[Java] Object-oriented syntax-Package

What is a package?

• ** Affiliation of class / interface, etc. **
• ** Fully qualified name **: A name that takes into account the namespace
• ** Declared with package instruction **
• Write ** only once ** at the beginning of the file
• All subsequent classes and interfaces belong to that package
• Role of package
• ** Classify types and identify names **
• ** Functional classification **
• java.text Package: Text / Date / Numeric Processing Class:
• java.sql package: SQL related classes
• ** Access control unit **
• Excessive information can be hidden from the outside by defining packages in appropriate units.

Naming rules

• ** Hierarchical structure is represented by . **
  • package com.example.neko.object;
• ** Package name set to lowercase **
• Easy to see class name in uppercase letters
• Each hierarchical element of the package (separated by.) And the simple name of the class / interface ** share the same namespace **
• ** Named based on domain name **
• Add ** Internet domain in reverse order to package prefix **
• Domains are unique so package names don't conflict
• ** Package hierarchy and file system hierarchy are in correspondence **

Default package

• ** Anonymous package **
• If the package declaration is omitted, the subordinate classes belong to the default package.
• I want to avoid it because there is a possibility of name collision

Name resolution

• ** Import command ** can be used to write fully qualified names as simple names

import com.example.neko.object.Animal;
Animal animal = new Animal();

• import imports ** name resolution information **
• It doesn't import the class file itself, so it doesn't make the class file bloated **
• Even if the import destination class is updated ** There is no need to recompile the import source class **

Import type

• ** Single type import **

  • import java.util.Map;

• ** On-demand import **

  • import java.util.*;

• Import all types under the package

• Note: Only java.util packages are imported, not packages such as java.util.regex, java.util.stream, etc.

• java.util and java.util.regex have a flat relationship with each other

• Benefits of single import

• ** Name resolution has different priorities **

• Preferred order

• 1. The type currently defined in the file
• 2. Single import type
• 3. Same package type
• 4. On-demand import type

• ** Be careful not to duplicate the standard library type name when defining it yourself **

• Easy to see the relationship between class and package

static import

• ʻImport static fully qualified name`
• You will be able to abbreviate types such as classes, structs, and enumerations as well as packages.
• Simple application with frequently used static methods / constants / static member classes

import static java.lang.Math.abs;
//All static methods under Math class are statically imported
//import static java.lang.Math.*;

public class ImportStatic {

  public static void main(String[] args) {
    System.out.println(abs(-10));
  }
}