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);
}复制代码
/**
* @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 = Objects.requireNonNull(array);
}
@Override
public int size() {
return a.length;
}
@Override
public Object[] toArray() {
return a.clone();
}
@Override
@SuppressWarnings("unchecked")复制代码
AbstractList源码：
复制代码
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
* {@link #add(int, Object) add(int, E)} is overridden.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link 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();
}复制代码
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 = Objects.requireNonNull(array);
}
看一段这样的代码：
复制代码
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());
}}
复制代码
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 Object[] toArray() {
return a.clone();
}复制代码
@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;
}复制代码
对于第一个重载方法，是将list直接转为Object[] 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:
复制代码
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();
复制代码
结果一运行，报错：
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
第一个方法不能将Object[] 转化为String[]，我们需要修改为：
1 Object[] 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：
复制代码
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+" ");
}
}}
复制代码
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
