In App Engine, according to the JavaDoc , the getTypeRank
method has this signature:
public static int getTypeRank(java.lang.Class<? extends java.lang.Comparable> datastoreType)
In the method signature there is a question mark inside the angle brackets:
<? extends java.lang.Comparable>
What does it signify?
?
essentially indicates a wildcard. <? extends java.lang.Comparable>
<? extends java.lang.Comparable>
means "any type that extends java.lang.Comparable (or Comparable itself) can be used here".
It's called bounded wildcard
<? extends Comparable>
<? extends Comparable>
is an example of a bounded wildcard. The ? stands for an unknown type, just like the wildcards we saw earlier. However, in this case, we know that this unknown type is in fact a subtype of Comparable. (Note: It could be Comparableitself, or some subclass; it need not literally extend Comparable.)
More details you find here
它意味着“任何实现Comparable
接口的类。因此,调用将看起来像getTypeRank(String.class)
。
? refers to any subclass of java.lang.Comparable. In other words, any class that extends java.lang.Comparable.
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