Enumeration 

package main;

public enum Apple {
    Jonathan, GoldenDel, RedDel, Winesap, Cortland
}
package main;

public class Main {
    public static void main(String[] args) {
        Apple ap = Apple.RedDel;
        System.out.println("Value of ap: " + ap);

        ap = Apple.GoldenDel;
        if(ap == Apple.GoldenDel)
            System.out.println("ap contains GoldenDel");

        switch (ap){
            case Jonathan:
                System.out.println("Jonathan is red.");
                break;
            case GoldenDel:
                System.out.println("Goldel Delicious is yellow");
                break;
            case Winesap:
                System.out.println("Winesap is red");
                break;
        }
    }
}
Output is:
"C:\Program Files\Java\jdk1.8.0_281\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=7482:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_281\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\rt.jar;C:\Users\LENOVO\Desktop\Project\untitled\target\classes" main.Main
Value of ap: RedDel
ap contains GoldenDel
Goldel Delicious is yellow

Process finished with exit code 0


The values() and valueOf() Methods
All enumerations automatically contain two predefined methods: values() and valueOf(). 
The values() method returns an array that contains a list of the enumeration constants. 
The valueOf() method returns the enumeration constant whose value corresponds to the string 
passed in str.

package main;

public enum Apple {
    Jonathan, GoldenDel, RedDel, Winesap, Cortland
}
package main;

public class Main {
    public static void main(String[] args) {
        System.out.println("Here are all Apple constants: ");

        Apple[] apples = Apple.values();
        for(Apple a : apples) {
            System.out.println(a);
        }

        Apple ap = Apple.valueOf("Winesap");
        System.out.println("ap contains " + ap);
    }
}
Outcome:
"C:\Program Files\Java\jdk1.8.0_281\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=7794:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_281\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\rt.jar;C:\Users\LENOVO\Desktop\Project\untitled\target\classes" main.Main
Here are all Apple constants: 
Jonathan
GoldenDel
RedDel
Winesap
Cortland
ap contains Winesap

Process finished with exit code 0

As explained, valueOf() returns the enumeration value associated with the name of the constant
represented as a string.

Java Enumerations Are Class Types.

It is important to understand that each enumeration constant is an object of its enumeration 
type. Thus, when you define a constructor for an enum, the constructor is called when each
enumeration constant is created. Also, each enumeration constant has its own copy of any 
instance variables defined by the enumeration.

package main;

public enum Apple {
    Jonathan(10),
    GoldenDel(9),
    RedDel(12),
    Winesap(15),
    Cortland(8);

    private int price;

    Apple(int price) {
        this.price = price;
    }

    public int getPrice() {
        return price;
    }
}
package main;

public class Main {
    public static void main(String[] args) {
        for(Apple a : Apple.values()) {
            System.out.println(a + " costs " + a.getPrice() + " cents.");
        }
    }
}
Outcome:
"C:\Program Files\Java\jdk1.8.0_281\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=10983:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_281\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\rt.jar;C:\Users\LENOVO\Desktop\Project\untitled\target\classes" main.Main
Jonathan costs 10 cents.
GoldenDel costs 9 cents.
RedDel costs 12 cents.
Winesap costs 15 cents.
Cortland costs 8 cents.

Process finished with exit code 0

Here are two restrictions that apply to enumerations. First, an enumeration can't inherit 
another class. Second, an enumeration cannot be a superclass. This means that an enum can't 
be extended. Otherwise, enum acts much like any other class type. The key is to remember that 
each of the enumeration constants is an object of the class in which it is defined.
Enumerations Inherit Enum
Although you can't inherit a superclass when declaring an enum, all enumerations automatically
inherit one: java.lang.Enum. 
You can obtain a value that indicates an enumeration constant's position in the list of 
constants, and it is retrieved by calling the ordinal() method. Ordinal values begin at zero.
You can compare the ordinal value of two constants of the same enumeration by using
comparyTo() method.
You can compare for equality an enumeration constant with any other object by using equals(),
which overrides the equals() method defined by Object. 
Remember, you can compare two enumeration references for equality by using ==.

package main;

public enum Apple {
    Jonathan, GoldenDel, RedDel, Winesap, Cortland
}
package main;

public class Main {
    public static void main(String[] args) {
        Apple ap, ap2, ap3;

        System.out.println("Here are all apple constants and their ordinal values: ");
        for(Apple a : Apple.values()) {
            System.out.println(a.ordinal());
        }

        ap = Apple.RedDel;
        ap2 = Apple.GoldenDel;
        ap3 = Apple.RedDel;

        if(ap.compareTo(ap2) < 0)
            System.out.println(ap + " comes before " + ap2);

        if(ap.compareTo(ap2) > 0)
            System.out.println(ap2 + " comes before " + ap);

        if(ap.compareTo(ap3) == 0)
            System.out.println(ap + " comes before " + ap3);

        if(ap.equals(ap2))
            System.out.println("Error!");

        if(ap.equals(ap3))
            System.out.println(ap + " equal " + ap3);

        if(ap == ap3)
            System.out.println(ap + " == " + ap3);
    }
}
Outcome:
"C:\Program Files\Java\jdk1.8.0_281\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=6718:C:\Program Files\JetBrains\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_281\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_281\jre\lib\rt.jar;C:\Users\LENOVO\Desktop\Project\untitled\target\classes" main.Main
Here are all apple constants and their ordinal values: 
0
1
2
3
4
GoldenDel comes before RedDel
RedDel comes before RedDel
RedDel equal RedDel
RedDel == RedDel

Process finished with exit code 0

In Enum our constructor is already private. 

package com;


public enum Menu {
    SUM(1, '+'),
    SUBTRACT(2, '-'),
    DIVIDE(3, '/'),
    MULTIPLY(4, '*');

    int number;
    char value;

    Menu(int number, char value) {
        this.number = number;
        this.value = value;
    }

    public static Menu define(Character str) {
        Menu[] menus = Menu.values();
        for (int i = 0; i < menus.length; i++) {
            if (menus[i].name().equals(str)) {
                return menus[i];
            }
        }
        return null;
    }

    public static Menu defineByNumbar(int number) {
        Menu[] menus = Menu.values();
        for (int i = 0; i < menus.length; i++) {
            if (menus[i].ordinal() == number) {
                return menus[i];
            }
        }
        return null;
    }

    public static Menu defineBySpecificNumber(int number){
        Menu[] menus = Menu.values();
        for (int i = 0; i < menus.length; i++) {
            if (menus[i].number == number) {
                return menus[i];
            }
        }
        return null;
    }
}

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