https://www.geeksforgeeks.org/enum-in-java/
Enumerations serve the purpose of representing a group of named constants in a programming language. For example, the 4 suits in a deck of playing cards may be 4 enumerators named Club, Diamond, Heart, and Spade, belonging to an enumerated type named Suit. Other examples include natural enumerated types (like the planets, days of the week, colors, directions, etc.).
Enums are used when we know all possible values at compile time, such as choices on a menu, rounding modes, command-line flags, etc. It is not necessary that the set of constants in an enum type stay fixed for all time.
A Java enumeration is a class type. Although we don’t need need to instantiate an enum using new, it has the same capabilities as other classes. This fact makes Java enumeration a very powerful tool. Just like classes, you can give them constructor, add instance variables and methods, and even implement interfaces.
One thing to keep in mind is that, unlike classes, enumerations neither inherit other classes nor can get extended(i.e become superclass).
In Java (from 1.5), enums are represented using enum data type. Java enums are more powerful than C/C++ enums. In Java, we can also add variables, methods, and constructors to it. The main objective of enum is to define our own data types(Enumerated Data Types).
Declaration of enum in Java: Enum declaration can be done outside a Class or inside a Class but not inside a Method.
- Java
// A simple enum example where enum is declared
// outside any class (Note enum keyword instead of
// class keyword)
enum Color {
RED,
GREEN,
BLUE;
}
public class Test {
// Driver method
public static void main(String[] args)
{
Color c1 = Color.RED;
System.out.println(c1);
}
}
|
RED
- Java
// enum declaration inside a class.
public class Test {
enum Color {
RED,
GREEN,
BLUE;
}
// Driver method
public static void main(String[] args)
{
Color c1 = Color.RED;
System.out.println(c1);
}
}
|
RED
- The first line inside the enum should be a list of constants and then other things like methods, variables, and constructors.
- According to Java naming conventions, it is recommended that we name constant with all capital letters
Important Points of enum:
- Every enum is internally implemented by using Class.
/* internally above enum Color is converted to
class Color
{
public static final Color RED = new Color();
public static final Color BLUE = new Color();
public static final Color GREEN = new Color();
}*/
- Every enum constant represents an object of type enum.
- enum type can be passed as an argument to switch statements.
- Java
// A Java program to demonstrate working on enum
// in switch case (Filename Test. Java)
import java.util.Scanner;
// An Enum class
enum Day {
SUNDAY,
MONDAY,
TUESDAY,
WEDNESDAY,
THURSDAY,
FRIDAY,
SATURDAY;
}
// Driver class that contains an object of "day" and
// main().
public class Test {
Day day;
// Constructor
public Test(Day day) { this.day = day; }
// Prints a line about Day using switch
public void dayIsLike()
{
switch (day) {
case MONDAY:
System.out.println("Mondays are bad.");
break;
case FRIDAY:
System.out.println("Fridays are better.");
break;
case SATURDAY:
case SUNDAY:
System.out.println("Weekends are best.");
break;
default:
System.out.println("Midweek days are so-so.");
break;
}
}
// Driver method
public static void main(String[] args)
{
String str = "MONDAY";
Test t1 = new Test(Day.valueOf(str));
t1.dayIsLike();
}
}
|
Mondays are bad.
- Every enum constant is always implicitly public static final. Since it is static, we can access it by using the enum Name. Since it is final, we can’t create child enums.
- We can declare the main() method inside the enum. Hence we can invoke enum directly from the Command Prompt.
- Java
// A Java program to demonstrate that we can have
// main() inside enum class.
enum Color {
RED,
GREEN,
BLUE;
// Driver method
public static void main(String[] args)
{
Color c1 = Color.RED;
System.out.println(c1);
}
}
|
RED
Enum and Inheritance:
- All enums implicitly extend java.lang.Enum class. As a class can only extend one parent in Java, so an enum cannot extend anything else.
- toString() method is overridden in java.lang.Enum class, which returns enum constant name.
- enum can implement many interfaces.
values(), ordinal() and valueOf() methods:
- These methods are present inside java.lang.Enum.
- values() method can be used to return all values present inside the enum.
- Order is important in enums.By using the ordinal() method, each enum constant index can be found, just like an array index.
- valueOf() method returns the enum constant of the specified string value if exists.
- Java
// Java program to demonstrate working of values(),
// ordinal() and valueOf()
enum Color {
RED,
GREEN,
BLUE;
}
public class Test {
public static void main(String[] args)
{
// Calling values()
Color arr[] = Color.values();
// enum with loop
for (Color col : arr) {
// Calling ordinal() to find index
// of color.
System.out.println(col + " at index "
+ col.ordinal());
}
// Using valueOf(). Returns an object of
// Color with given constant.
// Uncommenting second line causes exception
// IllegalArgumentException
System.out.println(Color.valueOf("RED"));
// System.out.println(Color.valueOf("WHITE"));
}
}
|
RED at index 0 GREEN at index 1 BLUE at index 2 RED
enum and constructor:
- enum can contain a constructor and it is executed separately for each enum constant at the time of enum class loading.
- We can’t create enum objects explicitly and hence we can’t invoke enum constructor directly.
enum and methods:
- enum can contain both concrete methods and abstract methods. If an enum class has an abstract method, then each instance of the enum class must implement it
- Java
// Java program to demonstrate that enums can have
// constructor and concrete methods.
// An enum (Note enum keyword inplace of class keyword)
enum Color {
RED,
GREEN,
BLUE;
// enum constructor called separately for each
// constant
private Color()
{
System.out.println("Constructor called for : "
+ this.toString());
}
public void colorInfo()
{
System.out.println("Universal Color");
}
}
public class Test {
// Driver method
public static void main(String[] args)
{
Color c1 = Color.RED;
System.out.println(c1);
c1.colorInfo();
}
}
|
Constructor called for : RED Constructor called for : GREEN Constructor called for : BLUE RED Universal Color
Next Article on enum: Enum with Customized Value in Java
This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
原文地址：http://www.cnblogs.com/kungfupanda/p/16928902.html
