Java Generics



  • Avoids annoying casts
  • More accurately expresses intent; can't even put in the wrong type… or pass the wrong type of collection/impl.

Key Concepts

  • Type Parameter
  • Type Token
  • Erasure — compiler removes type parameters when it generates bytecodes to support backward compatibility.
    • Also means you don't have type information at runtime. :(
    • Required for backward compatibility with Java 1.4 code.
  • Generic Method
  • Wildcards
    • upper bound (e.g. "? extends Set" matches any class/interface that implements Set)
    • lower bound (e.g. ? super

Use-Case Decisions

  • Generic Methods vs. Wildcard Parameters
    • Wildcard Parameters express the desire to have polymorphic handling of generics. Preferred whenever possible.
    • Generic Methods are used to express dependencies between the types of parameters or return values.
  • Class<T> — aka "Class Literal as a Run-time type Token"
    • T, here is the compile-time type variable of the class being represented by Class. (e.g. String.class == Class<String>).
    • This makes it possible to use the type in a generic context (think of the query example where MyDomainObject.class.newInstance() would be called, but returns an Object… instead you can declare references of type T… where T could be MyDomainObject.class)
  • Wrapping legacy collections with Collections.checked___(Collection<?> c, Class) for fail-fast.
  • Annotations make heavy used of Generics.


  • You cannot create arrays of generics (e.g. List<String>[10] is not allowed).
    • Avoid casting to Generics; since generics are not runtime variables, it doesn't help.
    • While you can use Type Variables in casts they are not safe casts.
      • Ditto with casting from one Generic to another (e.g. Collection<String> and Collection<StringBuffer>).

Uses of Generics

  • Collections
  • Type-Oriented classes (e.g. DAOs)
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