java - detailed introduction to object-oriented (1)

1. What is object-oriented?

Object-Oriented (OO) is a common program structure design method.
The basis of object-oriented thinking is to put related data and methods together, combine them into a new composite data type, and then use the newly created composite data type as the basis of the project.

Object-oriented is a very abstract concept, which is relative to process-oriented.

Processes and objects are both ideas for solving problems.

Process-oriented: Emphasizes functional behavior, a process, what to do first, and then what to do;

Object-oriented: Encapsulates functions into objects, emphasizes having a certain function Object;

According to object-oriented thinking, anything can be regarded as an object!

Three characteristics of object-oriented:

Encapsulation (Encapsulation);

Inheritance (Inheritance);

Polymorphism ( Polymorphism).

My summary:

Process-oriented: emphasizing the specific function implementation; (executor)

Object-oriented: emphasizing the ability to Functional object. (Manager)

2. Class

Class (class) is the smallest programming unit of the Java language and the basis for designing and implementing Java programs. This part will go into depth. Introduce relevant knowledge of the class.
The concept of class
A class is a description of the common characteristics and functions of a group of things. A class is an overall description of a group of things. It is the smallest unit when designing according to object-oriented technology, and it is also the most basic module that makes up a project. The concept of class is abstract, similar to drawings in architectural design, and is an abstraction of the specific content that needs to be represented in reality. Classes only contain skeletal structures, not specific data. Therefore, the class represents the whole, not a specific individual.

My summary: Classes are abstract, objects are concrete, real!

Definition of class:

[modifier] class class name {

1~n construction methods;

0~n fields;

     0~n methods

}

Define a class, which is actually to define the objects in the class

Objects include:

State; (Attribute)

Function, behavior; (Method)

Describe the object through the class;

State--------Member variable;

Function, behavior - method;

Eg：

class Person{
//属性
   private String name;
   private int age;
   private int sal;
   //方法
   public void show(){
      System.out.println("个人情况："+name+age+sal);
   }
}

3. Construction method

Construction method: use To construct an instance of a class (each class has a parameterless constructor by default, which must be called with new)

Field: the data contained in the class or object, a description of the class state;

Method: Characteristics or behaviors of a class or object

Function:

Initialize the fields in the class and can be used to create objects.

Features:

The method name is the same as the class name

No need to define the return value type

No need to write a return statement

Note:

Characteristics of the default construction method.

Multiple constructors exist in the form of overloading.

Overloading of the constructor method: (Adapt whichever one is needed and call whichever one is needed)

this([actual parameter]); Call the constructor method of the current class

Note: this([actual parameter]); must be placed in the first line of the constructor;

Object generation format:

Class Name Object name = new Class name ();

Because there is (), it is a method. In fact, it is a constructor, and it is a non-private constructor.

For example:

CellPhone cp = new CellPhone();

Eg：
 
class Person{
 
   private String name;
 
   private int age;
 
   private int sal;
 
   public void show(){
 
      System.out.println("个人情况："+name+age+sal);
 
   }
 
 
 
   public Person(String name) {
 
      super();
 
      this.name = name;
 
   }
 
 
 
   public Person(String name, int age) {
 
      super();
 
      this.name = name;
 
      this.age = age;
 
   }
 
 
 
   public Person(String name, int age, int sal) {
 
      super();
 
      this.name = name;
 
      this.age = age;
 
      this.sal = sal;
 
   }
 
}

4. static keyword

Features:

Loaded as the class is loaded

Priority to object existence

Been owned by all objects Sharing

can be called directly by the class name

Usage Note:

Static methods can only access static members

But non-static member methods can access static members;

This cannot be used in static methods, and the super keyword

The main method (main) is static (you can use the class name to It is normal to call the static main method! But it will fall into an infinite loop, causing memory overflow and the jvm to stop automatically!)

public static void main(String[] agrs){}

Can modify fields and methods.

Members modified with static indicate that they belong to this class, rather than to a single instance of the class.

# STATIC modified field == class field

STATIC modified method == class method

illegal field modified fields and methods, members belong to a single instance of the class, Does not belong to class.

