Java 8-5 Abstraction

1, abstract class overview:
Animals should not be defined as specific things, and animals in the eating, sleeping and so should not be specific.
We call a feature that is not a specific function, but a class that, if it has abstract functionality, must be an abstract class.

Features of abstract classes:
A: Abstract classes and abstract methods must be decorated with the abstract keyword, which cannot have a body
B: Abstract classes do not necessarily have abstract methods, but classes with abstract methods must be defined as abstract classes
C: Abstract class cannot be instantiated
Because it's not specific.
Abstract classes have a construction method, but they cannot be instantiated.
What is the function of the construction method?
Initialization for subclass access to the parent class data
D: Abstract sub-class
A: If you do not want to override the abstract method, the subclass is an abstract class.
B: Rewrite all the abstract methods, this time the subclass is a concrete class.

The instantiation of an abstract class is actually achieved by a specific subclass. is a polymorphic way.
Animal a = new Cat ();

1 //Abstract class Animal//declarative format for abstract classes2 Abstract classAnimal {3 //Abstract Methods4 //Public abstract void Eat () {}//empty method body, this will be an error. Abstract methods cannot have principals5  Public Abstract voideat ();6 7  PublicAnimal () {}8 }9 Ten //The first type: Subclasses are abstract classes One Abstract classDogextendsAnimal {} A  - //The second type: Subclasses are concrete classes, overriding abstract methods - classCatextendsAnimal { the  Public voideat () { -System.out.println ("Cat Eats fish"); - } - } +  - classAbstractdemo { +  Public Static voidMain (string[] args) { A //Creating Objects at //animal is abstract; unable to instantiate - //Animal a = new Animal (); - //by polymorphic means -Animal A =NewCat (); - a.eat (); - } in}

2, the characteristics of the abstract class Members:
Member variable: can be either a variable or a constant.
Construction method: there is.
Initializes the child class to access the parent class data.
Member methods: can be either abstract or non-abstract.

Member method attributes for abstract classes:
A: Abstract method: Forcing a subclass to do something. The sub-class does not have to error.
B: Non-abstract method: Subclass inherited things, improve code reusability.

1 Abstract classAnimal {2  Public intnum = 10;3  Public Final intnum2 = 20;4 5  PublicAnimal () {}6 7  PublicAnimal (String name,intAge ) {}8 9  Public Abstract voidShow ();Ten  One  Public voidmethod () { ASystem.out.println ("Method"); - } - } the  - classDogextendsAnimal { -  Public voidShow () { -System.out.println ("Show Dog"); + } - } +  A classAbstractDemo2 { at  Public Static voidMain (string[] args) { - //Creating Objects -Animal A =NewDog (); -A.num = 100; - System.out.println (a.num); - //a.num2 = $; in System.out.println (a.num2); -System.out.println ("--------------"); to a.show (); + A.method (); - } the}

3. Interview questions:
If there is no abstract method for a class, can it be defined as an abstract class? If so, what's the point?
A: Yes.
B: Don't let the object be created. This object can only be used by accessing its subclasses

What keywords does abstract not coexist with?
Private: conflict, private proprietary is inherited, cannot be overridden, and abstract inheritance requirements are overridden, so conflicts
Final: conflict, final variable is the final variable, cannot be rewritten, ibid.
Static: Meaningless, abstract method is no method body, and static static is accessed by the class name method body, but. To access a method without a method body, meaningless.

1 Abstract classFu {2 //Public abstract void Show ();3 //illegal modifier combinations: abstract and private4 //private abstract void Show ();5 6 //illegal modifier combination7 //final abstract void show (); 8 9 //illegal modifier combinationTen Static Abstract voidShow (); One  A  Public Static voidmethod () { -System.out.println ("Method"); - } the } -  - classZiextendsFu { -  Public voidShow () {} + } -  + classAbstractDemo3 { A  Public Static voidMain (string[] args) { at Fu.method (); - } -}

Java 8-5 Abstraction