我使用cqengine进行集合索引:
<VirtualHost *:80>
ServerAdmin stuart.brown@example.co.uk
ServerName stuart.com
DocumentRoot /var/www/stuart.com/public_html
Alias /stuart.com "/var/www/stuart.com/public_html"
<Directory "/var/www/stuart.com/public_html">
Options None
AllowOverride None
Order allow,deny
Allow from all
</Directory>
</VirtualHost>
此代码未编译,因为:
Attribute<UtilizationEntry, LocalDate> startDateAttr = new SimpleAttribute<UtilizationEntry, LocalDate>() {
@Override
public LocalDate getValue(UtilizationEntry object, QueryOptions queryOptions) {
return object.getStartDate();
}
};
IndexedCollection<UtilizationEntry> entries = new ConcurrentIndexedCollection<>();
entries.addIndex(NavigableIndex.onAttribute(startDateAttr)); // compilation error in this line
可能是缺少库设计:Error:(61, 70) java: no suitable method found for onAttribute(com.googlecode.cqengine.attribute.Attribute<entities.UtilizationEntry,java.time.LocalDate>)
method com.googlecode.cqengine.index.navigable.NavigableIndex.<A,O>onAttribute(com.googlecode.cqengine.attribute.Attribute<O,A>) is not applicable
(inferred type does not conform to equality constraint(s)
inferred: java.time.chrono.ChronoLocalDate
equality constraints(s): java.time.chrono.ChronoLocalDate,java.time.LocalDate)
method com.googlecode.cqengine.index.navigable.NavigableIndex.<A,O>onAttribute(com.googlecode.cqengine.index.support.Factory<java.util.concurrent.ConcurrentNavigableMap<A,com.googlecode.cqengine.resultset.stored.StoredResultSet<O>>>,com.googlecode.cqengine.index.support.Factory<com.googlecode.cqengine.resultset.stored.StoredResultSet<O>>,com.googlecode.cqengine.attribute.Attribute<O,A>) is not applicable
(cannot infer type-variable(s) A,O
(actual and formal argument lists differ in length))
实现扩展LocalDate
的{{1}},这意味着必须在泛型方法声明中使用通配符边界。我想,在这种情况下,创建实现ChronoLocalDate
的{{1}}包装器是一种解决方法。
也许,这个问题有其他一些解决方案吗?
答案 0 :(得分:5)
我认为,创建新类(如包装器)会导致修复通用签名错误的开销很大。我会创建一个像
这样的委托方法@SuppressWarnings("unchecked")
static <A extends Comparable<A>,O>
NavigableIndex<A,O> onAttribute(Attribute<O,? extends A> attribute) {
return NavigableIndex.onAttribute((Attribute)attribute);
}
实际上,问题在于A extends Comparable<A>
声明应该是A extends Comparable<? super A>
,但是在这里修复它无济于事,因为这种模式在整个API中传播并允许这样的返回类型只是在下一个API使用时引发错误。但是,据我所知,Attribute
仅生成A
但不消费它们,我们可以在此处放宽签名并接受Attribute<…,? extends A>
,跟随PECS模式。
使用此帮助程序,您的代码应该顺利编译:
IndexedCollection<UtilizationEntry> entries = new ConcurrentIndexedCollection<>();
entries.addIndex(onAttribute(startDateAttr));