值的列表|4110690|List<Integer>+list+=+Arrays.asList(1,+2,+null,+4,+5);|4110691|以下是处理null的三个选项|4110692|4110693|在映射之前过滤掉null值。|4110694|4110695|int[]+arr+=+list.parallelStream().filter(Objects::nonNull).mapToInt(Integer::intValue).toArray();|4110696|Map将null值设置为默认值。|4110697|4110698|int[]+arr+=+list.parallelStream().map(i+->+I+==+null？-1+:+lambda表达式中的i).mapToInt(Integer::intValue).toArray();|4110699|Handle+null。|4110700|4110701|i.intValue()).toArray();+arr+=+list.parallelStream().mapToInt(i+->+I+==+null？-1+:null|4110702|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html|1|https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html^0|6|7|G|3|R|H|0|0|0|0|0|0|0|0|0|0|2|D|0|0|0|0|5|8|1|0|0|0|0|0|6|4|0|0|5|4|0|0|8|4|0|0|0|4|4|0|0|0|7|4|0|0|0^^$0|@$1|2|3|4|5|6|7|28|8|@$9|29|A|2A|B|C]|$9|2B|A|2C|B|C]]|D|@$9|2D|A|2E|1|2F]]|E|$]]|$1|F|3|G|5|H|7|2G|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|2H|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|2I|8|@]|D|@]|E|$]]|$1|N|3|O|5|P|7|2J|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|2K|8|@]|D|@]|E|$]]|$1|R|3|-4|5|6|7|2L|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|2M|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|2N|8|@]|D|@]|E|$]]|$1|V|3|W|5|6|7|2O|8|@$9|2P|A|2Q|B|C]]|D|@]|E|$]]|$1|X|3|Y|5|Z|7|2R|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|2S|8|@]|D|@]|E|$]]|$1|11|3|12|5|6|7|2T|8|@]|D|@]|E|$]]|$1|13|3|14|5|15|7|2U|8|@]|D|@$9|2V|A|2W|1|2X]]|E|$]]|$1|16|3|-4|5|6|7|2Y|8|@]|D|@]|E|$]]|$1|17|3|18|5|6|7|2Z|8|@]|D|@]|E|$]]|$1|19|3|1A|5|6|7|30|8|@]|D|@]|E|$]]|$1|1B|3|-4|5|6|7|31|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|6|7|32|8|@$9|33|A|34|B|C]]|D|@]|E|$]]|$1|1E|3|1F|5|H|7|35|8|@]|D|@]|E|$I|J]]|$1|1G|3|1H|5|6|7|36|8|@$9|37|A|38|B|C]]|D|@]|E|$]]|$1|1I|3|-4|5|6|7|39|8|@]|D|@]|E|$]]|$1|1J|3|1K|5|Z|7|3A|8|@$9|3B|A|3C|B|C]]|D|@]|E|$]]|$1|1L|3|-4|5|6|7|3D|8|@]|D|@]|E|$]]|$1|1M|3|1N|5|6|7|3E|8|@]|D|@]|E|$]]|$1|1O|3|1P|5|15|7|3F|8|@$9|3G|A|3H|B|C]]|D|@]|E|$]]|$1|1Q|3|-4|5|6|7|3I|8|@]|D|@]|E|$]]|$1|1R|3|1S|5|6|7|3J|8|@]|D|@]|E|$]]|$1|1T|3|1U|5|15|7|3K|8|@$9|3L|A|3M|B|C]]|D|@]|E|$]]|$1|1V|3|-4|5|6|7|3N|8|@]|D|@]|E|$]]|$1|1W|3|1X|5|6|7|3O|8|@]|D|@]|E|$]]|$1|1Y|3|-4|5|6|7|3P|8|@]|D|@]|E|$]]]|1Z|$20|$5|21|22|23|E|$24|25]]|26|$5|21|22|23|E|$24|27]]]]

If you are simply mapping an <code>Integer</code> to an <code>int</code> then you should consider <a href="https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html" rel="noreferrer">using parallelism</a>, since your mapping logic does not rely on any variables outside its scope.

