Java learning summary: 17

Package and access control permissions

Package definition

In Java, use the package keyword to define a package. This statement must be written on the first line of the xxx.java file.
Example: Definition package

package com.study.Demo;	//定义程序所在包，此语句必须放在首行

public class Hello {
    public static void main(String args[]){
        System.out.println("Hello World");
    }
}

Package import

Example: define a class

package com.study.A;

public class Message {
    public void print(){
        System.out.println("666");
    }
}

Example: define another class, this class should use the previously defined class

package com.study.test;
import com.study.A.Message;	//导入所需要的类
public class TestMessage {
    public static void main(String args[]){
        Message msg=new Message();	//实例化对象
        msg.print();				//调用方法
    }
}
//结果
//666

Example: Import multiple classes in a package

package com.study.test;
import com.study.A.*;	//自动导入指定包中所需的类
public class TestMessage {
    public static void main(String args[]){
        Message msg=new Message();
        msg.print();
    }
}

If there is a conflict with the same name in different packages, you must add the package name when using the class.
Such as:

com.study.test.Message msg=new com.study.A.Message();

System common package

Access control authority

For the above table, it can be simply understood that
private can only be accessed in one class;
default can only be accessed in one package;
protected can be accessed in subclasses of different packages;
public is all.

For beginners, grasp the following two basic principles of access rights:
attribute declarations mainly use private declarations,
method declarations mainly use public declarations

Example:

package com.study.A;

public class A {
    protected String info="Hello";	//使用protected权限定义
}

package com.study.Demo;
import com.study.A.A;
public class B extends A {	//是A不同包的类
    public void print(){
        System.out.println("A类的info="+super.info);	//直接访问父类中的protected属性
    }
}

package com.study.Demo;
import com.study.Demo.B;
public class Test {
    public static void main(String args[]){
        new B().print();
    }
}
//结果
//A类的info=Hello

Naming convention (although I am not very careful)

Class name: capitalization of each word at the beginning, for example: TestDemo;
variable name: capitalization of the first letter of the first word, capitalization of the first letter of each word thereafter, for example: studetName;
method name: the first letter of the first word Lowercase, capitalize the first letter of each word after it, for example: printInfo ();
constant name: capitalize each letter, for example: FLAG;
package name: lowercase all letters, for example: com.study