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  • Java Packages

    2021-03-15 21:37:07
    Java packages are a mechanism to group Java classes that are related to each other, into the same "group" (package). When a Java project grows bigger, for instance an app or API, it is useful to split...

    Java packages are a mechanism to group Java classes that are related to each other, into the same "group" (package). When a Java project grows bigger, for instance an app or API, it is useful to split the code into multiple Java classes, and the classes into multiple Java packages. When you divide classes into multiple Java packages, it becomes easier to figure out where a certain class you are looking for is.

    A Java package is like a directory in a file system. In fact, on the disk a package is a directory. All Java source and class files of classes belonging to the same package are located in the same directory.

    Java packages can contain subpackages. Java packages can thus make up what is called a package structure. A Java package structure is like a directory structure. Its a tree of packages, subpackages and classes inside these classes. A Java package structure is indeed organized as directories on your hard drive, or as directories inside a zip file (JAR files).

    Here is a screenshot of an example Java package structure:

    packages-1.png

    An example Java package structure.

    At the top you see a directory called "src". This is the source root directory. It is not a Java package itself. Inside this directory, all subdirectories correspond to Java packages. Thus, "collections", "com", "concurrency" etc. are all Java packages (which are also directories on the disk). In the screenshot above the Java packages are illustrated with a folder icon.

    I have expanded two of the sublevel Java packages, so you can see the classes inside. The classes are illustrated using a little blue circle with a C inside, in the screenshot above.

    The full path of a subpackage is its name, with all ancestor package names in front of it, separated by dots. For instance, the full path of "navigation" subpackage is:

    com.jenkov.navigation

    Similarly, the fully qualified name of a Java class includes its package name. For instance, the full qualified name of the "Page" class, is:

    com.jenkov.navigation.Page

    Creating a Java Package Structure

    To create a Java package you must first create a source root directory on your hard disk. The root directory is not itself part of the package structure. The root directory contains all the Java sources that need to go into your package structure.

    Once you have created a source root directory you can start adding subdirectories to it. Each subdirectory corresponds to a Java package. You can add subdirectories inside subdirectories to create a deeper package structure.

    Adding Classes to Packages

    In order to put add Java classes to packages, you must do two things:

    Put the Java source file inside a directory matching the Java package you want to put the class in.

    Declare that class as part of the package.

    Putting the Java source file inside a directory structure that matches the package structure, is pretty straightforward. Just create a source root directory, and inside that, create directories for each package and subpackage recursively. Put the class files into the directory matching the package you want to add it to.

    When you have put your Java source file into the correct directory (matching the package the class should belong to), you have to declare inside that class file, that it belongs to that Java package. Here is how you declare the package inside a Java source file:

    package com.jenkov.navigation;

    public class Page {

    ...

    }

    The first line in the code above (in bold) is what declares the class Page as belonging to the package com.jenkov.navigation.

    Java Package Naming Conventions

    Java packages are always written in lowercase letters. Not like Java classes, where the first letter is usually a capital letter.

    To avoid creating packages with the same names as other public Java packages it is recommended that you start your package hierarchy with the reverse domain name of your company. For instance, since the domain name of my company is jenkov.com I should start with a package structure called com.jenkov. In other words, a top level package named com with a subpackage inside called jenkov.

    Importing Classes From Other Java Packages

    If a class A needs to use the class B, you must reference class B inside class A. If class A and B are located in the same Java package, the Java compiler will accept references between the two classes. Here is an example of that:

    public class B {

    public void doIt() {

    // do something...

    }

    }

    public class A {

    public static void main(String[] args){

    B theBObj = new B();

    b.doIt();

    }

    }

    If the classes A and B are located in the same Java package, there is no problem with the code above. However, if class A and B are located in different Java packages, then class A must import class B in order to use it. Here is how that could look:

    import anotherpackage.B;

    public class A {

    public static void main(String[] args){

    B theBObj = new B();

    b.doIt();

    }

    }

    It is the first line in the example that imports class B. The example assumes that class B is located in a Java package named anotherpackage.