<pre><code>int[] arr = list.parallelStream().mapToInt(Integer::intValue).toArray();
</code></pre>

Just be aware of this 

<blockquote>
 Note that parallelism is not automatically faster than performing operations serially, although it can be if you have enough data and processor cores. While aggregate operations enable you to more easily implement parallelism, it is still your responsibility to determine if your application is suitable for parallelism.
</blockquote>

<hr>

There are two ways to map Integers to their primitive form:

<ol>
<li>Via a <code>ToIntFunction</code>.

<pre><code>mapToInt(Integer::intValue)
</code></pre></li>
<li>Via explicit <a href="https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html" rel="noreferrer">unboxing</a> with lambda expression.

<pre><code>mapToInt(i -&gt; i.intValue())
</code></pre></li>
<li>Via implicit (auto-) unboxing with lambda expression.

<pre><code>mapToInt(i -&gt; i)
</code></pre></li>
</ol>

<hr>

Given a list with a <code>null</code> value

<pre><code>List&lt;Integer&gt; list = Arrays.asList(1, 2, null, 4, 5);
</code></pre>

Here are three options to handle <code>null</code>:

<ol>
<li>Filter out the <code>null</code> values before mapping.

<pre><code>int[] arr = list.parallelStream().filter(Objects::nonNull).mapToInt(Integer::intValue).toArray();
</code></pre></li>
<li>Map the <code>null</code> values to a default value.

<pre><code>int[] arr = list.parallelStream().map(i -&gt; i == null ? -1 : i).mapToInt(Integer::intValue).toArray();
</code></pre></li>
<li>Handle <code>null</code> inside the lambda expression.

<pre><code>int[] arr = list.parallelStream().mapToInt(i -&gt; i == null ? -1 : i.intValue()).toArray();
</code></pre></li>
</ol>

blocks|key|9634|text|如果您有一个类型为java.util.List<Integer>的列表，则可以使用Eclipse+Collections执行以下操作|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|9635|List<Integer>+integers+=+Lists.mutable.with(1,+2,+3,+4,+5);
int[]+ints+=+LazyIterate.adapt(integers).collectInt(i+->+i).toArray();

Assert.assertArrayEquals(new+int[]{1,+2,+3,+4,+5},+ints);|code-block|syntax|javascript|9636|如果您已经有一个像MutableList这样的Eclipse+Collections类型，那么您可以执行以下操作：|9637|MutableList<Integer>+integers+=+Lists.mutable.with(1,+2,+3,+4,+5);
int[]+ints+=+integers.asLazy().collectInt(i+->+i).toArray();

Assert.assertArrayEquals(new+int[]{1,+2,+3,+4,+5},+ints);|9638|注意:我是Eclipse+Collections的提交者|9639|entityMap|0|LINK|mutability|MUTABLE|url|https://www.eclipse.org/collections/|1|https://www.eclipse.org/collections/javadoc/7.0.0/org/eclipse/collections/api/list/MutableList.html^0|9|N|15|J|0|0|0|9|B|9|B|1|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@$9|1A|A|1B|1|1C]]|E|$]]|$1|M|3|N|5|H|7|1D|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1E|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|1F|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]|Y|$5|T|U|V|E|$W|Z]]]]

With <a href="https://www.eclipse.org/collections/" rel="nofollow">Eclipse Collections</a>, you can do the following if you have a list of type <code>java.util.List&lt;Integer&gt;</code>:

<pre><code>List&lt;Integer&gt; integers = Lists.mutable.with(1, 2, 3, 4, 5);
int[] ints = LazyIterate.adapt(integers).collectInt(i -&gt; i).toArray();

Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, ints);
</code></pre>

If you already have an Eclipse Collections type like <a href="https://www.eclipse.org/collections/javadoc/7.0.0/org/eclipse/collections/api/list/MutableList.html" rel="nofollow"><code>MutableList</code></a>, you can do the following:

<pre><code>MutableList&lt;Integer&gt; integers = Lists.mutable.with(1, 2, 3, 4, 5);
int[] ints = integers.asLazy().collectInt(i -&gt; i).toArray();

Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, ints);
</code></pre>

Note: I am a committer for Eclipse Collections

blocks|key|4110757|text|这是Java8的单行代码|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|4110758|public+int[]+toIntArray(List<Integer>+intList){
+++++++return+intList.stream().mapToInt(Integer::intValue).toArray();
}|code-block|syntax|javascript|4110759|entityMap^0|2|5|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Here is Java 8 single line code for this

<pre><code>public int[] toIntArray(List&lt;Integer&gt; intList){
 return intList.stream().mapToInt(Integer::intValue).toArray();
}
</code></pre>

blocks|key|4110777|text|Java8为我们提供了一种通过streams实现这一点的简单方法……|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|4110778|使用集合stream()函数，然后映射到int，您将获得一个IntStream。使用IntStream，我们可以调用toArray()，它为我们提供了int+[]|CODE|4110779|int+[]+ints+=+list.stream().mapToInt(Integer::intValue).toArray();|code-block|syntax|javascript|4110780|to+int+[]|4110781|to+IntStream|4110782|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html|1|https://stackoverflow.com/questions/24633913/how-do-i-get-an-intstream-from-a-listinteger^0|0|5|0|4|8|16|9|23|6|0|0|0|9|0|0|0|C|1|0^^$0|@$1|2|3|4|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|14|8|@$9|15|A|16|B|H]|$9|17|A|18|B|H]|$9|19|A|1A|B|H]]|D|@]|E|$]]|$1|I|3|J|5|K|7|1B|8|@]|D|@]|E|$L|M]]|$1|N|3|O|5|6|7|1C|8|@]|D|@$9|1D|A|1E|1|1F]]|E|$]]|$1|P|3|Q|5|6|7|1G|8|@]|D|@$9|1H|A|1I|1|1J]]|E|$]]|$1|R|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]]|S|$T|$5|U|V|W|E|$X|Y]]|Z|$5|U|V|W|E|$X|10]]]]

Java 8 has given us an easy way to do this via streams...

Using the collections <code>stream()</code> function and then mapping to ints, you'll get an IntStream. With the <code>IntStream</code> we can call toArray() which gives us <code>int []</code>

<pre><code>int [] ints = list.stream().mapToInt(Integer::intValue).toArray();
</code></pre>

<a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html" rel="noreferrer">to int []</a>

<a href="https://stackoverflow.com/questions/24633913/how-do-i-get-an-intstream-from-a-listinteger">to IntStream</a>

blocks|key|9690|text|这个简单的循环总是正确的！没有bug|type|unstyled|depth|inlineStyleRanges|entityRanges|data|9691|++int[]+integers+=+new+int[myList.size()];
++for+(int+i+=+0;+i+<+integers.length;+i%2B%2B)+{
++++++integers[i]+=+myList.get(i);
++}|code-block|syntax|javascript|9692|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This simple loop is always correct! no bugs

<pre><code> int[] integers = new int[myList.size()];
 for (int i = 0; i &lt; integers.length; i++) {
 integers[i] = myList.get(i);
 }
</code></pre>

This is similar to this question:
<a href="https://stackoverflow.com/questions/880581/java-convert-int-to-integer">How to convert int[] to Integer[] in Java?</a>

I'm new to Java. How can i convert a <code>List&lt;Integer&gt;</code> to <code>int[]</code> in Java? I'm confused because <code>List.toArray()</code> actually returns an <code>Object[]</code>, which can be cast to nether <code>Integer[]</code> or <code>int[]</code>.

Right now I'm using a loop to do so:

<pre><code>int[] toIntArray(List&lt;Integer&gt; list){
 int[] ret = new int[list.size()];
 for(int i = 0;i &lt; ret.length;i++)
 ret[i] = list.get(i);
 return ret;
}
</code></pre>

I'm sure there's a better way to do this.

How to convert List<Integer> to int[] in Java?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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这类似于这个问题：我是Java新手。如何在Java中将List<Integer>转换为int[]？我很困惑，因为List.toArray()实际上返回了一个Object[]，它可以转换为Integer[]或int[]。现在我正在使用一个循环来做到这一点：int[] toIntArray(List<Integer> li...

问如何在Java中将List<Integer>转换为int[]？
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