Java object oriented

Posted by dvdflashbacks on Wed, 16 Feb 2022 08:25:57 +0100


object-oriented programming
Essence: organize code in the form of classes and organize (encapsulate) data with objects.
Characteristics: encapsulation, inheritance, polymorphism.


break: jump out of the switch and end the loop
Return: end the method. The result has been returned. The return value and return value type are the same
Parameter list: (parameter type, parameter name)
Method call:
Static method: called through (class name. Method) in main method
Dynamic method: instantiate this class
Object type object name = object value;
Student student= new Student();
Examples of calling static and non static methods:

//Demo02 class
public class Demo02 {
	public static void main(String[] args) {
		//Call of non static method
		//Instantiate this class
		//Object type object name = object value;
		Student student = new Student();
		//Call of static method
//Student class
public class Student {
	//Non static method
	public void say() {
		System.out.println("Class is over");
	//Static method
	public static void say1() {
		System.out.println("Class is over");

be careful:
//static is loaded with the class,
//public static void a() {b();} An existing call and a non-existent call will report an error
public void a() {b();}
//Class does not exist until it is instantiated
public void b() {}
Value passing and reference passing:

//Reference passing: object, essence or value passing
public class Demo05 {
	public static void main(String[] args) {
		Person person = new Person();
		System.out.println(;//Qin Jiang
	public static void change(Person person) {"xyz";
class Person{
	String name;

Relationship between class and object

Class: it is an abstract data type. It is the overall description / definition of a certain thing, but it cannot represent a specific transaction.
Object: a concrete instance of an abstract concept.
Example 1
Student class:

//Student class
public class Student {
	String name;
	int age;
	public void study() {
		System.out.println("I'm learning");

Application class:

//There should be only one main method in a project
public class Application {
	public static void main(String[] args) {
		//Class: abstract, instantiated
		//Class will return its own object after instantiation
		//The stu object is a concrete instance of the Student class
		Student xm= new Student();
		Student xh= new Student();"Xiao Ming";
		xm.age=18;"Xiao Hong";

Parametric structure and nonparametric structure


public class Person {
	//Even if a class is not written, it will have a method
	//Display definition constructor
	String name;	
	//Nonparametric construction method
	//1. Using the new keyword is essentially calling the constructor
	//2. Used to initialize values
	public Person() {		
	//Structure with parameters: once a structure with parameters is defined, a structure without parameters must display the definition
	//The first name refers to the upper String name;
	//The second name refers to the parameter passed in
	//heavy load
	public Person(String name) {;
 public static void main(String[] args) { 
	//new Instantiated an object
	Person person = new Person("kuangshen");//Call the parameterized construct in Person

1. Same as class name
2. No return value
effect: essentially calls the construction method again
2. Initialize the value of the object
1. After defining a parameterized structure, if you want to use a parameterless structure, a parameterless structure will be defined.
eclipse shortcuts:
1.alt + / generate parameterless constructor or promotion information
2.alt+shift+s+o generation zone parameter structure

Create object memory analysis


1. Class and object
Class is a template, abstract
The object is concrete
2. Method:
Call: static and dynamic
3. Reference of object:
reference type
Basic type: 8
Objects are operated by reference: stack - > heap
4. Attributes:
Fields, member variables
Default initialization:
Number: 0.0
Reference type: null
Definition of attribute: modifier attribute type attribute name = attribute value;
5. Creation and use of objects
1) You must use the new keyword to create an object, and the constructor Person xyz = new Person();
2) Object properties: XYZ name
3) Object method: xyz,sleep();
6. Category:
Static properties
Dynamic behavior method

Topics: Java Back-end