    If class B had been located in a subpackage of anotherpackage you would have to list the full package and subpackage path to class B. For instance, if class B was located in package anotherpackage.util then the import statement would have looked like this:

    import anotherpackage.util.B;

    Importing All Classes From Another Package

    If you need to use a lot of classes from a certain Java package, importing them one at a time results in a lot of import statements. It is possible to import all classes of a package using the * character instead of a class name. Here is how such an import statement looks:

    import anotherpackage.util.*;

    Using Classes via Their Fully Qualified Class Name

    It is possible to use a class from another package without importing it with an import statement. You can write the fully qualified class name instead of just the name of the class itself. The fully qualified class name consists of the full package path down to the subclass containing the class, and also including the class name itself. The fully qualified class name is also what you would write in an import statement. For instance:

    anotherpackage.util.TimeUtil;

    You could use this fully qualified class name to reference the TimeUtil class inside another class, like this:

    public class A {

    public static void main(String[] args){

    anotherpackage.util.TimeUtil timeUtil =

    new anotherpackage.util.TimeUtil();

    timeUtil.startTimer();

    }

    }

    How to Divide Your Classes Into Packages

    You might be wondering how you decide on what Java packages to create, and how to divide your classes into these packages. There is no official standard for that, but there are two commonly used methods.

    Divide by Layer

    The first method is to divide classes after what "layer" of the application they belong to. For instance, your application may have a database layer. Then you would create a database package. All classes involved in communication with the database would then be located in the database package.

    Divide by Application Functionality

    The second method is to divide your classes based on what part of the application functionality they belong to. Thus, if your application has a functionality area which calculates pensions, you might create a Java package named pension . All classes involved in one way or another in the pension calculations would go into that package (or subpackages, if the number of classes in pension grow large).

    Combined with my domain name, the package structure for a pension package would be:

    com.jenkov.pension

    A total of three packages, two nested inside other packages (jenkov inside com, and pension inside jenkov).

    The "divide by application functionality" method of dividing classes into Java packages tends to work better than "divide by layer", as your application grows in number of classes. Instead of having a fixed number of layer packages which keep growing in number of classes, you get a growing number of application functionality packages with a smaller number of classes inside. The "divide by application functionality" is the method I use, and which most of my colleagues are using too.

    Built-in Java Packages

    The Java platform comes with a lot of built-in Java packages. These packages contain classes for all kinds of purposes that programmers often need, like reading and writing files from the local hard disk, sending and receiving data over networks and the internet, connecting to databases, and many, many other things. I have covered many of these packages (APIs) in separate tutorials. If you go the frontpage of my tutorial website, you can see a list of what I have covered so far:

    展开全文
  • Upgrade 1 Package (+1 Dependent package) Total download size: 2.8 M Is this ok [y/d/N]: y Downloading packages: No Presto metadata available for updates (1/2): httpd-tools-2.4.6-97.el7.centos.1.x86_...

     安装报错,报错内容如下

    [root@test ~]# yum install httpd -y

    error: rpmdb: BDB0113 Thread/process 8835/139774366890048 failed: BDB1507 Thread died in Berkeley DB library

    error: db5 error(-30973) from dbenv->failchk: BDB0087 DB_RUNRECOVERY: Fatal error, run database recovery

    error: cannot open Packages index using db5 -  (-30973)

    error: cannot open Packages database in /var/lib/rpm

    CRITICAL:yum.main:

    Error: rpmdb open failed

    解决方法如下:

