为什么T扩展了Comparable <? super T> 包括T？ 含义包括可比较 <T> ？

Question

class BinarySearch<T extends Comparable<? super T> >

Why is T extends Comparable <? super T> T extends Comparable <? super T> including T , meaning including Comparable<T> and not just the super class hierarchy? I am confused on the super keyword as I thought super would only include super class items. I am new to java and in the Java book it had the following example:

This is regarding a method in a class that was trying to limit the upper and lower bound of the hierarchy using the Java.awt.Component so the class had extends Container

class CustomComponent<T extends Container>

in the class they have the following method

void describeComponent<CustomComponent<? super JPasswordField> ref)

and then goes on to say

Note that JPasswordField, which is a superclass of JTextField, itself is omitted in the list of permissible objects.

Answer 1

From Lower Bounded Wildcards , a section of the Generics section of The Java Tutorial :

... a lower bounded wildcard restricts the unknown type to be a specific type or a super type of that type.

^{(bold mine, emphasis theirs)}

Thus, the set of classes that match T extends Comparable<T> is a subset of the set of classes that match T extends Comparable<? super T> T extends Comparable<? super T> . The wildcard ? super T ? super T itself matches T and any superclasses of T .

In other words, the assumption that " super would only include super class items" is simply incorrect.

Your confusion in the example may also arise from the fact that JTextField is a superclass of JPasswordField ; in other words, JPasswordField extends JTextField . The example would match any of the following classes:

• javax.swing.JPasswordField
• javax.swing.JTextField
• javax.swing.JTextComponent
• javax.swing.JComponent
• java.awt.Container
• java.awt.Component
• java.lang.Object

The example would make much more sense as the following:

 void describeComponent(CustomComponent<? super JTextField> ref) {...} 

Note that JPasswordField, which is a subclass of JTextField, itself is omitted in the list of permissible objects, because it is not a superclass of JTextField.

Answer 2

我相信“<T extends Comparable <？super T >>”的语义是T或T的超类必须实现Comparable