No static modified field == Example field

There is no static modifier method == instance method

new Person().age = 17;//使用一次之后就被销毁了

6、this关键字

特点：this表示当前对象。

当前对象 ←→ 当前正在调用实例成员的对象

换言之：谁调用了方法，谁就是当前对象。

什么时候使用this关键字呢？

方法间的相互调用;

this.字段;

构造器中相互调用，但是此时this([参数])必须写在构造方法第一行。

this不能用在static修饰的方法里和static修饰的代码块里;

Eg：构造方法中的this.name = name;

7、面向对象之封装

封装的两个含义：

1.把对象的状态和行为看成一个统一的整体，将二者存放在一个独立的模块中(类)；

2."信息隐藏", 把不需要让外界知道的信息隐藏起来,尽可能隐藏对象功能实现细节,字段;

封装机制在程序中的体现是：

把描述对象的状态用字段表示，描述对象的行为用方法表示，把字段和方法定义在一个类中，并保证外界不能任意更改其内部的字段值，也不允许任意调动其内部的功能方法。

程序中的一种体现：

通常将类中的成员变量私有化(private)，通过对外提供方法（setXxx,getXxx）,可对该变量(xxx)进行访问。

boolean 类型的变量没有getXX,只有 isXX;

 
Eg：
 
class Person1{
 
   private String name;
 
   private int age;
 
   private int sal;
 
  
 
   public String getName() {
 
      return name;
 
   }
 
 
 
   public void setName(String name) {
 
      this.name = name;
 
   }
 
 
 
   public int getAge() {
 
      return age;
 
   }
 
 
 
   public void setAge(int age) {
 
      this.age = age;
 
   }
 
}

8、访问修饰符

private 类访问权限:本类内部可以访问,不能继承到子类;

default 什么都不写,包访问权限:本类内部可以访问，同包其他类也可以访问,同包可继承;

protected 子类访问权限:本类内部可以访问，不同包的子类也可以访问，同包其他类也可以访问,能继承到子类;

public 公共访问权限:任何地方都可以访问,能继承到子类;

总结：

9. Class design analysis

Analysis ideas:

Write the class according to the requirements Contained fields;

All fields must be privatized;

The encapsulated fields can be set and obtained through setters and getters;

Several fields can be added as needed Constructor method;

Corresponding methods can be added according to requirements;

All methods in the class should not be processed directly (output and print), but should be left to the caller for processing.

10. Object-oriented inheritance

First there are classes that reflect the characteristics of general things, and then on this basis, classes that reflect special things ;

That is to say: inheritance is a relationship from general to special;

Features:

1. Improved code reusability.

2. Create a relationship between classes. Only with this inheritance relationship can we have the characteristics of polymorphism.

3. Java language only supports single inheritance (different from C language).

Because multiple inheritance can easily bring security risks (if there are too many parent classes and the functions are the same, will there be uncertainty in the call? Overwriting a method, who is overriding it?).

ps: The interface can realize multiple inheritance

4. Java supports multi-layer inheritance. Object is the super class of each class and implements a tree structure.

My summary:

Inheritance is the prerequisite for polymorphism.

For classes, only single inheritance is supported.

Format:

[Modifier] class SubClass extends SuperClass

According to this relationship, we The SuperClass class is called the parent class or base class, and the SubClass is called the subclass or derived class or extended class;

My summary:

java.lang.Object It is the parent class of all classes.

Object is either a direct parent class or an indirect parent class.

Eg:

Students belong to a special situation of human beings. At this time, I write the common characteristics of people in the Person class, in order to let students have these common characteristics (others such as teachers can also have These commonalities), then I ask students to expand the Person category.

My summary:

The relationship between the subclass and the parent class:

The subclass extends the parent class (the subclass is a special case of the parent class)

Mainly based on the parent class, and then add its own fields and methods.

The private members of the parent class cannot be inherited by subclasses;

The construction method of the parent class cannot be inherited;

Java only supports single inheritance, not multiple inheritance;/ /Otherwise, for example, if the show method inherits multiple methods, I don’t know which one to call.

A class has one and only one direct parent class;

When a class does not explicitly inherit another class, the default direct parent class is the Object class;

The direct parent class of Student is Person, and the Object class is also the parent class of Student class, but it is an indirect parent class;

Once a class explicitly inherits another class, the default direct parent class is Object will be cancelled;

A class in Java can only have one direct parent class;

java.lang.Object is the parent class of all classes, Object is either a direct parent class or an indirect parent class.

Subclass object instantiation process

In the inheritance operation, for the instantiation of subclass objects:

The subclass object must first call the parent class before instantiation 's constructor and then calls its own constructor.

The above is part of the content of this article. We will continue to organize object-oriented knowledge for everyone in the future. Please point out anything wrong, thank you!

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