What are wildcards?

What are wildcards?

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In Java, wildcards mean unknown types. They are represented using the ? symbol. Wildcards are used in Java to provide flexibility of using different type parameters with the generics code.

Wildcards are of the following two types:

• Unbounded wildcards: These are the wildcards without any upper or lower bound. An unbounded wildcard is the same as <? extends Object>.



• Bounded wildcards: These wildcards have either an upper or a lower bound. They are of the following two types:
  
  
  • Wildcards with an upper bound: These are the wildcards with the upper bound specified. An example of such wildcards is as follows:
    
    <? extends ClassOne>: It represents a family of subtypes of type ClassOne. This means that an unknown type is a subtype of ClassOne.
  
  
  
  • Wildcards with a lower bound: These are the wildcards with the lower bound specified. An example of such wildcards is as follows:
    
    <? super ClassOne>: It represents a family of supertypes of type ClassOne. This means that an unknown type is a supertype of ClassOne.

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