The article directories

• Classes and objects
• • 1. Look at a cat problem
  • 2. Use existing technologies
  • 3. Analysis of shortcomings solved by existing technologies
  • 4. A program is a world with many things (objects [properties, actions]).
  • 5. Schematic diagram of the relationship between classes and objects
  • 6. Quick Start – Object oriented solution to cat problems
  • 7. Differences and relationships between classes and objects
  • 8. Objects have form in memory (important)
  • 9. Attributes/member variables/fields
  • 10. Notes and details of attributes
  • 11. How do I create objects
  • 12. How do I access properties
  • 13. Memory allocation mechanisms for classes and objects (critical)
  • 14. Object mechanism exercises

Classes and objects

1. Look at a cat problem

2. Use existing technologies

Object01.java

1. Define variables separately
2. Using arrays

3. Analysis of shortcomings solved by existing technologies

	// Separate variables to solve => bad data management (you take a cat's information apart)
	
	// First cat information
	
	String cat1Name = "White";
	int cat1Age = 3;
	String cat1Color = "White";
	
	// The second cat
	String cat2Name = "Flower";
	int cat2Age = 100;
	String cat2Color = "Design and color";
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	// Array ===>(1) Data type is not reflected
			//(2) only [subscript] can be used to obtain information, resulting in the corresponding relationship between variable name and content is not clear
			//(3) do not reflect the cat's behavior
	
	String[] cat1 = {"White"."3"."White"}; 
	String[] cat2 = {"Flower"."100"."Design and color"};
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4. A program is a world with many things (objects [properties, actions]).

5. Schematic diagram of the relationship between classes and objects

6. Quick Start – Object oriented solution to cat problems

• Using Object Orientation

	// Define a Cat -> custom datatype
	class Cat {
	// Attributes/member variables
	String name; / / name
	int age; / / age
	String color; / / color
	double weight; / / weight
	/ / behavior
	}
	
	// Use OOP object-oriented solution
	// Create a cat object
	/ / read
	//1. new Cat() creates a Cat(Cat object)
	//2. Cat cat1 = new Cat(); Assign the created cat to cat1
	//3. Cat1 is an object
	Cat cat1 = new Cat();
	cat1.name = "White";
	cat1.age = 3;
	cat1.color = "White";
	cat1.weight = 10;
	// Create a second cat and assign it to cat2
	// Cat2 is also an object (cat object)
	Cat cat2 = new Cat();
	cat2.name = "Flower";
	cat2.age = 100;
	cat2.color = "Design and color";
	cat2.weight = 20;
	
	// How to access the properties of the object
	System.out.println("The first cat message." + cat1.name 
		+ "" + cat1.age + "" + cat1.color + "" + cat1.weight);
	
	System.out.println("The second cat message." + cat2.name 
		+ "" + cat2.age + "" + cat2.color + "" + cat2.weight);
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7. Differences and relationships between classes and objects

8. Objects have form in memory (important)

9. Attributes/member variables/fields

class Car{
    String name;// Attribute, member variable, field
    double price;
    String color;
    String[] master;// Attributes can be primitive data types or reference types (objects, arrays)
}
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10. Notes and details of attributes

PropertiesDetail.java

1. Property definition syntax same as variable, example: access modifier Property type Property name;
   
   Here’s a brief introduction to access modifiers:Controls the access scope of attributes
   
   There are four types of access modifiersPublic proctected default private
2. The definition type of an attribute can be any type, including a base type or a reference type
3. Properties have default values if they are not assigned, the same rules as arrays. Specifically:

int 0.short 0.byte 0.long 0.float 0.0.double 0.0.char\ u0000,boolean falseThe Stringnull
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• Example: [Person class]

public class PropertiesDetail { 

	// Write a main method
	public static void main(String[] args) {
		// Create Person object
		//p1 is the object name (object reference)
		// The object space (data) created by new Person() is the real object
		Person p1 = new Person();

		// The default value for the property of the object, following the array rules:
		//int 0, short 0, byte 0, long 0, float 0.0,double 0.0, char \u0000, Boolean false, String null;
	
		System.out.println("\n Information about the current person");
		System.out.println("age=" + p1.age + " name=" 
				+ p1.name + " sal=" + p1.sal + " isPass="+ p1.isPass) ; }}class Person {
	// Four attributes
	int age;
	String name;
	double sal;
	boolean isPass;
}
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11. How do I create objects

Cat cat ; // Declare the object cat
cat = new Cat(); / / create
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Cat cat = new Cat();
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12. How do I access properties

• The basic grammar

Object name. Attribute name.Copy the code

Examples demonstrate assignment and output

cat.name;
cat.age;
cat.color;
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• Memory allocation mechanisms for classes and objects (important)

Look at a thought question

• Define a human being (Person)(including name, age). (Object03.java)
  
  
  
  

	public class Object03 { 
	
		// Write a main method
		public static void main(String[] args) {
	
			Person p1 = new Person();
			p1.age = 10;
			p1.name = "Xiao Ming";
			Person p2 = p1; // assign p1 to P2 so that P2 points to P1
			System.out.println(p2.age); / / 10}}class Person {
		String name;
		int age; 
	}
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13. Memory allocation mechanisms for classes and objects (critical)

14. Object mechanism exercises

• Look at an exercise and analyze the memory layout
  
  
  
  
• Code:

	public class Object04 {
	    public static void main(String[] args) {
	        Person02 a = new Person02();
	        a.name = "Xiao Ming";
	        a.age = 10;
	        Person02 b;
	        b = a;
	        System.out.println(b.name);
	        b.age = 200;
	        b = null; System.out.println(a.age); System.out.println(b.age); }}class Person02{
	    String name;
	    int age;
	}
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