一. ArrayList.subList()方法
ArrayList.subList(int fromIndex, int toIndex)方法返回一個List: 此List的值包含fromIndex所在的值, 但不包含toIndex所在的值. 如果fromIndex等于toIndex, 那么會返回一個沒有任何元素的空List. 返回的List支持java.util.List接口的所有操作.**
ArrayList.subList()方法返回的List不能強轉為ArrayList. **
ArrayList.subList()方法返回的List實際類型是ArrayList的一個內部類SubList, 即java.util.ArrayList$SubListArrayList.subList()方法返回的List是ArrayList中某段數據的一個視圖. 因此, 在操作此方法返回的List時, 同樣會改變ArrayList的數據. 我們來看看ArrayList.SubList這個類的幾個方法就知道了:
public E set(int index, E e) {
rangeCheck(index);
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}
public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}
我們知道ArrayList的內部是用對象數組(成員: elementData)存儲數據的, 可以看到ArrayList的內部類SubList的add和get方法操作的就是ArrayList內部的對象數組. 因此對ArrayList.subList()方法的返回值的任何操作都會反映到ArrayList對象本身.
如果原ArrayList對象有改動, 那么
ArrayList.subList()方法返回的視圖將會無效. 也就是說在操作ArrayList.subList()方法的返回值前不能對原ArrayList做任何修改, 否則你將得到一個java.util.ConcurrentModificationException異常.ArrayList的subList方法定義如下:
/**
* Returns a view of the portion of this list between the specified
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If
* {@code fromIndex} and {@code toIndex} are equal, the returned list is
* empty.) The returned list is backed by this list, so non-structural
* changes in the returned list are reflected in this list, and vice-versa.
* The returned list supports all of the optional list operations.
*
* <p>This method eliminates the need for explicit range operations (of
* the sort that commonly exist for arrays). Any operation that expects
* a list can be used as a range operation by passing a subList view
* instead of a whole list. For example, the following idiom
* removes a range of elements from a list:
* <pre>
* list.subList(from, to).clear();
* </pre>
* Similar idioms may be constructed for {@link #indexOf(Object)} and
* {@link #lastIndexOf(Object)}, and all of the algorithms in the
* {@link Collections} class can be applied to a subList.
*
* <p>The semantics of the list returned by this method become undefined if
* the backing list (i.e., this list) is <i>structurally modified</i> in
* any way other than via the returned list. (Structural modifications are
* those that change the size of this list, or otherwise perturb it in such
* a fashion that iterations in progress may yield incorrect results.)
*
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
更多細節可參考JDK中java.util.ArrayList類的源碼
二. Arrays.asList()方法
-
Arrays.asList(T ... args)方法將一個數組轉換成一個List.Arrays.asList()方法返回的對象是Arrays類的一個內部類, 并沒有實現修改集合的相關方法 (看源碼可以知道java.util.Arrasy$ArrayList類支持set()方法, 所以還是可以修改的). Arrays.asList()體現的是適配器模式, 只是轉換接口, 后臺數據仍是被轉換的數組. 因此: 對原數組的改動會影響返回的List, 反之亦然!! - 看源碼可以知道
Arrays.asList()的返回值是ArrayList, 但是請注意, 此ArrayList是Arrays類的一個內部類(java.util.Arrays$ArrayList), 而不是java.util.ArrayList. 因此不要把返回值強轉成java.util.ArrayList -
Arrays.asList()方法的返回值不支持add/remove/clear方法 (返回的List沒有實現修改操作), 如果調用, 你會得到一個UnsupportedOperationException異常. -
Arrays.asList()方法定義如下:
/**
* 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<String> 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);
}
此方法主要作用是: 用已存在的數組初始化一個固定大小的List.
三. 總結
- 對API使用有疑問的話, 最好是去看源碼(如果有源碼) 或 API文檔.
- 定期閱讀JDK源碼 (把其中常用的API吃透, 也就不會出現由于對API理解有誤而導致的錯誤!!)
