Use and implementation of enumeration classes

Use of enumeration class

• Class has only a limited number of objects , affirmatory .
• When you need to define a set of constants , It is highly recommended to use enumeration classes

Implementation of enumeration class

• JDK1.5 You need to customize the enumeration class before
• JDK 1.5 Newly added enum Keyword is used to define enumeration classes
• If enumeration has only one object , It can be used as a single instance mode .

Enumerate the properties of a class

• The properties of enumeration class objects should not be allowed to be changed , So you should use private final modification
• Use of enumeration class private final The decorated property should be assigned a value in the constructor
• If the enumeration class explicitly defines a constructor with parameters , When listing enumeration values, the corresponding input parameters must also be listed

Custom enumeration class

1. The constructor of the privatized class , Ensure that its objects cannot be created outside of the class
2. Create an instance of the enumeration class inside the class . Declare as ：public static final
3. Object if it has instance variables , Should be declared as private final, And initialize... In the constructor

Use enum Define an enumeration class

• Instructions ：
  • Use enum The enumeration class defined inherits by default java.lang.Enum class , So you can't inherit other classes
  • Constructors of enumeration classes can only use private Permission modifier
  • All instances of an enumeration class must be explicitly listed in the enumeration class (, Separate ; ending ). The listed instance system will automatically add public static final modification
  • The enumeration class object must be declared on the first line of the enumeration class
• JDK 1.5 Can be found in switch Use in expressions Enum Defines the object of the enumeration class as an expression , case Clause can directly use the name of the enumeration value , You don't need to add an enumeration class as a qualification .

Enum Class

• values() Method ： Returns an array of objects of type enumeration . This method can easily traverse all enumeration values .
• valueOf(String str)： You can turn a string into a corresponding enumeration class object . It is required that the string must be an enumeration class object “ name ”. If not , There will be runtime exceptions ：IllegalArgumentException.
• toString()： Returns the name of the current enumeration class object constant

Enumeration classes that implement interfaces

• And the general Java Similar to class , Enumeration classes can implement one or more interfaces
• If each enumeration value behaves the same way when calling the implemented interface method , Then, as long as the method is implemented uniformly .
• If you need each enumeration value to behave differently in the interface method of the call implementation , Let each enumeration value implement the method separately

package com.atguigu.java;

/** *  One 、 Use of enumeration class  * 1. Enumeration class understanding ： Class has only a limited number of objects , affirmatory . We call this class an enumeration class  * 2. When you need to define a set of constants , It is highly recommended to use enumeration classes  * 3. If there is only one object in the enumeration class , Can be used as the implementation method of singleton mode  * *  Two 、 How to define enumeration class  *  Mode one ：jdk5.0 Before , Custom enumeration class  *  Mode two ：jdk5.0, have access to enum Keyword definition enumeration class  * *  3、 ... and 、Enum Common methods in class ： * values() Method ： Returns an array of objects of type enumeration . This method can easily traverse all enumeration values . * valueOf(String str)： You can turn a string into a corresponding enumeration class object . It is required that the string must be an enumeration class object “ name ”. *  If not , There will be runtime exceptions ：IllegalArgumentException. * toString()： Returns the name of the current enumeration class object constant  * *  Four 、 Use enum The enumeration class defined by the keyword implements the interface  *  Situation 1 ： Implementation interface , stay enum Class to implement abstract methods  *  Situation two ： Let the objects of enumeration class implement the abstract methods in the interface  * * @Author damao * @Date 2022/7/13 14:51 * @Version 1.0 */
public class SeasonTest {
    

    public static void main(String[] args) {
    
        Season spring = Season.SPRING;
        System.out.println(spring);
    }

}

// Custom enumeration class 
class Season{
    
    //1. Statement Season Object properties 
    private final String seasonName;
    private final String seasonDesc;

    //2. Privatize the constructor of the class and assign values to object properties 
    private Season(String seasonName,String seasonDesc){
    
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //3. Provides multiple objects of the current enumeration class ：public static final Of 
    public static final Season SPRING = new Season(" spring "," Spring flowers ");
    public static final Season SUMMER = new Season(" summer "," It's hot in summer ");
    public static final Season AUTUMN = new Season(" autumn "," autumn ");
    public static final Season WINTER = new Season(" winter "," a world of ice and snow ");

    //4. Other demands 1： Get the properties of the enumeration class object 

    public String getSeasonName() {
    
        return seasonName;
    }

    public String getSeasonDesc() {
    
        return seasonDesc;
    }

    // Other demands 2： Provide toString()

    @Override
    public String toString() {
    
        return "Season{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

package com.atguigu.java;

/** *  Use enum Keyword definition enumeration class  *  explain ： The enumeration class defined is inherited from by default class java.lang.Enum class  * * @Author damao * @Date 2022/7/13 15:57 * @Version 1.0 */
public class SeasonTest1 {
    
    public static void main(String[] args) {
    
        Season1 summer = Season1.SUMMER;
        System.out.println(summer);

// System.out.println(Season1.class.getSuperclass());
        System.out.println("=================");

        //values():
        Season1[] values = Season1.values();
        for (int i = 0; i < values.length; i++) {
    
            System.out.println(values[i]);
            values[i].show();
        }

        System.out.println("=================");
        Thread.State[] values1 = Thread.State.values();
        for (int i = 0; i < values1.length; i++) {
    
            System.out.println(values1[i]);
        }

        System.out.println("=================");
        //valueOf(String objName)： On the basis of objName, Returns the name of the object in the enumeration class objName The object of 
        // without objName Enumeration class objects of , Throw exception ：IllegalArgumentException
        Season1 winter = Season1.valueOf("WINTER");
// Season1 winter = Season1.valueOf("WINTER1");
        System.out.println(winter);
        winter.show();
    }
}

interface Info{
    
    void show();
}

// Use enum Keyword definition enumeration class 
enum  Season1 implements Info{
    
    //1. Provides the object of the current enumeration class , Use... Between multiple objects “,” separate , The end object uses “;” end 
    SPRING(" spring ", " Spring flowers "){
    
        @Override
        public void show() {
    
            System.out.println(" Where is spring ?");
        }
    },
    SUMMER(" summer ", " It's hot in summer "){
    
        @Override
        public void show() {
    
            System.out.println(" summer ");
        }
    },
    AUTUMN(" autumn ", " autumn "){
    
        @Override
        public void show() {
    
            System.out.println(" autumn ");
        }
    },
    WINTER(" winter ", " a world of ice and snow "){
    
        @Override
        public void show() {
    
            System.out.println(" winter ");
        }
    };

    //2. Provides multiple objects of the current enumeration class ：public static final Of 
    private final String seasonName;
    private final String seasonDesc;

    //3. Privatize the constructor of the class and assign values to object properties 
    private Season1(String seasonName, String seasonDesc) {
    
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //4. Other demands 1： Get the properties of the enumeration class object 

    public String getSeasonName() {
    
        return seasonName;
    }

    public String getSeasonDesc() {
    
        return seasonDesc;
    }

// // Other demands 2： Provide toString()
//
// @Override
// public String toString() {
    
// return "Season1{" +
// "seasonName='" + seasonName + '\'' +
// ", seasonDesc='" + seasonDesc + '\'' +
// '}';
// }

    /*@Override public void show(){ System.out.println(" It's a season "); }*/
}