Regain the basics of Java (9): Summary of abstract classes and interfaces

Regain the basics of java (9): Summary of abstract classes and interfaces

1. Final keyword

1. Can modify classes, attributes, methods

2. Modified class: final class final class Pet{}

3. Modified attributes: the value of the attribute cannot be modified final Modified attributes must be manually assigned PI

4. Modification method: This method cannot be overridden 5. All methods in the final class default to final

2. Abstract class

1. Pet Dog Cat has no realistic meaning

2. The parent class only acts as a code template

3. abstract, abstract, keyword

4. Modified class abstract class Pet{ //Abstract class }

5. Abstract class cannot be instantiated. Instance == object. The process of creating an object is called instantiation

6. After a class becomes an abstract class, what impact will it have on the members in the class? a. Attribute No change b. Construction method No change c. Get/set method No change d. Ordinary functional method No change 7. It is recommended that all parent classes be changed to abstract classes in the future, abstract parent classes

3. Abstract methods

1. can modify methods, and functional methods in parent classes can generally be changed to abstract methods

2. Characteristics a. Cannot have Method body b. Abstract methods must appear in abstract classes c. Abstract methods must be overridden

3. There can be both ordinary methods and abstract methods in abstract classes

4. It is recommended to change the functional methods in the parent class to abstract methods

5. What keywords cannot the abstract keyword abstract coexist with?

final: If the method is abstracted, it needs to be rewritten, but final cannot be rewritten, so there is a conflict.

private: If the method is private and subclasses cannot inherit it, how to rewrite it?

static: No object is needed. Abstract methods can be called through class names, but calling abstract methods is meaningless.

4. Preliminary understanding of interfaces

1. In real life, what is an interface? Water pipe interface socket USB interface

2. The function is similar, and it is named "interface"

3. Function a. To ensure scalability and maintainability, the interface It is a form of polymorphism (USB interface) b. Practical application: unified standards and specifications (USB interface)

4. In Java, what exactly is an interface? The essence of the interface is a more special abstract class

5. Define an interface: interface = abstract class

6. The class that inherits the interface is not called a subclass, but is called an implementation class. Replace extends with implements.

7. Compared with abstract classes, interfaces are more special:

a. Interfaces cannot be instantiated and cannot have constructors

b. All attributes in the interface They are all modified by public static final. If you don’t write it, it will be automatically added.

c. All methods in the interface must be abstract methods, and the public abstract keyword will be automatically added.

d. A class can Implement multiple interfaces

e. Multiple inheritance is possible between interfaces

f. The interface does not inherit the Object class

5. Interface is a form of polymorphism

1. Function: Ensure that the program has good scalability and maintainability

2. Upward transformation of the interface: Interface name Object name = new implementation class();

3. Application scenario:

a. If the passive party needs to use attributes, then inheritance-polymorphism can only be used

b. If The passive party does not need to use attributes, or the focus is on functions (methods), then interfaces should be used. Case: changing to a different encryption algorithm (Company A needs to encrypt data, and initially used the algorithm provided by Company B, but later found that this algorithm is easy was cracked, and then replaced with the encryption algorithm of Company C...)

6. Relationship-Summary

1. ##Classes and classes Inheritance Single inheritance, multi-layer Inheritance

2. Interfaces and Interfaces Inheritance Multiple Inheritance

3. Classes and Interfaces Implementation Multiple Implementations

4. class A extends B implements C, D

7. Practical application of interface

An interface is a standard or specification that is used to unify the method naming of all parties

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