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Generics, or parameterized types, or parametric polymorphism, is one of the major features introduced in J2SE 5.0. Before generics were introduced, it was required to declare all the types explicitly. With generics, it became possible to work in a similar manner with different types without declaring the exact types. The main purpose of generics is to ensure type safety and to detect runtime errors during compilation. Unlike C#, information on the used parameters is not available at runtime due to type erasure.cite-ref-9[9]

Classes can be parameterized by adding a type variable inside angle brackets (< and >) following the class name. It makes possible the use of this type variable in class members instead of actual types. There can be more than one type variable, in which case they are declared in a comma-separated list.

It is possible to limit a type variable to a subtype of some specific class or declare an interface that must be implemented by the type. In this case the type variable is appended by the extends keyword followed by a name of the class or the interface. If the variable is constrained by both class and interface or if there are several interfaces, the class name is written first, followed by interface names with & sign used as the delimiter.

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