Java数组转集合与集合转数组的坑

Java数组转集合与集合转数组的坑
在Java中将数组转为集合，会用到Arrays.asList()的方法，然而，这个方法却与我们的预期期望存在一些出入，当用到asList方法将数组转化成List列表时，对得到的List列表进行add()和remove()操作, JVM会抛出异常：java.lang.UnsupportedOperationException异常

Arrays.asList返回的是同样的ArrayList，为什么就不能使用add和remove方法呢？

接下来我们来看一下Arrays.asList 源码

public static List asList(T... a) {

    return new ArrayList<>(a);}

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/**

 * @serial include */private static class ArrayList<E> extends AbstractList<E>    implements RandomAccess, java.io.Serializable{    private static final long serialVersionUID = -2764017481108945198L;    private final E[] a;    ArrayList(E[] array) {        a =  s.requireNonNull(array);    }    @Override    public int size() {        return a.length;    }    @Override    public  [] toArray() {        return a.clone();    }    @Override    @SuppressWarnings("unchecked")

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AbstractList源码：

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public abstract class AbstractList extends AbstractCollection implements List {

/** * Sole constructor.  (For invocation by subclass constructors, typically * implicit.) */protected AbstractList() {}/** * Appends the specified element to the end of this list (optional * operation). * * <p>Lists that support this operation may place limitations on what * elements may be added to this list.  In particular, some * lists will refuse to add null elements, and others will impose * restrictions on the type of elements that may be added.  List * classes should clearly specify in their documentation any restrictions * on what elements may be added. * * <p>This implementation calls {@code add(size(), e)}. * * <p>Note that this implementation throws an * {@code UnsupportedOperationException} unless * {@  #add(int,  ) add(int, E)} is overridden. * * @param e element to be appended to this list * @return {@code true} (as specified by {@  Collection#add}) * @throws UnsupportedOperationException if the {@code add} operation *         is not supported by this list * @throws ClassCastException if the class of the specified element *         prevents it from being added to this list * @throws NullPointerException if the specified element is null and this *         list does not permit null elements * @throws IllegalArgumentException if some property of this element *         prevents it from being added to this list */public boolean add(E e) {    add(size(), e);    return true;}/** * {@inheritDoc} * * @throws IndexOutOfBoundsException {@inheritDoc} */abstract public E get(int index);/** * {@inheritDoc} * * <p>This implementation always throws an * {@code UnsupportedOperationException}. * * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException            {@inheritDoc} * @throws NullPointerException          {@inheritDoc} * @throws IllegalArgumentException      {@inheritDoc} * @throws IndexOutOfBoundsException     {@inheritDoc} */public E set(int index, E element) {    throw new UnsupportedOperationException();}/** * {@inheritDoc} * * <p>This implementation always throws an * {@code UnsupportedOperationException}. * * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException            {@inheritDoc} * @throws NullPointerException          {@inheritDoc} * @throws IllegalArgumentException      {@inheritDoc} * @throws IndexOutOfBoundsException     {@inheritDoc} */public void add(int index, E element) {    throw new UnsupportedOperationException();}/** * {@inheritDoc} * * <p>This implementation always throws an * {@code UnsupportedOperationException}. * * @throws UnsupportedOperationException {@inheritDoc} * @throws IndexOutOfBoundsException     {@inheritDoc} */public E remove(int index) {    throw new UnsupportedOperationException();}

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由源码可见，UnsupportedOperationException 是AbstractList抛出的，因为Arrays中的ArrayList并没有实现remove()和add()方法，所以抛出了异常。
所以说 Arrays.asList 返回的 List 是一个不可变长度的列表，此列表不再具备原 List 的很多特性，因此慎用 Arrays.asList 方法。

ArrayList中构造方法：

ArrayList(E[] array) {

        a =  s.requireNonNull(array);    }

看一段这样的代码：

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import java.util.Arrays;
import java.util.List;

public class Test2{

public static void main(String[] args) {    int[] array = new int[]{1,2,3,4,5};    List list = Arrays.asList(array);    System.out.println(list.size());}

}
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Arrays 的内部类 ArrayList 构造方法接收的是一个泛型数组，即数组类型不能为基本类型，应该为其对应的包装类型(传即入引用类型)，否则传入的基本类型的整个数组，将被编译器视为一个引用参数，

所以原始类型不能作为 Arrays.asList 方法的参数，否则会被当做一个参数

再来看看在Java中将集合转为数组，ArrayList提供了一个将List转为数组的一个非常方便的方法toArray。toArray有两个重载的方法：

@Override

    public  [] toArray() {        return a.clone();    }

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@Override

    @SuppressWarnings("unchecked")    public <T> T[] toArray(T[] a) {        int size = size();        if (a.length < size)            return Arrays.copyOf(this.a, size,                                 (Class<? extends T[]>) a.getClass());        System.arraycopy(this.a, 0, a, 0, size);        if (a.length > size)            a[size] = null;        return a;    }

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对于第一个重载方法，是将list直接转为 [] 数组；

第二种方法是将list转化为你所需要类型的数组，当然我们用的时候会转化为与list内容相同的类型。

若采用第一种toArray方法，是这样写:

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1 ArrayList list=new ArrayList();
2 for (int i = 0; i < 10; i++) {
3 list.add(""+i);
4 }
5
6 String[] array= (String[]) list.toArray();
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结果一运行，报错：

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang. ; cannot be cast to [Ljava.lang.String;

第一个方法不能将 [] 转化为String[]，我们需要修改为：

1 [] arr = list.toArray();
2 for (int i = 0; i < arr.length; i++) {
3 String e = (String) arr[i];
4 System.out.println(e);
5 }
建议用第二种方法：

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import java.util.ArrayList;
import java.util.List;

public class Test2{

public static void main(String[] args) {    List<Integer> list = new ArrayList();    list.add(1);    list.add(2);    list.add(3);    Integer[] array = new Integer[list.size()];    list.toArray(array);    for (int i:array){        System.out.print(i+" ");    }}

}
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或者

set.toArray(new Integer[set.size()]);
set好处是去除重复元素
原文地址https://www.cnblogs.com/hetaoyuan/archive/2019/07/24/11236999.html