What's the purpose behind wildcards and how are they different from generics?

Question

I\'d never heard about wildcars until a few days ago and after reading my teacher\'s Java book, I\'m still not sure about what\'s it for and why would I need to use it.

Answer 1

Java generics are invariant.

Suppose we have B extends A:

• B is a subtype of A
• an instanceof B is also an instanceof A

Since Java arrays are covariant:

• B[] is a subtype of A[]
• an instanceof B[] is also an instanceof A[]

However, Java generics are invariant:

• List is NOT a subtype of List
• a instanceof List is NOT an instanceof List.

Wildcards are used to make it more flexible while preserving type safety.

• a List is a List

References

• Java Tutorials/Generics
  • Subtyping
  • More fun with wildcards

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