Java中List与Array的转换

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在Java项目开发过程中，集合之间的互相转换是非常常见的，其中两个比较典型的转换是List和Array之间的转换，本文主要介绍这二者之间的转换、其中存在的一些问题以及解决方案，本文JDK版本为1.8。

文章作者：Tyan
博客：noahsnail.com

    在Java项目开发过程中，集合之间的互相转换是非常常见的，其中两个比较典型的转换是List和Array之间的转换，本文主要介绍这二者之间的转换、其中存在的一些问题以及解决方案，本文JDK版本为1.8。

1.Array To List

Array To List在Java中的方法是Arrays.asList()方法，这是在Java开发中常用的方法，在一般情况下使用这个方法将Array转为List都没问题，但要对转换后的List进行修改时会出现下面的异常： 

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Exception in thread "main" java.lang.UnsupportedOperationException
   at java.util.AbstractList.add(AbstractList.java:148)
   at java.util.AbstractList.add(AbstractList.java:108)
   at com.liu.test.Test.main(Test.java:17)

测试代码为： 

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public class Test {
   public static void main(String[] args) {
       String[] str = {"134", "test", "list"};
       List<String> list = Arrays.asList(str);
       list.add("");
    }
}

为什么会出现这个错误呢？看一下Arrays.asList()方法的源码及介绍

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/**
 * Returns a fixed-size list backed by the specified array.  (Changes to
 * the returned list "write through" to the array.)  This method acts
 * as bridge between array-based and collection-based APIs, in
 * combination with {@link Collection#toArray}.  The returned list is
 * serializable and implements {@link RandomAccess}.
 *
 * <p>This method also provides a convenient way to create a fixed-size
 * list initialized to contain several elements:
 * <pre>
 *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
 * </pre>
 *
 * @param <T> the class of the objects in the array
 * @param a the array by which the list will be backed
 * @return a list view of the specified array
 */
@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

注释中第一句话就说了，Arrays.asList()方法返回的是一个固定大小的List，如何来使返回的List可添加或删除元素呢?
方案一： 

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List<String> list = new ArrayList<String>(Arrays.asList(str));

方案二：使用Google Guava，需要import Lists： 

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List<String> list = Lists.newArrayList(str);

方案三：使用Apache Commons Collections，需要import CollectionUtils： 

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List<String> list = new ArrayList<String>();
CollectionUtils.addAll(list, str);

上面三种方案根据项目需要自己选择即可。

2.List To Array

List To Array在Java中的方法是list.toArray()方法，但这个方法有个问题是返回的数组对象为Object[]，直接用String[]去强制转换会报语法错误，直接点击toArray()方法去查看源码，当然也可以直接看官方API文档: 

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/**
 * Returns an array containing all of the elements in this list in proper
 * sequence (from first to last element).
 *
 * <p>The returned array will be "safe" in that no references to it are
 * maintained by this list.  (In other words, this method must
 * allocate a new array even if this list is backed by an array).
 * The caller is thus free to modify the returned array.
 *
 * <p>This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this list in proper
 *         sequence
 * @see Arrays#asList(Object[])
 */
Object[] toArray();
/**
 * Returns an array containing all of the elements in this list in
 * proper sequence (from first to last element); the runtime type of
 * the returned array is that of the specified array.  If the list fits
 * in the specified array, it is returned therein.  Otherwise, a new
 * array is allocated with the runtime type of the specified array and
 * the size of this list.
 *
 * <p>If the list fits in the specified array with room to spare (i.e.,
 * the array has more elements than the list), the element in the array
 * immediately following the end of the list is set to <tt>null</tt>.
 * (This is useful in determining the length of the list <i>only</i> if
 * the caller knows that the list does not contain any null elements.)
 *
 * <p>Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs.  Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * <p>Suppose <tt>x</tt> is a list known to contain only strings.
 * The following code can be used to dump the list into a newly
 * allocated array of <tt>String</tt>:
 *
 * <pre>{@code
 *     String[] y = x.toArray(new String[0]);
 * }</pre>
 *
 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 * <tt>toArray()</tt>.
 *
 * @param a the array into which the elements of this list are to
 *          be stored, if it is big enough; otherwise, a new array of the
 *          same runtime type is allocated for this purpose.
 * @return an array containing the elements of this list
 * @throws ArrayStoreException if the runtime type of the specified array
 *         is not a supertype of the runtime type of every element in
 *         this list
 * @throws NullPointerException if the specified array is null
 */
<T> T[] toArray(T[] a);

从源码说明中可以看出，如果想要list.toArray()方法返回指定类型的数组，需要在toArray方法中加上一个数组类型的参数，如果参数数组个数大于list数组则将数组中多余的元素赋为空，如果数组大小不够会重新分配一个指定类型与list大小一致的数组。因此，List To Array可以写成： 

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list.toArray(new String[0]);
or
list.toArray(new String[list.size()]); //better
or 
list.stream().toArray(String[]::new); //JDK 1.8
如果有收获，可以请我喝杯咖啡！