Chapter 12 generics

Catalog

One 、 The use of generics

1.jdk 5.0 New features

2. Summary of using generics in Collections

3. How to customize generic structure ： Generic classes 、 Generic interface ; Generic methods

1. About custom generic classes 、 Generic interface

2. Generic methods

​ Editor II 、 The embodiment of generics in inheritance

3、 ... and 、 The embodiment of generics in generics

Four 、 The use of wildcards

(1) wildcard

 (2) The use of restricted wildcards

   5、 ... and 、 Examples of generic applications : Generic nesting

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One 、 The use of generics

1.jdk 5.0 New features

Design the type of the element as a parameter , This type parameter is called generics .Collecton<E>,List<E>,ArrayList<E>, This <E> That's the type parameter , That is, generics . 

The significance of generics ：① Solve the security problem of element storage .② Solve the problem of type cast when getting data elements .

 

2. Summary of using generics in Collections

① Set interface or collection class in jdk5.0 It is modified to a generic structure .

② When instantiating a collection class , Specific generic types can be specified

③ After that, I will , In a collection class or interface, whenever a class or interface is defined , internal structure ( such as ： Method 、 Constructors 、 Properties, etc ) Where to use generics of classes , Are specified as instantiated generic types . Such as ：add(E e) ---> After instantiation ：add(Integer e)

④ Be careful ： The type of a generic must be a class , Cannot be a basic data type . Where basic data types are needed , Replace with packing .

⑤ If instantiated , The type of the generic is not specified . The default type is java.lang.Object type .

3. How to customize generic structure ： Generic classes 、 Generic interface ; Generic methods

1. About custom generic classes 、 Generic interface

If a generic class is customized , Instantiation does not indicate the generic type of the class , The generic type is considered to be Object type

         requirement ： If a class is defined to be generic , It is suggested to specify the generic type of the class when instantiating .

References of different generics cannot be assigned to each other .          When a generic method is called , Indicates the type of the generic parameter .

The internal structure of the class can use the generics of the class         Class generics cannot be used in static methods          Exception classes cannot be declared as generic classes

2. Generic methods

There is a generic structure in the method , Generic parameters have nothing to do with the generic parameters of the class . let me put it another way , It doesn't matter whether the generic method belongs to a generic class or not . Generic methods can be declared static . reason ： Generic parameters are determined when the method is called , Not determined when instantiating a class .

Two 、 The embodiment of generics in inheritance

3、 ... and 、 The embodiment of generics in generics

         Although the class A It's a class B Parent class of , but G<A> and G<B> They don't have the relationship of children and parents , The two are juxtaposed .

         Add ： class A It's a class B Parent class of ,A<G> yes B<G> Parent class of

Four 、 The use of wildcards

(1) wildcard

? class A It's a class B Parent class of ,G<A> and G<B> It doesn't matter. , The common parent of the two is : G<?>

 (2) The use of restricted wildcards

? extends A:  G<? extends A> It can be used as G<A> and G<B> Parent class of , among B yes A Subclasses of

? super A:  G<? super A> It can be used as G<A> and G<B> Parent class of , among B yes A Parent class of

   5、 ... and 、 Examples of generic applications : Generic nesting