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In the Java Collections Framework, the class <code>List<MyClass></code> represents an ordered collection of objects of type <code>MyClass</code>.
Upper bounds are specified using <code>'''extends'''</code>:
A <code>List<? '''extends''' MyClass></code> is a list of objects of some subclass of <code>MyClass</code>, i.e. any object in the list is guaranteed to be of type <code>MyClass</code>, so one can iterate over it using a variable of type <code>MyClass</code><ref>[[Inheritance (object-oriented programming)]]</ref>
<syntaxhighlight lang="java">
public void doSomething(List<? extends MyClass> list) {
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</syntaxhighlight>
The converse is true for lower bounds, which are specified using <code>'''super'''</code>:
A <code>List<? '''super''' MyClass></code> is a list of objects of some superclass of <code>MyClass</code>, i.e. the list is guaranteed to be able to contain any object of type <code>MyClass</code>, so one can add any object of type <code>MyClass</code>:
<syntaxhighlight lang="java">
public void doSomething(List<? super MyClass> list) {
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</syntaxhighlight>
In order to be able to do both add objects of type <code>MyClass</code> to the list and iterate over it using a variable of type <code>MyClass</code>, a <code>List<MyClass></code> is needed, which is the only type of <code>List</code> that is both <code>List<? '''extends''' MyClass></code> and <code>List<? '''super''' MyClass></code>.<ref>[[Java syntax|Java syntax(Generics)]]</ref>
The mnemonics PECS (Producer Extends, Consumer Super) from the book '''Effective Java''' by [[Joshua Bloch]] gives an easy way to remember when to use wildcards (corresponding to Covariance and Contravariance) in Java.{{sfn|Bloch|2018|loc=Chapter §5 Item 31: Use bounded wildcards to increase API flexibility|pp=139-145}}
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