Overview of packaging

Design idea of packaging

• Hide the internal complexity of the object, and only disclose simple interfaces to facilitate external calls, so as to improve the scalability and maintainability of the system
• High cohesion: the internal data operation details of the class are completed by themselves, and external interference is not allowed
• Low coupling: only a small number of methods are exposed for use

Implementation of encapsulation

public class Student {
  private String name;
  private int age;

  public void setName(String n) {
    name = n;
  }

  public String getName() {
    return name;
  }

  public void setAge(int a) {
    age = a;
  }

  public int getAge() {
    return age;
  }
}

Permission modifier

Constructor (Construct0r)

Function of constructor

• Initialize the instance object and assign the initial value to the member variable of the object

Example

public class Student {
	private String name;
	private int age;
	// Nonparametric structure
  	public Student() {} 
 	// Parametric structure
  	public Student(String n,int a) {
		name = n;
    	age = a; 
	}
    //Other codes are omitted here
}

be careful

• Java will automatically provide a parameterless constructor. Once the constructor is defined, the default parameterless constructor provided by Java will become invalid
• The constructor name must be the same as the class name in which it resides
• It has no return value, so void is not required
• Constructors cannot be modified by static, final, synchronized, Abstractct, or native

this keyword

The meaning of this

• This is used in the constructor to represent the example object being created, that is, who is new, this represents who
• This is used in the instance method to represent the object calling the method, that is, this represents who is calling

Standard JavaBean s

public class Student {
	// Member variable
	private String name;
	private int age;

	// Construction method
	public Student() {
	}

	public Student(String name, int age) {
		this.name = name;
		this.age = age;
	}

	// get/set member method
	public void setName(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public int getAge() {
		return age;
	}
    
    //List of other member methods
    public String getInfo(){
        return "full name:" + name + "，Age:" + age;
    }
}

package

Function of package

• Avoid duplicate class names
• The classification organization manages a large number of classes

Common packages of JDK

• java.lang ---- contains some core classes of Java language, such as String, Math, Integer, System and Thread, and provides common functions
• java.io - contains classes that can provide multiple input / output functions
• java.util - contains some utility classes, such as collection framework class, date and time, array tool class, Arrays, text Scanner, Scanner, Random value generation tool, Random
• java.sql and javax SQL ---- contains the relevant classes / interfaces for JDBC database programming with Java
• java.net ---- contains classes and interfaces that perform network related operations

Declare the syntax format of the package

package Package name;

be careful

• Must be on the first line of source code
• A source file can only have one statement declaring a package
• Each word is lowercase and used between words separate
• Accustomed to using company domain name inversion

Classes that use other packages

Use the full name of the type

• java.util.Scanner input = new java.util.Scanner(System.in);

Import using import

import package.Class name;
import package.*;
import static package.Class name.Static member; 

be careful

• Using Java Classes under Lang package can be used directly without import statement
• The import statement must be under package and above class
• When using classes with the same name in two different packages, for example: Java util. Date and Java sql. Date. One uses a full name and one uses a simple name

Example

package com.atguigu.test;

import java.util.Scanner;
import java.util.Date;
import com.atguigu.bean.Student;

public class Test{
    public static void main(String[] args){
        Scanner input = new Scanner(System.in);
        Student stu = new Student();
        String str = "hello";
        
        Date now = new Date();
        java.sql.Date d = new java.sql.Date(346724566);        
    }
}