Java Enum with Constructor Example
Many Java developers don’t know that Java Enum can have a constructor to pass data while creating Enum constants. This feature allows you to associate related data together. One example of passing arguments to enum Constructor is our TrafficLight Enum where we pass the action to each Enum instance e.g. GREEN is associate with go, RED is associated with stop, and ORANGE is associated with the slow down.
This is really useful because it provides more context and meaning to your code. If you want, you can also provide one or more constructors to your Enum as it also supports constructor overloading like normal Java classes. This is very different from the enum you have seen in C or C++, which is just a collection of fixed things without any OOP power.
Just remember that constructor in enums an only be either private or package level it can’t be public or protected hence access modifier public and protected are not allowed to Enum constructor, it will result in a compile-time error.
By the way, this is not our first tutorial on Java Enum where we have explained a key feature, we have also ready covered some important features on Enum in our previous examples like Java Enum Switch Example explains that you can use enum constants inside switch block.
Similarly, Java Enum valueOf Example and Enum to String Example explain that how you can get the String representation of Enum. These short tutorials are a good way to learn some useful features of enum in Java.
Java Enum with Constructor Example
Here is a complete code example of using Constructor with Java Enum. Here is our TrafficLight constructor accepts a String argument which is saved to action field which is later accessed by getter method getAction().
As I explained, we have an Enum TrafficSignal, which has three enum constants, RET, GREEN, and ORANGE, and we have associated, wait, go and slow down with them by passing values into the constructor. You can also see these free Java Programming courses to learn more about enum in Java.
/**
* Java enum with the constructor for example.
* Constructor accepts one String argument action
*/
public enum TrafficSignal <
//this will call enum constructor with one String argument
RED ( «wait» ) , GREEN ( «go» ) , ORANGE ( «slow down» ) ;
private String action ;
public String getAction () <
return this . action ;
>
// enum constructor — can not be public or protected
TrafficSignal ( String action ) <
this . action = action ;
>
>
/**
*
* Java Enum example with the constructor.
* Java Enum can have a constructor but it can not
* be public or protected
*
* @author http://java67.com
*/
public class EnumConstructorExample
public static void main ( String args [])
//let’s print name of each enum and there action
// — Enum values() examples
TrafficSignal [] signals = TrafficSignal. values () ;
for ( TrafficSignal signal : signals ) <
//Java name example — Java getter method example
System. out . println ( «name : »
+ signal. name ()
+ » action: »
+ signal. getAction ()) ;
>
This was our Java Enum example with Constructor. Now, you know that Enum can have a constructor in Java that can be used to pass data to Enum constants, just like we passed action here. Though Enum constructor cannot be protected or public, it can either have private or default modifier only.
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Java Enum Tutorial
Java enum, also called Java enumeration type, is a type whose fields consist of a fixed set of constants. The very purpose of an enum is to enforce compile-time type safety. The enum keyword is one of the reserved keywords in Java.
We should use an enum when we know all possible values of a variable at compile time or design time, though we can add more values in the future as and when we identify them. In this java enum tutorial, we will learn what enums are and what problems they solve.
Enumerations (in general) are generally a set of related constants. They have been in other programming languages like C++ from the beginning. After JDK 1.4, Java designers decided to support it in Java also, and it was officially released in JDK 1.5 release.
The enumeration in Java is supported by keyword enum . The enums are a special type of class that always extends java.lang.Enum.
1.1. The enum is a Reserved Keyword
The enum is a reserved keyword which means we cannot define a variable with the name enum . e.g. It will result in compile time error «invalid VariableDeclaratorId» .
1.2. Syntax to Create Enums
As we know, generally we deal with four directions in daily life. Their names, angles and other properties are fixed. So, in programs, we can create an enum for them. The syntax to create an enum is as below:
Logically, each enum is an instance of enum type itself. So given enum can be seen as the below declaration. JVM internally adds ordinal and value methods to this class which we can call while working with an enum.
final class Direction extends Enum
By convention, enums are constants. In Java, constants are defined in all UPPER_CASE letters. This follows for enums also.
- Enum name should be in title case (same as class names).
- Enum fields should be in all UPPER CASE (same as static final constants).
We can create a variable of specified enum type just like we use final static class fields.
Direction north = Direction.NORTH; System.out.println(north); //Prints NORTHBy default, enums don’t require constructor definitions and their default values are always the string used in the declaration. Though, you can give define your own constructors to initialize the state of enum types.
For example, we can add angle attribute to direction. All directions have some angle.
enum Direction < EAST(0), WEST(180), NORTH(90), SOUTH(270); // constructor private Direction(final int angle) < this.angle = angle; >// internal state private int angle; public int getAngle() < return angle; >>If we want to access the angle for any direction, we can make a simple method call in the enum field reference.
Direction west = Direction.WEST; System.out.println(west); System.out.println(west.getAngle()); //or System.out.println(Direction.WEST.getAngle());By default, an enum will have the following methods to access its constants.
The ordinal() method returns the order of an enum instance among the list of constants. It represents the sequence in the enum declaration, where the initial constant is assigned an ordinal of ‘0’ . It is very much like array indices.
Ordinal is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap .
Direction.EAST.ordinal(); //0 Direction.NORTH.ordinal(); //2The enum values() method returns all the enum values as an enum array.
