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Java Generics FAQs - Language Features of Java Generics

This is a collection of answers to frequently asked questions (FAQs) about Java Generics, a new language feature added to the Java programming language in version 5.0 of the Java Standard Edition (J2SE 5.0).

If you want to provide feedback or have any questions regarding Java generics, to which you cannot find an answer in this document, feel free to send me EMAIL or use the GENERICS FAQ form.
A printable version of the FAQ documents is available in PDF format (4.5MB).

Java Generics FAQs - Language Features of Java Generics

Language Features of Java Generics

Which language features are related to Java generics?

Generic Types

Fundamentals

What is a parameterized or generic type?

How do I define a generic type?

Are there any types that cannot have type parameters?

How is a generic type instantiated?

Why do instantiations of a generic type share the same runtime type?

Can I cast to a parameterized type?

Can I use parameterized types in exception handling?

Can generic types have static members?

Concrete Instantiations

What is a concrete instantiation?

Are different concrete instantiations of the same generic type compatible?

Can I use a concrete parameterized type like any other type?

Can I create an array whose component type is a concrete parameterized type?

Can I declare a reference variable of an array type whose component type is a concrete parameterized type?

How can I work around the restriction that there are no arrays whose component type is a concrete parameterized type?

Why is there no class literal for the concrete parameterized type?

Raw Types

What is the raw type?

Why are raw types permitted?

Can I use a raw type like any other type?

Wildcard Instantiations

What is a wildcard instantiation?

What is the unbounded wildcard instantiation?

What is the difference between the unbounded wildcard parameterized type and the raw type?

Which methods and fields are accessible/inaccessible through a reference variable of a wildcard type?

Can I use a wildcard parmeterized type like any other type?

Can I create an object whose type is a wildcard parameterized type?

Can I create an array whose component type is a wildcard parameterized type?

Can I declare a reference variable of an array type whose component type is a bounded wildcard parameterized type?

Why is it allowed to create an array whose component type is an unbounded wildcard parameterized type?

Can I declare a reference variable of an array type whose component type is an unbounded wildcard parameterized type?

Can I derive from a wildcard parameterized type?

Why is there no class literal for wildcard parameterized type?

Generic Methods

Fundamentals

What is a generic method?

How do I invoke a generic method?

Type Parameters

Fundamentals

What is a type parameter?

Where is a type parameter visible (or invisible)?

Type Parameter Bounds

What is a type parameter bound?

Which types are permitted as type parameter bounds?

Can I use a type parameter as a type parameter bound?

Can I use different instantiations of a same generic type as bounds of a type parameter?

How can work around the restriction that a type parameter cannot have different instantiations of a same generic type as its bounds?

Does a bound that is a class type give access to all its public members?

How do I decrypt "Enum<E extends Enum<E>> "?

Usage

Can I use a type parameter like a type?

Can I create an object whose type is a type parameter?

Can I create an array whose component type is a type parameter?

Can I cast to the type that the type parameter stands for?

Can I use a type parameter in exception handling?

Can I derive from a type parameter?

Why is there no class literal for a type parameter?

Scope

Where is a type parameter visible (or invisible)?

Can I use a type parameter as part of its own bounds or in the declaration of other type parameters?

Can I use the type parameter of an outer type as part of the bounds of the type parameter of an inner type or a method?

Static Context

Is there one instances of a static field per instantiation of a generic type?

Why can't I use a type parameter in any static context of the generic class?

Type Arguments

Fundamentals

What is a type argument?

Which types are permitted as type arguments?

Are primitive types permitted as type arguments?

Are wildcards permitted as type arguments?

Are type parameters permitted as type arguments?

Do type parameter bounds restrict the set of types that can be used as type arguments?

Do I have to specify a type argument when I want to use a generic type?

Do I have to specify a type argument when I want to invoke a generic method?

Wildcards

Wildcard Bounds

Which language features are related to Java generics?

*Features for definition and use of generic types and methods.*
Java Generics support definition and use of generic types and methods. It provides language features for the following purposes: definition of a generic type definition of a generic method type parameters type parameter bounds type arguments wildcards wildcard bounds wildcard capture instantiation of a generic type raw type concrete instantiation wildcard instantiation instantiation of a generic method automatic type inference explicit type argument specification
LINK TO THIS	LanguageFeatures.FAQ001
REFERENCES	What is a parameterized or generic type? What is a generic method? What is a type parameter? What is a bounded type parameter? What is a type argument? What is a wildcard? What is a bounded wildcard? What is the capture of a wildcard? How is a generic type instantiated? What is the raw type? What is a concrete instantiation? What is a wildcard instantiation? How is a generic method instantiated? What is type argument inference? What is explicit type argument specification?

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