    [root@test ~]# cd /var/lib/rpm

    [root@test rpm]# ll

    total 178512

    -rw-r--r--. 1 root root   6684672 Oct 12 14:35 Basenames

    -rw-r--r--. 1 root root     16384 Oct 12 14:35 Conflictname

    -rw-r--r--  1 root root    311296 Nov  4 17:35 __db.001

    -rw-r--r--  1 root root     90112 Nov  4 17:35 __db.002

    -rw-r--r--  1 root root   1318912 Nov  4 17:35 __db.003

    -rw-r--r--. 1 root root   2265088 Oct 12 14:35 Dirnames

    -rw-r--r--. 1 root root     40960 Oct 12 14:35 Group

    -rw-r--r--. 1 root root     45056 Oct 12 14:35 Installtid

    -rw-r--r--. 1 root root    118784 Oct 12 14:35 Name

    -rw-r--r--. 1 root root     24576 Oct 12 14:35 Obsoletename

    -rw-r--r--. 1 root root 167387136 Oct 12 14:35 Packages

    -rw-r--r--. 1 root root   3330048 Oct 12 14:35 Providename

    -rw-r--r--. 1 root root    921600 Oct 12 14:35 Requirename

    -rw-r--r--. 1 root root    212992 Oct 12 14:35 Sha1header

    -rw-r--r--. 1 root root    131072 Oct 12 14:35 Sigmd5

    -rw-r--r--. 1 root root      8192 Oct 12 14:35 Triggername

    [root@test rpm]# rm -f __db.00*

    [root@test rpm]# rpm --rebuilddb  重建rpm数据库

    [root@test rpm]# yum install httpd
    
    Loaded plugins: fastestmirror, langpacks
    
    Determining fastest mirrors
    
    epel                                                                                                                                                | 4.7 kB  00:00:00    
    
    extras                                                                                                                                              | 2.9 kB  00:00:00    
    
    os                                                                                                                                                  | 3.6 kB  00:00:00    
    
    salt-latest-repo                                                                                                                                    | 2.9 kB  00:00:00    
    
    updates                                                                                                                                             | 2.9 kB  00:00:00    
    
    (1/4): epel/7/x86_64/updateinfo                                                                                                                     | 1.0 MB  00:00:00    
    
    (2/4): salt-latest-repo/primary_db                                                                                                                  |  42 kB  00:00:00    
    
    (3/4): epel/7/x86_64/primary_db                                                                                                                     | 7.0 MB  00:00:01    
    
    (4/4): updates/7/x86_64/primary_db                                                                                                                  |  12 MB  00:00:01    
    
    Resolving Dependencies
    
    --> Running transaction check
    
    ---> Package httpd.x86_64 0:2.4.6-97.el7.centos will be updated
    
    ---> Package httpd.x86_64 0:2.4.6-97.el7.centos.1 will be an update
    
    --> Processing Dependency: httpd-tools = 2.4.6-97.el7.centos.1 for package: httpd-2.4.6-97.el7.centos.1.x86_64
    
    --> Running transaction check
    
    ---> Package httpd-tools.x86_64 0:2.4.6-97.el7.centos will be updated
    
    ---> Package httpd-tools.x86_64 0:2.4.6-97.el7.centos.1 will be an update
    
    --> Finished Dependency Resolution
    
    
    
    Dependencies Resolved
    
    
    
    ===========================================================================================================================================================================
    
     Package                                 Arch                               Version                                              Repository                           Size
    
    ===========================================================================================================================================================================
    
    Updating:
    
     httpd                                   x86_64                             2.4.6-97.el7.centos.1                                updates                             2.7 M
    
    Updating for dependencies:
    
     httpd-tools                             x86_64                             2.4.6-97.el7.centos.1                                updates                              93 k
    
    
    
    Transaction Summary
    
    ===========================================================================================================================================================================
    
    Upgrade  1 Package (+1 Dependent package)
    
    
    
    Total download size: 2.8 M
    
    Is this ok [y/d/N]: y
    
    Downloading packages:
    
    No Presto metadata available for updates
    
    (1/2): httpd-tools-2.4.6-97.el7.centos.1.x86_64.rpm                                                                                                 |  93 kB  00:00:00    
    
    (2/2): httpd-2.4.6-97.el7.centos.1.x86_64.rpm                                                                                                       | 2.7 MB  00:00:01    
    
    ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    
    Total                                                                                                                                      2.2 MB/s | 2.8 MB  00:00:01    
    
    Running transaction check
    
    Running transaction test
    
    Transaction test succeeded
    
    Running transaction
    
      Updating   : httpd-tools-2.4.6-97.el7.centos.1.x86_64                                                                                                                1/4
    
      Updating   : httpd-2.4.6-97.el7.centos.1.x86_64                                                                                                                      2/4
    
      Cleanup    : httpd-2.4.6-97.el7.centos.x86_64                                                                                                                        3/4
    
      Cleanup    : httpd-tools-2.4.6-97.el7.centos.x86_64                                                                                                                  4/4
    
      Verifying  : httpd-2.4.6-97.el7.centos.1.x86_64                                                                                                                      1/4
    