Direction[] directions = Direction.values(); for (Direction d : directions)
The enum valueOf() method helps to convert a string to an enum instance.
Direction east = Direction.valueOf("EAST"); System.out.println(east);Remember that an enum is basically a special class type, and can have methods and fields just like any other class. We can add methods that are abstract as well as non-abstract methods. Both methods are allowed in the enum.
Adding a concrete method in an enum is similar to adding the same method in any other class. We can use any access specifier e.g. public , private or protected . We can return values from enum methods or simply use them to perform internal logic.
We can call the message() method as a simple method call on an enum instance.
We can also add abstract methods in enums. In this case, we must implement the abstract method at each enum field, individually.
public enum Direction < EAST < @Override public String message() < return "You are moving in east. You will face sun in morning time."; >>, WEST < @Override public String message() < return "You are moving in west. You will face sun in evening time."; >>, NORTH < @Override public String message() < return "You are moving in north. Sea behind."; >>, SOUTH < @Override public String message() < return "You are moving in south. Sea ahead."; >>; public abstract String message(); >Direction.WEST.message(); //You are moving in west. You will face sun in evening time. Direction.NORTH.message(); //You are moving in north. Sea behind.We can enforce a contract for all enums to be created in this way. It can serve as a template for enum creation.
For example, If we want that each enum type of Direction should be able to print the direction name with a custom message when needed. This can be done by defining a abstract method inside Direction , which each enum has to override.
As mentioned earlier, enum extends Enum class. The java.lang.Enum is an abstract class. This is the common base class of all Java enumeration types.
public abstract class Enum> extends Object implements Constable, Comparable, Serializable < //. >It means that all enums are comparable and serializable implicitly. Also, all enum types in Java are singleton by default.
As noted all enums extends java.lang.Enum , so enum cannot extend any other class because Java does not support multiple inheritance this way. But enums can implement any number of interfaces.
All enums are by default comparable and singletons as well. It means you can compare them with equals() method, even with «= wp-block-code»> Direction east = Direction.EAST; Direction eastNew = Direction.valueOf(«EAST»); System.out.println( east == eastNew ); //true System.out.println( east.equals( eastNew ) ); //true
We can compare enum types using ‘==’ operator or equals() method, because enums are singlton and comparable by default.
Two classes have been added to java.util package in support of enums – EnumSet (a high-performance Set implementation for enums; all members of an enum set must be of the same enum type) and EnumMap (a high-performance Map implementation for use with enum keys).
EnumSet class is a specialized Set implementation for use with enum types. All of the elements in an enum set must come from a single enum type that is specified, explicitly or implicitly, when the set is created.
Set enumSet = EnumSet.of(Direction.EAST, Direction.WEST, Direction.NORTH, Direction.SOUTH );Like most collection implementations EnumSet is not synchronized. If multiple threads access an enum set concurrently, and at least one of the threads modifies the set, it should be synchronized externally.
Note that null elements are not permitted in EnumSet. Also, these sets guarantee the ordering of the elements in the set based on their order in the enumeration constants is declared. Performance and memory benefits are very high in comparison to a regular set implementation.
EnumMap is a specialized Map implementation for use with enum type keys. Also, all of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created.
Like EnumSet , null keys are not permitted and is not synchronized as well.
Map enumMap = new EnumMap(Direction.class); enumMap.put(Direction.EAST, Direction.EAST.getAngle()); enumMap.put(Direction.WEST, Direction.WEST.getAngle()); enumMap.put(Direction.NORTH, Direction.NORTH.getAngle()); enumMap.put(Direction.SOUTH, Direction.SOUTH.getAngle());In this article, we explored the Java enum from the language basics to more advanced and interesting real-world use cases.
Constructor for enum java
В примере, где было показано, как программисты выкручивались до появления такой замечательной конструкции, как enum, необходимо добавить модификатор `final` к переменным, иначе теряется весь смысл. Похоже, что этот момент просто упустили. Если мы говорим об именованных константах, то они всегда должны быть public static final (именно такими они и являются в классах-перечислениях).
В первом же примере, про то как в «Java 1.5 программистам приходилось, откровенно говоря, выкручиваться» с помощью класса и приватного конструктора, сами константы объявлены как public static Month, без final, видимо опечатка
насколько я понимаю Enum должен быть приват статик константой, что с геттерами и сеттерами неоч сходится, а иначе зачем вообще создавать жесткое ограничение выбора
System.out.println(Month.FEBRUARY); // Month Month.FEBRUARY.setDaysCount(10); Month.FEBRUARY.setName("Ololo"); System.out.println(Month.FEBRUARY); // Month 
«Ну, например, старая конструкция не позволяла нам применять свой набор значений в switch-выражениях.» Не совсем понял. У меня, к примеру, есть public final class Constants, в котором есть статические публичные константы: и я спокойно могу его использовать в свиче: Т.е. в части enum и моего класса констант (с приватным конструктром, который я явно сделал, чтобы нельзя было создавать объекты) разницы нет. В обоих случаях я спокойно использую в свичах. Что я не так понял? Основная моя идея — сделать классы-справочники, чтобы они содержали уникальные данные (переменные), к котором я могу обращаться, чтобы использовать в своих нуждах. Что в таком случае лучше: енам или справочник-класс-констант? Пытаюсь в реальной жизни определить круг использования енам.