Arrays工具類提供了一些比較實用的方法，比如sort, binarySearch, fill等。其中還有一個asList方法，此方法能夠將一個變長參數或者數組轉換成List。
但是，這個生成的List，它是固定長度的，如果對其進行add或者remove的操作，會拋出UnsupportedOperationException，為什么會這樣呢?
帶著疑問，查看一下Arrays的源碼，可以得到問題的結果。
Java代碼
/**
* 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 Collection.toArray. The returned list is
* serializable and implements {@link RandomAccess}.
*
*
This method also provides a convenient way to create a fixed-size
* list initialized to contain several elements:
*
* List stooges = Arrays.asList("Larry", "Moe", "Curly");
*
*
* @param a the array by which the list will be backed.
* @return a list view of the specified array.
* @see Collection#toArray()
*/
public static List asList(T... a) {
return new ArrayList(a);
方法asList返回的是new ArrayList(a)。但是，這個ArrayList并不是java.util.ArrayList，它是一個Arrays類中的重新定義的內部類。
具體的實現如下：
Java代碼
/**
* @serial include
*/
private static class ArrayList extends AbstractList
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
if (a[i]==null)
return i;
} else {
for (int i=0; i
if (o.equals(a[i]))
return i;
}
return -1;
}
public boolean contains(Object o) {
return indexOf(o) != -1;
}
}
從這個內部類ArrayList的實現可以看出，它繼承了類AbstractList,但是沒有重寫add和remove方法，沒有給出具體的實現。查看一下AbstractList類中對add和remove方法的定義，如果一個list不支持add和remove就會拋出UnsupportedOperationException。
Java代碼
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).
*
* This implementation calls add(size(), o).
*
* Note that this implementation throws an
* UnsupportedOperationException unless add(int, Object)
* is overridden.
*
* @param o element to be appended to this list.
*
* @return true (as per the general contract of
* Collection.add).
*
* @throws UnsupportedOperationException if the add 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).
*
* 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 add 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 (index <
* 0 || index > size()).
*/
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.
*
* This implementation always throws an
* UnsupportedOperationException.
*
* @param index the index of the element to remove.
* @return the element previously at the specified position.
*
* @throws UnsupportedOperationException if the remove method is
* not supported by this list.
* @throws IndexOutOfBoundsException if the specified index is out of
* range (index < 0 || index >= size()).
*/
public E remove(int index) {
throw new UnsupportedOperationException();
}
}
至此，為什么Arrays.asList產生的List是不可添加或者刪除，否則會產生UnsupportedOperationException，就可以得到解釋了。
如果我們想把一個變長或者數據轉變成List， 而且tb期望這個List能夠進行add或者remove操作，那該怎么做呢?
我們可以寫一個類似的方法，里面直接采用java.util.ArrayList即可。
比如：
Java代碼
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class MyArrays {
public static List asList(T... a) {
List list = new ArrayList();
Collections.addAll(list, a);
return list;
}
}
測試代碼如下：
Java代碼
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Test {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
List stooges = Arrays.asList("Larry", "Moe", "Curly");
print(stooges);
List> seasonsList = Arrays.asList(retrieveSeasonsList());
print(seasonsList);
/*
* 自己實現一個asList方法，能夠添加和刪除。
*/
List list = MyArrays.asList("Larry", "Moe", "Curly");
list.add("Hello");
print(list);
}
private static void print(List list) {
System.out.println(list);
}
private static List retrieveSeasonsList() {
List seasonsList = new ArrayList();
seasonsList.add("Spring");
seasonsList.add("Summer");
seasonsList.add("Autumn");
seasonsList.add("Winter");
return seasonsList;
}
}
輸出結果：
[Larry, Moe, Curly]
[[Spring, Summer, Autumn, Winter]]
[Larry, Moe, Curly, Hello]