关于jdk的java.util.RandomAccess

 

RandomAccess是一个声明接口，主要用来区分对List不同实例的访问策略。比如ArrayList是顺序访问的策略最快，也就是RandomAccess随机访问；LinkedList是链表实现，使用迭代器一个个的访问，不去实现RandomAccess接口，表示它不支持随机访问（下标位置）。具体内容看下面jdk的描述。在使用List的接口的时候最好可以根据RandomAccess来判断实例的访问策略，加快访问list的性能。

举例代码如下：

    public static <T> List<T> unmodifiableList(List<? extends T> list) {
        return (list instanceof RandomAccess ?
                new UnmodifiableRandomAccessList<>(list) :
                new UnmodifiableList<>(list));
    }

 

 

jdk的原文描述 写道

Marker interface used by List implementations to indicate that they support fast (generally constant time) random access. The primary purpose of this interface is to allow generic algorithms to alter their behavior to provide good performance when applied to either random or sequential access lists.

The best algorithms for manipulating random access lists (such as ArrayList) can produce quadratic behavior when applied to sequential access lists (such as LinkedList). Generic list algorithms are encouraged to check whether the given list is an instanceof this interface before applying an algorithm that would provide poor performance if it were applied to a sequential access list, and to alter their behavior if necessary to guarantee acceptable performance.

It is recognized that the distinction between random and sequential access is often fuzzy. For example, some List implementations provide asymptotically linear access times if they get huge, but constant access times in practice. Such a List implementation should generally implement this interface. As a rule of thumb, a List implementation should implement this interface if, for typical instances of the class, this loop:

for (int i=0, n=list.size(); i < n; i++)
list.get(i);

runs faster than this loop:
for (Iterator i=list.iterator(); i.hasNext(); )
i.next();

This interface is a member of the Java Collections Framework.

Since:
1.4