Overview of Object-Oriented Objects

It is an idea that emphasizes the way of thinking that human beings use in their daily logical thinking, such as abstraction, separation, inheritance, aggregation, polymorphism, etc.

1.3 Three main object-oriented features:

encapsulation
inherit
polymorphic

1.4 The saying that everything is an object when facing an object

Because in Java, we can encapsulate functions, structures, and so on into classes, and implement specific functions by instantiating classes. It involves the interaction between front-end web page data and back-end database. When interacting, they all have corresponding classes and objects, which correspond to events in real life one by one.

1.5 Each object-oriented overall process takes three steps

Create Class
Class instantiation, that is, creating objects
Call class method properties for operation

1.6 Classes and Objects Their Relationships

Classes are abstractions of things that have a series of common properties in life

A class is a collection of objects with common attributes and behaviors

The relationship between a class and an object is a high-level summary of the common characteristics of the object, which is a concrete and real entity.

That is, the class we often compare is a template, a drawing, and an object is a made entity.

1.7 Instance Description

Our behavioral activities in life can be divided into one person (someone) or something (bicycle), what attributes are there, what is done

Programs are designed to simulate the human world and help people deal with things, such as

1. Xiao Ming goes out by bike

real life	Program
One person, transportation	Class name: Human, Transportation
Xiao Ming, Bicycle	Object: Xiao Ming, Bike
Cycling out to play	Method: Go out and play

Xiao Ming, 2 or 12, goes to fly kites

real life	Program
People, toys (kites can also be used, Xiao Ming's kite belongs to one of the kites)	Class name: human, toy (kites can also be used, Xiaoming's kite belongs to one of the kites)
12 years old	Xiao Ming's attributes: 12 years old
Xiao Ming, Kite	Object: Xiao Ming, Kite
Fly a kite	Method: Fly a kite

Composition of 2 classes

2.1 Attributes:

2.1.1 Meaning

Member variables, whether or not the initial value is OK, are defined outside the methods in the class.

2.1.2 Writing Format

//Variable Type Variable Name
String name;

2.2 Behavior:

2.2.1 Meaning

Includes construction methods and custom methods, which in general must be written and will not error if not written, since Virtual Opportunities provide a parameterless construction method by default.

2.2.2 Writing Format

//Parameterless constructor
		public Class name(){}		//No void, no formal parameters

//Parametric constructor
		public Class name(Variable Type Variable Name){		//Like parameterless constructors, there can be no void, and there can be multiple formal parameters.
    		Arguments = Formal parameters;
		}

//A parameterless constructor and a parameterized constructor constitute an overload

//Custom methods
		public void method(){		//Method name and return value, return value type depends on you
		}

Three Call Methods for 2.2.3 Constructor

//  Method 1, also known as anonymous call;
        new Animal("Lin Qingxia").show();

// Method 2. Normally call a class without parameters, create an object, and set member variables in the class through the setter method.
        Animal an = new Animal();
        an.setName("Lin Qingxia");
        an.show();

// Method 3. Create objects by calling classes with parameters and assign values directly.
        Animal an1 = new Animal("Lin Qingxia");
        an1.show();

2.2.4 Explanation of the existence of parametric and parametric constructors

There is no constructor in the class, and the virtual machine will automatically provide a parameterless constructor to use
A parameterized constructor is provided in the class, and no parameterized constructor is automatically provided by the system

Purpose of the 2.2.5 constructor:

Class initialization is done by calling the constructor, that is, when the class is initialized, the code in the constructor executes.
Create Object with

2.2.6 Note:

A parameter cannot be added to a parameterless constructor.
The name of the constructor and the class name should be highly consistent or an error will be invalidated.
public can be removed, but it is not recommended. Once removed, it can only be called in this package. There will be an explanation in the permission modifier later
The construction method name here must be the same as the class name, and different words will be considered writing methods, so it is important to determine whether void has no parameter return value or a certain type of parameter return value.

2.27 Modifiers that can change variable permissions

After final modification, the variable becomes a constant, which cannot be modified elsewhere and is written in full capitals
After the static modification, the variable becomes a common property, which is accessed by the variable for each object in the class.
The this keyword is used inside a method, and when local and member variables are repeated, the this modifier is a member variable

public static void main(String[] args) {
		StaticTest.setSchool("Henan University");

        StaticTest test = new StaticTest("Zhang San");
        StaticTest test2 = new StaticTest("Li Si");

        System.out.println(test.getName()+"..."+ StaticTest.getSchool());
        System.out.println(test2.getName()+"..."+ StaticTest.getSchool());
        //The output is Zhang San...Henan University
        //            Li Si...Henan University
 }


public class StaticTest {
    private static String school;		//school is decorated with static, and the get method for each object yields the same result 											 school results before this change
    private String name;

    public StaticTest() {
    }

    public StaticTest(String name) {
        this.name = name;
    }

    public static String getSchool() {
        return school;
    }

    public static void setSchool(String school) {
        StaticTest.school = school;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

3 A standard java bean class

public class animal{
    
}
//***This constitutes a class, but generally it is not, because there is no practical point in changing it to an abstract class******

class Animal {
   
    private String name;

    public Animal() {		//Parameterless constructor
    }

    public Animal(String name) {		//Parametric constructor
        this.name = name;
    }

    public void show() {			//Defined Method
        System.out.println(name + "Eureka");
    }

    public String getName() {		//The getter method provided to make it easy to call variables in this class elsewhere
        return name;
    }

    public void setName(String name) {		//Provided setter method to facilitate setting variables in other classes
        this.name = name;
    }
}

Tags: Java Back-end

Posted on Thu, 25 Nov 2021 12:25:38 -0500 by buroy

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