A summary of Generic Type

There are 2 levels of Generic Type: class level & method level.

Class Level:

public class ExampleClass<T>{}

Method Level:

public <T> ReturnType exampleMethod(ParameterType parameter){return null;}

Generic Type works very much like any kind of variables. You need to declare them before using. The above Generic Type place holders <T> are the only places you can declare generic types. It means any “generic types” appear else where are not declarations, but references of <T>.

For example:
public class ExampleClass<T> extends SuperClass<T>{}
The first <T> is declaration, and the second is just a reference. As an effect, you can’t have something like this:


public class ExampleClass<T> extends SuperClass<T extends Type>{}

because if you want to bound a type parameter on a generic type, you have to do it while it’s declared, like this:


public class ExampleClass<T extends Type> extends SuperClass<T>{}

Another trick with this example is, if <T> of SuperClass is bounded to a type in its own definition, the <T> of ExampleClass must narrow its type to fit the <T> of SuperClass.


public class SuperClass<T extends Type>{}
public class ExampleClass<T> extends SuperClass<T>{}


public class SuperClass<T extends Type>{}
public class ExampleClass<T extends Type> extends SuperClass<T>{}

For the ExampleClass<T extends Type>, T can also extend from a subclass of Type since subclass has an “is” relationship to its parent class.


super is opposite to extends, and is commonly used with consumer pattern.

Wildcard extends:

List<? extends T>

It indicates each element in the list is an instanceof T.

Wildcard super:

List<? super T>

It means the type of each element equals the type of T, or the types of its parents.

*Search PECS (Producer Extends Consumer Super) for details about its usage.




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