abstract：今天看代码时, 发现书上使用了Arrays.asList()方法, 将一个数组转成了List, 然后说到得到的List不能调用add(), remove()方法添加元素或者删除,带着疑问看了下内部实现原理, 看了别人博客明白了, 为啥不能调用add(), remove()方法了.下面是转载的一篇文章.Arrays工具类提供了一些比较实用的方法，比如sort, binarySearch, fill等
今天看代码时, 发现书上使用了Arrays.asList()方法, 将一个数组转成了List, 然后说到得到的List不能调用add(), remove()方法添加元素或者删除,带着疑问看了下内部实现原理, 看了别人博客明白了, 为啥不能调用add(), remove()方法了.下面是转载的一篇文章.
Arrays工具类提供了一些比较实用的方法，比如sort, binarySearch, fill等。其中还有一个asList方法，此方法能够将一个变长参数或者数组转换成List。
但是，这个生成的List，它是固定长度的，如果对其进行add或者remove的操作，会抛出UnsupportedOperationException，为什么会这样呢？
带着疑问，查看一下Arrays的源码，可以得到问题的结果。
/**
* 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 <tt>Collection.toArray</tt>. 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<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
* </pre>
*
* @param a the array by which the list will be backed.
* @return a list view of the specified array.
* @see Collection#toArray()
*/
public static <T> List<T> asList(T... a) {
return new ArrayList<T>(a);
}方法asList返回的是new ArrayList<T>(a)。但是，这个ArrayList并不是java.util.ArrayList，它是一个Arrays类中的重新定义的内部类。
具体的实现如下：
/**
* @serial include
*/
private static class ArrayList<E> extends AbstractList<E>
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private Object[] a;
ArrayList(E[] array) {
if (array==null)
throw new NullPointerException();
a = array;
}
public int size() {
return a.length;
}
public Object[] toArray() {
return (Object[])a.clone();
}
public E get(int index) {
return (E)a[index];
}
public E set(int index, E element) {
Object oldValue = a[index];
a[index] = element;
return (E)oldValue;
}
public int indexOf(Object o) {
if (o==null) {
for (int i=0; i<a.length; i++)
if (a[i]==null)
return i;
} else {
for (int i=0; i<a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
public boolean contains(Object o) {
return indexOf(o) != -1;
}
}从这个内部类ArrayList的实现可以看出，它继承了类AbstractList<E>,但是没有重写add和remove方法，没有给出具体的实现。查看一下AbstractList类中对add和remove方法的定义，如果一个list不支持add和remove就会抛出UnsupportedOperationException。
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractList() {
}
/**
* Appends the specified element to the end of this List (optional
* operation). <p>
*
* This implementation calls <tt>add(size(), o)</tt>.<p>
*
* Note that this implementation throws an
* <tt>UnsupportedOperationException</tt> unless <tt>add(int, Object)</tt>
* is overridden.
*
* @param o element to be appended to this list.
*
* @return <tt>true</tt> (as per the general contract of
* <tt>Collection.add</tt>).
*
* @throws UnsupportedOperationException if the <tt>add</tt> method is not
* supported by this Set.
*
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this set.
*
* @throws IllegalArgumentException some aspect of this element prevents
* it from being added to this collection.
*/
public boolean add(E o) {
add(size(), o);
return true;
}
/**
* Inserts the specified element at the specified position in this list
* (optional operation). Shifts the element currently at that position
* (if any) and any subsequent elements to the right (adds one to their
* indices).<p>
*
* This implementation always throws an UnsupportedOperationException.
*
* @param index index at which the specified element is to be inserted.
* @param element element to be inserted.
*
* @throws UnsupportedOperationException if the <tt>add</tt> method is not
* supported by this list.
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this list.
* @throws IllegalArgumentException if some aspect of the specified
* element prevents it from being added to this list.
* @throws IndexOutOfBoundsException index is out of range (<tt>index <
* 0 || index > size()</tt>).
*/
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
/**
* Removes the element at the specified position in this list (optional
* operation). Shifts any subsequent elements to the left (subtracts one
* from their indices). Returns the element that was removed from the
* list.<p>
*
* This implementation always throws an
* <tt>UnsupportedOperationException</tt>.
*
* @param index the index of the element to remove.
* @return the element previously at the specified position.
*
* @throws UnsupportedOperationException if the <tt>remove</tt> method is
* not supported by this list.
* @throws IndexOutOfBoundsException if the specified index is out of
* range (<tt>index < 0 || index >= size()</tt>).
*/
public E remove(int index) {
throw new UnsupportedOperationException();
}
}至此，为什么Arrays.asList产生的List是不可添加或者删除，否则会产生UnsupportedOperationException，就可以得到解释了。
同时我们可以用List来接受Arrays.asList(array)返回的参数、原因是Arrays.asList(array)返回的虽然不是java.util.ArrayList但是返回的ArrayList同理也继承自AbstractList
我们的AbstractList实现自List接口、所以可以用list接口来引用Arrays.asList(array)；
如果我们想把一个变长或者数据转变成List， 而且期望这个List能够进行add或者remove操作，那该怎么做呢？
我们可以写一个类似的方法，里面直接采用java.util.ArrayList即可。
比如：
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class MyArrays {
public static <T> List<T> asList(T... a) {
List<T> list = new ArrayList<T>();
Collections.addAll(list, a);
return list;
}
}测试代码如下：
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Test {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
print(stooges);
List<List<String>> seasonsList = Arrays.asList(retrieveSeasonsList());
print(seasonsList);
/*
* 自己实现一个asList方法，能够添加和删除。
*/
List<String> list = MyArrays.asList("Larry", "Moe", "Curly");
list.add("Hello");
print(list);
}
private static <T> void print(List<T> list) {
System.out.println(list);
}
private static List<String> retrieveSeasonsList() {
List<String> seasonsList = new ArrayList<String>();
seasonsList.add("Spring");
seasonsList.add("Summer");
seasonsList.add("Autumn");
seasonsList.add("Winter");
return seasonsList;
}
}输出结果：
[Larry, Moe, Curly] [[Spring, Summer, Autumn, Winter]] [Larry, Moe, Curly, Hello]