      Verifying  : httpd-tools-2.4.6-97.el7.centos.1.x86_64                                                                                                                2/4
    
      Verifying  : httpd-2.4.6-97.el7.centos.x86_64                                                                                                                        3/4
    
      Verifying  : httpd-tools-2.4.6-97.el7.centos.x86_64                                                                                                                  4/4
    
    
    
    Updated:
    
      httpd.x86_64 0:2.4.6-97.el7.centos.1                                                                                                                                    
    
    
    
    Dependency Updated:
    
      httpd-tools.x86_64 0:2.4.6-97.el7.centos.1                                                                                                                              
    
    
    
    Complete!

    遇到上述问题 需要重建rpm数据库

    展开全文
  • STOP: TOTAL NO. of ITERATIONS REACHED LIMIT.

    千次阅读 2021-11-27 12:36:01
    报错 C:\Users\taytay-POI\AppData\Roaming\Python\Python36\site-packages\sklearn\linear_model_logistic.py:939: ConvergenceWarning: lbfgs failed to converge (status=1): STOP: TOTAL NO. of ITERATIONS ...

    关于使用Jupyter Notebook做逻辑回归建模(LogisticRegressionCV)时遇到的问题

    使用LogisticRegressionCV建立逻辑回归模型报错
    C:\Users\taytay-POI\AppData\Roaming\Python\Python36\site-packages\sklearn\linear_model_logistic.py:939: ConvergenceWarning: lbfgs failed to converge (status=1):
    STOP: TOTAL NO. of ITERATIONS REACHED LIMIT.

    Increase the number of iterations (max_iter) or scale the data as shown in:
    https://scikit-learn.org/stable/modules/preprocessing.html.
    Please also refer to the documentation for alternative solver options:
    https://scikit-learn.org/stable/modules/linear_model.html#logistic-regression
    extra_warning_msg=_LOGISTIC_SOLVER_CONVERGENCE_MSG)

    解决办法
    意思是达到限制的迭代总数,只需要增加迭代次数(最大值)或缩放数据就可以。
    将代码改为(增加迭代次数):

    model = LogisticRegressionCV(multi_class='multinomial',max_iter=3000).fit(x_train, y_train)
    

    最大迭代次数默认值为1000,把它改为3000即可。但这样会增加运行时间,等了好一会儿才运行完这一行。

    展开全文
  • ERROR: Could not install packages due to an OSError: HTTPSConnectionPool(host='files.pythonhosted.org', port=443): Failed to establish a new connection: [Errno 8] nodename nor servname provided, or...

    ERROR: Could not install packages due to an OSError: HTTPSConnectionPool(host='files.pythonhosted.org', port=443):

    Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'

    Collecting matplotlib
      WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x10271bb20>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known')': /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz
      WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x10271bb50>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known')': /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz
      WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x10271ba90>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known')': /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz
      WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x102731190>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known')': /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz
      WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x102731640>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known')': /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz
    ERROR: Could not install packages due to an OSError: HTTPSConnectionPool(host='files.pythonhosted.org', port=443): Max retries exceeded with url: /packages/21/37/197e68df384ff694f78d687a49ad39f96c67b8d75718bc61503e1676b617/matplotlib-3.4.3.tar.gz (Caused by NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x102731820>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'))
    

    解决办法:

    (base) renfangri@IrvingonedeMacBook-Air 深度学习100例 % pip install --trusted-host pypi.org --trusted-host files.pythonhosted.org matplotlib
    Collecting matplotlib
      Downloading matplotlib-3.4.3.tar.gz (37.9 MB)
         |████████████████████████████████| 37.9 MB 923 kB/s

    展开全文
  • Install 69 Packages Total size: 45 M Total download size: 28 M Installed size: 292 M Is this ok [y/N]: 就是这样,该应用程序已经安装在你的系统上,你可以开始使用它了。 也可以直接从URL安装rpm软件包,...
  • Docker 安装和部署

