使用点运算符在Java中进行奇数方法调用以访问通用列表

Question

我遇到了一些高级Java代码（对我来说是高级的:)），我需要帮助来理解。

在一个类中，有一个嵌套的类，如下所示：

private final class CoverageCRUDaoCallable implements
        Callable<List<ClientCoverageCRU>>
{
    private final long oid;
    private final long sourceContextId;

    private CoverageCRUDaoCallable(long oid, long sourceContextId)
    {
        this.oid = oid;
        this.sourceContextId = sourceContextId;
    }

    @Override
    public List<ClientCoverageCRU> call() throws Exception
    {
        return coverageCRUDao.getCoverageCRUData(oid, sourceContextId);
    }
}

在外部类的后面，有一个正在创建的可调用类的实例。我不知道这是什么：

ConnectionHelper.<List<ClientCoverageCRU>> tryExecute(coverageCRUDaoCallable);

在我看来，它看起来不像Java语法。您能否详细说明这种神秘的语法是怎么回事？您可以在下面的代码摘录中看到它的使用情况。

CoverageCRUDaoCallable coverageCRUDaoCallable = new CoverageCRUDaoCallable(
        dalClient.getOid(), sourceContextId);

// use Connection helper to make coverageCRUDao call.
List<ClientCoverageCRU> coverageCRUList = ConnectionHelper
        .<List<ClientCoverageCRU>> tryExecute(coverageCRUDaoCallable);

编辑 添加了ConnectionHelper类。

public class ConnectionHelper<T>
{
    private static final Logger logger =
        LoggerFactory.getLogger(ConnectionHelper.class);

    private static final int    CONNECTION_RETRIES = 3;

    private static final int    MIN_TIMEOUT        = 100;

    public static <T> T tryExecute(Callable<T> command)
    { 
        T returnValue = null;
        long delay = 0;

        for (int retry = 0; retry < CONNECTION_RETRIES; retry++)
        { 
            try
            { 
                // Sleep before retry
                Thread.sleep(delay);

                if (retry != 0)
                {
                    logger.info("Connection retry #"+ retry);
                }

                // make the actual connection call
                returnValue = command.call();
                break;

            } 
            catch (Exception e)
            {
                Throwable cause = e.getCause();
                if (retry == CONNECTION_RETRIES - 1)
                {
                    logger.info("Connection retries have exhausted. Not trying "                        
                            + "to connect any more.");

                    throw new RuntimeException(cause);
                }

                // Delay increased exponentially with every retry.
                delay = (long) (MIN_TIMEOUT * Math.pow(2, retry));

                String origCause = ExceptionUtils.getRootCauseMessage(e);

                logger.info("Connection retry #" + (retry + 1)
                        + " scheduled in " + delay + " msec due to "
                        + origCause);
                        + origCause);
            }
        }
        return returnValue;
    }

Answer 1

您通常会认为类是通用的，但是方法也可以是通用的。一个常见的例子是Arrays.asList。

大多数时候，<...>即使在调用通用方法时，也不必在尖括号中使用语法，因为在某些情况下，这是Java编译器实际上能够进行基本类型推断的地方。例如，Arrays.asList文档中给出的代码片段省略了类型：

List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");

但这等效于显式给出泛型的该版本：

List<String> stooges = Arrays.<String>asList("Larry", "Moe", "Curly");