This learning content comes from station b: dark horse programmer
Note: I was speechless and failed to punch in. Normally, this should be the fourth day. After listening to the object-oriented part, I didn't understand it. I listened to it again. Now I feel a little on the road, but I know it's only the simplest, and there's more difficult to wait for me. There's a long way to go. Keep working hard! If you see this article can help you, giving me a praise or comment is also the biggest driving force for me to update! Thank you. If there is any mistake in the content, you are welcome to criticize and correct!!!
I What to master when facing the object
1. What is oriented programming
1.1 what are classes and objects
1.2 how to design class
Code implementation: car class
public class Car {
//Member variable
String name;
double price;
//method
public void start(){
System.out.println(name + "It's on");
}
public void run(){
System.out.println("The price is:" + price + "of" + name +"Run fast");
}
}
Code implementation: test class
//Learn to design objects and use them
public class Test1 {
public static void main(String[] args) {
//Create a car object
Car c = new Car();
c.name = "bmw";
c.price = 38.9;
System.out.println(c.name);
System.out.println(c.price);
c.start();
c.run();
System.out.println("=============");
Car c1 = new Car();
c1.name = "Benz";
c1.price = 39.5;
System.out.println(c1.name);
System.out.println(c1.price);
c1.start();
c1.run();
}
}
1.3 precautions for defining classes
Code implementation: Student class
public class Student {
public String name;
boolean flag;
}
class Animal{
}
class Dog{
}
Code implementation: test class
//Objective: to understand several supplementary precautions of definition class and prepare for subsequent knowledge learning
public class Test2 {
public static void main(String[] args) {
/*1,The first letter of the class is recommended to be capitalized to meet the hump mode, and keywords cannot be used,
Must be a legal identifier and meaningful
2,Multiple class classes can be defined in a Java file, but only one class is a public modifier,
Moreover, the class name modified by public must become the code file name
In actual development, it is recommended that only one class be defined in a code file
3,The complete definition format of member variable is: modifier data type variable name = initialization value:
Generally, there is no need to specify the initialization value, and there is a default value
Rules for default values:
byte short int long O
double flat 0.0
boolean false
String Reference type null
*/
Student s = new Student();
System.out.println(s.name);
System.out.println(s.flag);
}
}
2. Object memory diagram
2.1 two object memory diagram
2.2 two variables refer to the same object
Code implementation: Student class
public class Student {
String name;
char sex;
String hobby;
public void study(){
System.out.println("name:" + name + ",Gender:" + sex + ",Hobbies:" + hobby
+ "Start learning!");
}
}
Code implementation: test class
//Objective: to understand the memory mechanism of two variables pointing to one object
public class Test {
public static void main(String[] args) {
//1. Create student object
Student s1 = new Student();
s1.name = "Xiao Ming";
s1.sex = 'male';
s1.hobby = "Sleep, play, study";
s1.study();
//Assign the s1 variable to a student type variable s2
Student s2 =s1;
System.out.println(s2);
System.out.println(s1);
s2.hobby ="Love to ask questions";
System.out.println(s2.name);//Xiao Ming
System.out.println(s2.sex);//male
System.out.println(s1.hobby);//Love to ask questions
s2.study();
}
}
When the variable is assigned null, Java has an automatic garbage collector for regular cleaning
3. Learn to use constructors
3.1 function and classification format of constructor
Code implementation: car class
public class Car {
String name;
double price;
//Parameterless constructor
public Car(){
System.out.println("==The parameterless constructor was called=");
}
//Parametric constructor
public Car(String n,double p){
System.out.println("==A constructor with parameters was called=");
name = n;
price = p;
}
}Code implementation: test class
//Objective: to understand constructors and understand the role of two types of constructors
public class Test {
public static void main(String[] args) {
//Get the object by calling the constructor
Car c = new Car();
c.name = "bmw";
c.price = 38.9;
System.out.println(c.name);
System.out.println(c.price);
//Calling a parameterized constructor
Car c2 = new Car("Benz",50);
System.out.println(c2.name);
System.out.println(c2.price);
}
}
3.2 precautions for constructor
3.3 constructor summary
4.this keyword
4.1 what is this keyword
Code implementation: car class
public class Car {
String name;
double price;
//Parametric constructor
public Car() {
System.out.println("In a parameterless constructor this:" + this);
}
public Car(String name, double price) {
this.name = name;
this.price = price;
}
public void gowith(String name) {
System.out.println(this.name + "And" + name + "match");
}
public void run() {
System.out.println("In method this:" + this);
}
}
Code implementation: test class
//Objective: to understand the function of this keyword
public class Test {
public static void main(String[] args) {
//Create a car object
Car c = new Car();
c.run();
System.out.println(c);
System.out.println("==================");
Car c2 = new Car("Benz",50);
System.out.println(c2.name);
System.out.println(c2.price);
c2.gowith("bmw");
}
}4.2 function of this keyword
4.3 summary
5. Packaging
5.1 what is encapsulation
5.2 how to make packaging better
Code implementation: Student class
public class Student {
//1. Member variables are decorated with private and can only be accessed in this class
private int age;
//2. Provide a complete set of getter and setter methods to expose their value and assignment
public void setAge(int age){
if(age >= 0 && age <=200 ){
this.age = age;
}else {
System.out.println("There is a problem with your age data:");
}
}
public int getAge(){
return age;//You can directly return age in the same class
}
}
Code try: test class
//Objective: learn how to package better.
public class Test2 {
public static void main(String[] args) {
Student s = new Student();
s.setAge(23);//assignment
System.out.println(s.getAge());//Value
}
}
6. Standard JavaBean s
6.1 what is a JavaBean
Code implementation: User class
public class User {
//1. Use private for member variables
private String name;
private double height;
private double salary;
//3. A parameterless constructor is required, and a parameterless constructor is optional
public User() {
}
public User(String name, double height, double salary) {
this.name = name;
this.height = height;
this.salary = salary;
}
//2. A complete set of setter and getter methods must be provided for member variables
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getHeight() {
return height;
}
public void setHeight(double height) {
this.height = height;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
}
Code implementation: test class
//Objective: remember the writing requirements of JavaBean s
public class Test {
public static void main(String[] args) {
//1. The parameterless constructor creates an object that encapsulates a user's information
User u1 = new User();
u1.setName("Hangzhou Cai Xukun");
u1.setHeight(183);
u1.setSalary(19999);
System.out.println(u1.getName());
System.out.println(u1.getHeight());
System.out.println(u1.getSalary());
//3. A parameter constructor creates an object to encapsulate a user information
User u2 = new User("Hangzhou delireba",170,19999);
System.out.println(u2.getName());
System.out.println(u2.getHeight());
System.out.println(u2.getSalary());
}
}
7. Differences between member variables and local variables
II Summary
