java中使用点运算符访问通用列表的奇数方法调用 [英] Odd method call in java using a dot operator to access a generic list

问题描述

我遇到了一些高级 Java 代码(对我来说是高级的 :) )我需要帮助理解.

I came across some advanced java code (advanced for me :) ) I need help understanding.

在一个类中有一个嵌套类，如下所示:

In a class there is a nested class as below:

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);
    }
}

稍后在外部类中，创建了一个可调用类的实例.我不知道这是什么:

Later in the outer class, there is an instance of the callable class being created. I have no idea what this is:

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

对我来说它看起来不像 java 语法.你能详细说明一下这个神秘的语法是怎么回事吗?您可以在下面的代码摘录中看到它正在使用.

It doesn't look like java syntax to me. Could you please elaborate what's going on in this cryptic syntax? You can see it being used below in the code excerpt.

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

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

已编辑添加了 ConnectionHelper 类.

EDITED added the ConnectionHelper class.

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;
    }

推荐答案

您更多地认为类是通用的，但方法也可以是通用的.一个常见的例子是 <代码>Arrays.asList.

You more often think of classes as being generic, but methods can be generic too. A common example is Arrays.asList.

大多数情况下，您不必使用带尖括号 <...> 的语法，即使在调用泛型方法时也是如此，因为这是一个在某些情况下，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:

Most of the time, you don't have to use the syntax with angle brackets <...>, even when you're invoking a generic method, because this is the one place in which the Java compiler is actually capable of doing basic type inference in some circumstances. For example, the snippet given in the Arrays.asList documentation omits the type:

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

但它等同于显式给出泛型类型的这个版本:

But it's equivalent to this version in which the generic type is given explicitly:

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

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