26. Abstraction and template ideas

Catalog

One 、 Abstraction

One 、 Concept

Two 、 significance

3、 ... and 、 characteristic

Four 、 Example

Two 、 Template thought

1、 Concept

2、 characteristic

3、 Purpose

4、 advantage

5、 shortcoming

---

One 、 Abstraction

One 、 Concept

By abstract Keyword decorated class is called abstract class . When designing a parent class in an inheritance relationship , If a public method in the parent class, the parent class is not sure about the specific implementation of the method , Then the parent class simply does not implement , Define it as an abstract method , Let subclasses implement themselves , Make it concrete .

Two 、 significance

1、 Because abstract classes cannot instantiate objects , So you must have subclasses to implement it before you can use . In this way, some components with the same properties and methods can be abstracted , This is more conducive to the maintenance of code and program .

2、 When another component with similar characteristics is generated , You only need to implement the abstract class to get those properties and methods of the abstract class .

therefore ,Java Abstract classes in are very helpful for code maintenance and reuse , It's also Java An important embodiment of object-oriented .

3、 ... and 、 characteristic

1、 An abstract parent class cannot directly create an object , If you create , Compile failed and error reported . Only objects with non Abstract subclasses can be created .

2、 In an abstract class , There can be construction methods , Is when subclasses create objects , Initializes the .

3、 Abstract classes don't necessarily contain abstract methods , But it must be an abstract class that has abstract methods , Abstract classes need to be added abstract.

4、 A subclass of an abstract class , All abstract methods in the abstract parent class must be overridden , Otherwise, subclasses must also be defined as abstract classes , Compile failed and error reported .

5. Abstract classes exist in order to be inherited by subclasses , Abstract classes embody the idea of templates .

Four 、 Example

If I need to define new drivers and old drivers now , Both new and old drivers have the function of driving , The steps of driving are the same , It's just that the driving posture is a little different , New driver : Open door , ignition , Hold the steering wheel with both hands , brake , Stalling ; old hand : Open door , ignition , Hold the steering wheel with your right hand and smoke with your left hand , brake , Stalling . We can write the fixed process to the parent class , Different places are defined as abstract methods , Let different subclasses override , The code is as follows ：

analysis ： The only difference between the driving process of new drivers and old drivers here is the posture of grasping the steering wheel , So here we can abstract this step , Define it as a pose method , Then override with Subclasses , Write in every posture .

Two 、 Template thought

1、 Concept

The abstract class as a whole can be regarded as a template , What cannot be determined in a template is defined as an abstract method , Let the classes that use templates ( Inheriting subclasses of abstract classes ) To rewrite abstract methods to implement requirements .

2、 characteristic

1、 Subclasses override abstracted methods in the parent class on demand .

2、 Call with the definition in the abstract parent class .

3、 Purpose

Defines the skeleton of an algorithm in an operation , Instead, defer some steps to subclasses . The template approach allows subclasses to redefine certain steps of an algorithm without changing its structure .

4、 advantage

1、 Encapsulate the invariant part , Extend the variable part .

2、 Extract public code , Easy to maintain .

3、 Behavior is controlled by the parent class , Subclass implementation .

5、 shortcoming

Each different implementation needs a subclass to implement it , Causes the number of classes to increase , Make the system bigger .