This is the 13th day of my participation in the November Gwen Challenge. Check out the event details: The last Gwen Challenge 2021

Continue today ~

The enumeration

• Commonly used to represent a set of constants, such as four seasons in a year
• Use keywordsenumdefine

enum Game{
    CSGO,LOL
}
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Enumerations are used in inner classes

public class Test{
    enum Game{
        CSGO,LOL
    }
    public static void main(String[] args){
        Game g1 = Game.CSGO;
        System.out.println(g1);/ / output CSGO}}Copy the code

Ps: Enumerations can be interpreted as lists of identical key-value pairs

Iterated enumeration element

enum Game{
    CSGO,LOL
}
public class Test{
    public static void main(String[] args){
        for(Game games : Game.values()){ System.out.println(games); }}}Copy the code

Ps: values() can fetch all the values, using Java to enforce the for loop to loop the values and print them out

Use enumerated classes in Switch

enum Game{
    CSGO,LOL
}
public class Test{
    public static void main(String[] args){
        Game g1 = Game.CSGO;
        switch(g1){
            case CSGO:
                System.out.println(counter-Strike);
                break;
            case LOL:
                System.out.println(League of Legends);
                break; }}}Copy the code

Ps: Enumeration instantiates the obtained values, the individual values themselves, which can be used directly in the switch for matching

values().ordinal() 和 valueOf()methods

values()– Returns all values in the enumeration

ordinal()– Returns all indexes in the enumeration

valueOf()– Returns an enumeration constant for the specified string

enum Game{
    CSGO,LOL
}
public class Test{
    public static void main(String[] args){
        Game[] arr = Game.values();
        
        for(Game gow : arr){
            System.out.println(gow);/ / value
            System.out.println(gow.ordinal());/ / index
        } 
        System.out.println(Game.valueOf("LOL")); }}Copy the code

Enumeration class

• Enumerations can have their own variables, methods, constructors
• The constructor of the enumeration must be private
• If an enumeration class has an abstract method in it, its instance must implement it

enum Game{
    CSGO,LOL;
    
    private Game(a)
    {
        System.out.println("the game is start");
    }
    public void GameMode(a)
    {
        System.out.println("wa ha ha"); }}Copy the code

package

• Used to distinguish namespaces
• Put functional-related classes or interfaces in a package for easy application
• Defined by the package keyword, write first

For example, a package with a path of code/ Java /hello. Java would be defined as:

package code.java
public class hello{}Copy the code

Java package

java.lang– Package basic classes

java.io– Functions that contain input/output functions

Create a package

Write two files with the same package:

package game;

interface Game{
    public void watch(a);
    public void play(a);
}
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package game;
public class Computer implements Game{
    public void watch(a){
        System.out.println("i watching you");
    }
    public void play(a){
        System.out.println("i want play");
    }
    public static void main(String[] args){
        Computer cm = newComputer(); cm.watch(); cm.play(); }}Copy the code

Then compile them and create the package with mkdir:

mkdir game
cp Game.class Computer.class game
java game/Computer
Computer watch
Computer play
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The import keyword

• Use the import keyword to import the desired package

importThe package name. *Copy the code

Ps: * represents the import of everything in the package. You can import the corresponding class by replacing the * with the corresponding class name

The directory structure of package

There are two situations when a class is placed in a package:

• The package name becomes part of the class name
• The package name must match the directory structure

Java data structure

The enumeration

• The enumeration interface defines a way to retrieve contiguous elements from a data structure

A collection of

• The bitset class implements a set of bits or flags that can be set and cleared individually
• Useful for setting and clearing booleans

vector

• Vectors are very similar to traditional arrays, but they can change dynamically

The stack

• Think of it as an open container, in and out

The dictionary

• It’s a collection of key-value pairs
• It is an abstract class

Hash table

• Is a means of organizing data based on user-defined key structures

attribute

• The value for each key in the property list is a string

That’s all for today. Good night