Java enum, also called Java enumeration type, is a type whose fields consist of a fixed set of constants. The very purpose of enum is to enforce compile time type safety. enum
keyword is reserved keyword in Java.
We should use enum when we know all possible values of a variable at compile time or design time, though we can add more values in future as and when we identify them. In this java enum tutorial, we will learn what enums are and what problems they solve?
Tóm Tắt
1. enum in Java
Enumerations (in general) are generally a set of related constants. They have been in other programming languages like C++ from beginning. After JDK 1.4, Java designers decided to support it in Java also, and it was officially released in JDK 1.5 release.
Enumeration in Java is supported by keyword enum
. enums are a special type of class that always extends java.lang.Enum.
1.1. enum is reserved keyword
enum
in Java is reserved keyword. It means you cannot define a variable of name enum
. e.g. It will result in compile time error "invalid VariableDeclaratorId"
.
1.2. Java enum declaration
A simple example to create enum. 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 enum for them. Syntax to create an enum is as below:
public enum Direction { EAST, WEST, NORTH, SOUTH; }
Logically, each enum is an instance of enum type itself. So given enum can be seen as below declaration. JVM internally adds ordinal and value methods to this class which we can call while working with enum.
final class Direction extends Enum<Direction> { public final static Direction EAST = new Direction(); public final static Direction WEST = new Direction(); public final static Direction NORTH = new Direction(); public final static Direction SOUTH = new Direction(); }
1.3. Java enum example
We can use the enum just like we use final
static
class fields.
public class EnumExample { public static void main(String[] args) { Direction north = Direction.NORTH; System.out.println(north); //Prints NORTH } }
1.4. enum ordinal()
The ordinal()
method returns the order of an enum instance. It represents the sequence in the enum declaration, where the initial constant is assigned an ordinal of '0'
. It is very much like array indexes.
It is designed for use by sophisticated enum-based data structures, such as EnumSet
and EnumMap
.
Direction.EAST.ordinal(); //0 Direction.NORTH.ordinal(); //2
1.5. enum values() and valueOf()
The enum values() method returns all the enum values in an enum array.
Direction[] directions = Direction.values(); for (Direction d : directions) { System.out.println(d); } //Output: EAST WEST NORTH SOUTH
The enum valueOf() method helps to convert string to enum instance.
Direction east = Direction.valueOf("EAST"); System.out.println(east); //Output: EAST
1.6. enum naming convention
By convention, enums are constants. In Java, constants are defined in all UPPER_CASE letters. This follows are 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).
2. enum constructors
By 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. So let’s add them.
public enum Direction { // enum fields 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 angle for any direction, we can make a simple method call in enum field reference.
Direction north = Direction.NORTH; System.out.println( north ); //NORTH System.out.println( north.getAngle() ); //90 System.out.println( Direction.NORTH.getAngle() ); //90
3. enum methods
Remember that enum is basically a special class type, and can have methods and fields just like any other class. You can add methods which are abstract as well as concrete methods as well. Both methods are allowed in enum.
3.1. concrete methods in enum
Adding a concrete method in enum is similar to add same method in any other class. You can use any access specifier e.g. public
, private
or protected
. You can return values from enum methods or simply use them to perform internal logic.
public enum Direction { // enum fields EAST, WEST, NORTH, SOUTH; protected String printDirection() { String message = "You are moving in " + this + " direction"; System.out.println( message ); return message; } }
You can call printDirection()
method as simple method calls on enum instance.
Direction.NORTH.printDirection(); //You are moving in NORTH direction Direction.EAST.printDirection(); //You are moving in EAST direction
3.2. abstract methods in enum
We can add abstract method in enums. In this case, we must implement the abstract method at each enum field, individually.
public enum Direction { // enum fields EAST { @Override public String printDirection() { String message = "You are moving in east. You will face sun in morning time."; return message; } }, WEST { @Override public String printDirection() { String message = "You are moving in west. You will face sun in evening time."; return message; } }, NORTH { @Override public String printDirection() { String message = "You are moving in north. You will face head in daytime."; return message; } }, SOUTH { @Override public String printDirection() { String message = "You are moving in south. Sea ahead."; return message; } }; public abstract String printDirection(); }
Re-run above example.
Direction.NORTH.printDirection(); //You are moving in north. You will face head in daytime. Direction.EAST.printDirection(); //You are moving in east. You will face sun in morning time.
You can enforce a contract for all enums to be created in this way. It can serve as 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. In future, in any more directions are added (really?), then we must add a custom message as well.
4. enum inheritance
As mentioned earlier, enums extends Enum class. java.lang.Enum
is an abstract class. This is the common base class of all Java enumeration types.
public abstract class Enum<E extends Enum<E>> extends Object implements Comparable<E>, 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.
5. Compare enums
All enums are by default comparable and singletons as well. It means you can compare them with equals()
method, even with "=="
operator.
Direction east = Direction.EAST; Direction eastNew = Direction.valueOf("EAST"); System.out.println( east == eastNew ); //true System.out.println( east.equals( eastNew ) ); //true
You can compare enum types using
'=='
operator orequals()
method, because enums are singlton and comparable by default.
6. Enum collections – EnumSet and EnumMap
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).
6.1. java.util.EnumSet
EnumSet class is defined as follows:
public abstract class EnumSet<E extends Enum<E>> extends AbstractSet<E> implements Cloneable, Serializable { }
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.
6.1.1. EnumSet Example
public class Test { public static void main(String[] args) { 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.
null
elements are not permitted. 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.
6.2. java.util.EnumMap
EnumMap is declared as:
public class EnumMap<K extends Enum<K>,V> extends AbstractMap<K,V> implements Serializable, Cloneable { }
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.
6.2.1. EnumMap Example
public class Test { public static void main(String[] args) { //Keys can be only of type Direction Map enumMap = new EnumMap(Direction.class); //Populate the Map 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()); } }
7. Summary
- enums are implicitly final subclasses of
java.lang.Enum
class - if an enum is a member of a class, it’s implicitly
static
new
keyword can not be used to intialize an enum, even within the enum type itselfname()
andvalueOf()
methods simply use the text of the enum constants, whiletoString()
method may be overridden to provide any content, if desired- for enum constants,
equals()
and"=="
evaluates to same result, and can be used interchangeably - enum constants are implicitly
public static final
- the order of appearance of list of enum constants is called their “natural order“, and defines the order used by other items as well :
compareTo()
method, iteration order of values inEnumSet
,EnumSet.range()
. - Enum constructors should be declared as
private
. The compiler allows non private constructors, but this seems misleading to the reader, since new can never be used with enum types. - Since these enumeration instances are all effectively singletons, they can be compared for equality using identity (
"=="
). - you can use enum in switch statement like int or char primitive data type
In this article, we explored the Java enum from the language basics to more advanced and interesting real-world use cases.
Happy Learning !!
References:
SO Thread
Enum Java Doc
Java 1.5 enumeration