Java package (learning java programming language)

Write if you want 2021-01-21 23:24:15
java package learning java programming


Java Allow the use of packages (package) Organize classes into a collection . With the help of packages, you can easily organize your own code , And separate their own code from the code base provided by others .

The standard Java Class libraries are distributed in multiple packages , Include java.long、java.util and java.net etc. . The standard Java Packages have a hierarchy . All standard Java The bags are all in java and javax In the package hierarchy .

1. Package name

The main reason to use a package is to ensure that the class name is unique . in fact , To ensure the absolute uniqueness of the package name , To use an Internet domain name ( This is obviously the only one ) Take the package name in reverse order , Then use different sub packages for different projects . for example , domain name xiang117.com, Write in reverse order , Get the package name com.xiang117. Then you can append a project name , Such as com.xiang117.corejava. If you're putting Employee Class in this bag , So this kind of “ Completely limited ” The name is com.xiang117.corejava.Employee.

From the compiler's point of view , There is no relationship between nested packages . for example ,java.util Bag and java.util.jar The bag has nothing to do with it . Each package is a separate collection of classes .

2. Class import

A class can use all the classes in its package , And the public classes in other packages (public class).
There are two ways to access public classes in another package :

  1. Use fully qualified names (fully qualified name), The package name is followed by the class name .
    java.time.LocalData today = java.time.LocalDate.now();
  2. Use import sentence .

have access to import Statement to import a specific class or entire package .import Statement should be at the top of the resource file ( But in package After statement ).

import Statement is a simple way to refer to the classes in a package . Once used import sentence , When using classes , You don't have to write the full name of the class .

import The only advantage of a sentence is its simplicity . You can use a short name instead of a full package name to refer to a class .

You can use the following statement to import java.time All classes in the package . import java.time.*;
You can use LocalDate today = localDate.now();
There is no need to prefix the package .

java.time.* Its grammar is relatively simple , There's no negative impact on the size of the code . Of course , If you can clearly indicate the imported class , It will make the readers of the code know more exactly which classes you are using .

It should be noted that , Only asterisks can be used (_) Import a package , They can't be used import java._ or import java._._ Import to java All packages for prefix .

You can import specific classes in a package : import java.time.LocalDate;

in the majority of cases , You can import only the packages you need , You don't have to think too much about them . But in the event of a naming conflict , Pay attention to the bag . for example ,java.util and java.sql All the bags have Date class . If you import these two packages into the program : import java.util.; import java.sql.;
Use... In the program Date Class time , A compilation error will occur : Date today; // Error--java.util.Date or java.sal.Date?
At this point, the compiler cannot determine which one the program is using Date class . You can add a specific import To solve this problem : import java.util.; import java.sql.; import iava.util.Date;
If these two Date Classes need to use , You need to prefix each class name with the full package name . java.util.Date deadline = new java.util.Date(); java.sql.Date today = new java.sql.Date();

Locating a class in a package is a compiler (compiler) The job of . Bytecode in a class file always references other classes with the full package name .

3. Static import

There is a kind of import Statement allows static methods and static fields to be imported , Not just classes .

If it's at the top of the source file , Add an instruction : import static java.lang.System.*;
You can use System Class static methods and static fields . Without prefixing the class name : out.println("Goodbye, World!"); // i.e., System.out exit(0); // i.e., System.exit

You can import specific methods or fields : import static java.lang.System.out;

actually , Is there a lot of Website monitoring Programmers want to abbreviate System.out or System.exit, It's doubtful . The code written in this way doesn't look very clear . however , sqrt(pow(x, 2) + pow(y, 2))
Looks like Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2))
Much clearer .

4. Add classes to the package

To put a class in a package , You have to put the package name at the beginning of the source file , That is, before defining the code of each class in this package . For example, in com.xiang017.corejava Add... To the bag Employee class :

Employee.java

package com.xiang017.corejava;

public class Employee

{

...

}

If it is not placed in the source file package sentence , The classes in this source file belong to _ Nameless bag (unnamed package)_. No name bag, no name bag .

PackageTest.java

import com.xiang017.javacore.*;

public class PackageTest {

public static void main(String[] args) {

Employee harry = new Employee("Harry Hacker", 50000, 1989, 10, 1);

}

}

Place the source file in a subdirectory that matches the full package name . for example ,com.xiang017.corejava All source files in the package should be placed in subdirectories com/xiang017/corejava in . The compiler places class files in the same directory structure . for example :

. ( Base directory )

PackageTest.java

└─com

└─xiang017

└─javacore

Employee.java

To compile this program , Just switch to the base directory , And run the command : javac PackageTest.java
The compiler will automatically look up the file com/xiang017/corejava/Employee.java And compile . The compiled result is as follows :

. ( Base directory )

PackageTest.class

PackageTest.java

└─com

└─xiang017

└─javacore

Employee.class

Employee.java

To run this program , You need to execute the command in the base directory : java PackageTest

If PackageTest.java The source file is not in the base directory , stay mycompany It's a bag , Such as :

.( Base directory )

|-- com/

|-- corejava/

| |-- Employe.java

| |-- Employe.class

|

|-- mycompany/

|-- PackageTest.java

|-- PackageTest.class

Still compile and run classes from the base directory , Which contains com Directory of directories : javac com/mycompany/PackageTest.java java com.mycompany.PackageTest

We need to pay attention to , Compiler to file ( Extension with file separator .java The file of ) To operate . and Java Interpreter load class ( with . The delimiter ).

Warning : The compiler does not check the directory structure when compiling the source file . for example , Suppose there is a source file with the following instructions at the beginning : package com.mycompany;
Even if the source file is not in a subdirectory com/mycompany Next , You can also compile . If it doesn't depend on other packages , You can compile without compile errors . however , The final program will not run , Unless you first move all the class files to the correct location . If the package does not match the directory , The virtual machine can't find the class .

PackageTest.java

import com.xiang017.javacore.*;

public class PackageTest {

public static void main(String[] args) {

Employee harry = new Employee("Harry Hacker", 50000, 1989, 10, 1);

System.out.println("name=" + harry.getName() +

",salary=" + harry.getSalary() +

",hireDay=" + harry.getHireDay());

}

}

com/mycompany/Employee.java

package com.xiang017.javacore;

import java.time.LocalDate;

import java.util.Objects;

public class Employee {

private String name;

private double salary;

private LocalDate hireDay;

public Employee(String name, double salary, int year, int month, int day) {

this.name = name;

this.salary = salary;

hireDay = LocalDate.of(year, month, day);

}

public String getName() {

return name;

}

public double getSalary() {

return salary;

}

public LocalDate getHireDay() {

return hireDay;

}

public void raiseSalary(double byPercent) {

double raise = salary * byPercent / 100;

salary += raise;

}

}

5. Package access

Marked as public Can be used by any class ; Marked as private Can only be used by the class that defines them . If not specified public or private, This part ( class 、 Method or variable ) Can be accessed by all methods in the same package .

A class without a tag modifier can only be accessed by other classes in the same package . For a class , This default is reasonable . however , It's not appropriate for variables , Variables must be explicitly marked as private, Otherwise, the package will be visible by default . This will destroy encapsulation .

By default , A package is not a closed entity . in other words , Anyone can add more classes to the package .

from 1.2 version ,JDK The implementer of has modified the class loader , Explicitly prohibit loading package names to “java.” The user-defined class at the beginning ! Of course , User defined classes don't benefit from this protection . Another mechanism makes JAR The file declares that the package is sealed (sealed), To prevent third parties from modifying , But this mechanism is out of date . Now it's time to use the module wrapper .

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