    2021-02-05 19:32:20
    3.el7.centos base 21 M httpd-tools x86_64 2.4.6-40.el7.centos.4 updates 83 k mailcap noarch 2.1.41-2.el7 base 31 k Transaction Summary Install 1 Package (+5 Dependent packages) Total download size:...
  • Install 1 Package Total download size: 23 k Installed size: 32 k Downloading Packages: epel-release-8-8.el8.noarch.rpm 33 kB/s | 23 kB 00:00 -----------------------------------------------------------...
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    2020-12-24 00:51:21
    Install 6 Packages Total download size: 3.3 M Installed size: 9.2 M Downloading Packages: (1/6): mariadb-connector-c-config-3.0.7-1.el8.noarch.rp 53 kB/s | 13 kB 00:00 (2/6): mariadb-connector-c-3.0.7...
  • Please install the gcc make perl packages from your distribution。 执行命令行:yum install gcc perl make 输出:Loaded plugins: product-id, search-disabled-repos, subscription-managerThis system is n.....
  • Upgrade 4 Packages Total download size: 3.6 M Is this ok [y/d/N]: y Downloading packages: ————————————————————————————————— Total 20 MB/s | 3.6 MB 00:00:00 Running ...
  • WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection....
  • Install 3 Packages Total download size: 20 M Installed size: 71 M Is this ok [y/N]: y Is this ok [y/N]: y Downloading Packages: [MIRROR] nodejs-full-i18n-10.24.0-1.module_el8.3.0+717+fa496f1d.x86_64....
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    2021-12-03 22:37:22
    [root@nod dockerfile]# docker run 4942d6be2fc5 ls -al total 0 drwxr-xr-x 1 root root 46 Dec 5 09:15 . drwxr-xr-x 1 root root 46 Dec 5 09:15 .. -rwxr-xr-x 1 root root 0 Dec 5 09:15 .dockerenv ...
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    2021-08-26 19:10:20
    x86_64 7.1 kB/s | 15 kB 00:02 Available Packages 3:安装指定版本 # sudo dnf install docker-ce-3:20.10.8-3.el8 Last metadata expiration check: 0:02:07 ago on Thu 26 Aug 2021 07:06:32 PM CST. ...
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    2021-06-27 13:28:39
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  • Install 4 Packages Total size: 1.9 M Installed size: 5.6 M Downloading Packages: Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running ...
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    2021-06-09 10:23:11
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  • -------------------------------------------- Total 559 kB/s | 799 kB 00:01 warning: /var/cache/dnf/nginx-stable-614ef03fac352fea/packages/nginx-1.18.0-2.el8.ngx.x86_64.rpm: Header V4 RSA/SHA1 ...
  • rpm -qa yum clean packages all --- 清除下载到本机的指定的软件包或所有软件包的yum源缓存 yum whatprovides /etc/fstab --- 查看这个文件是由那个包提供的 ——————————————————————————...
  • Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLError(1, u'[SSL: CERTIFICATE_VERIFY_FAI LED] certificate verify failed (_ssl.c:
  • community 436M Transaction Summary===================================================================================================...========================Install3Packages Total download size...
  • Install 13 Packages Total size: 90 M Total download size: 68 M Installed size: 377 M Is this ok [y/N]: y 启动测试均正常 [root@petrichor Docker]# systemctl start docker [root@petrichor Docker]# docker ...
  • 7.0.42/lib$ ls -lg total 6132 -rwxrwxrwx 1 nazar_art 15264 Jul 2 10:59 annotations-api.jar -rwxrwxrwx 1 nazar_art 54142 Jul 2 10:59 catalina-ant.jar -rwxrwxrwx 1 nazar_art 134215 Jul 2 10:59 catalina-...
  • WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3....
  • Webots2021b和ROS2调试笔记21-07-27

    热门讨论 2021-07-27 09:18:28
    Aborted [4.28s] Aborted [10.8s] Summary: 0 packages finished [11.5s] 1 package failed: webots_ros2_importer 2 packages aborted: webots_ros2_msgs webots_ros2_ur_e_description 2 packages had stderr ...
  • 1.WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.urllib3.connection.VerifiedHTTPSConnection ...
  • 题目地址:https://leetcode-cn.com/problems/capacity-to-ship-packages-within-d-days/ 题目难度:Medium 题目描述: 传送带上的包裹必须在 D 天内从一个港口运送到另一个港口。 传送带上的第 i个包裹的重量为...

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