java - Why does T extends Comparable <? super T> include T? Meaning includes Comparable<T>? -

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class binarysearch<t extends comparable<? super t> >

why t extends comparable <? super t> including t, meaning including comparable<t> , not super class hierarchy? confused on super keyword thought super include super class items. new java , in java book had following example:

this regarding method in class trying limit upper , lower bound of hierarchy using java.awt.component class had extends container 

class customcomponent<t extends container>

in class have following method

void describecomponent<customcomponent<? super jpasswordfield> ref)

and goes on

note jpasswordfield, superclass of jtextfield, omitted in list of permissible objects.

from lower bounded wildcards, section of generics section of the java tutorial:

... lower bounded wildcard restricts unknown type a specific type or super type of type.

(bold mine, emphasis theirs)

thus, set of classes match t extends comparable<t> subset of set of classes match t extends comparable<? super t>. wildcard ? super t matches t , superclasses of t.

in other words, assumption "super include super class items" incorrect.

your confusion in example may arise fact jtextfield superclass of jpasswordfield; in other words, jpasswordfield extends jtextfield. example match of 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 make more sense following:

void describecomponent(customcomponent<? super jtextfield> ref) {...}

note jpasswordfield, subclass of jtextfield, omitted in list of permissible objects, because not superclass of jtextfield.


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