Enums in java are mainly used for grouping similar kind of constants as a one unit. constants means static and final. Enums are introduced in JDK 1.5 onward. Before that similar kind of constants are grouped by declaring them as static and final in one class. Below example shows how the constants will look without enums.
class ConstantsWithoutEnums
{
public static final String north = "NORTH";
public static final String south = "SOUTH";
public static final String east = "EAST";
public static final String west = "WEST";
}
public class MainClass
{
public static void main(String[] args)
{
System.out.println(ConstantsWithoutEnums.north);
System.out.println(ConstantsWithoutEnums.south);
System.out.println(ConstantsWithoutEnums.east);
System.out.println(ConstantsWithoutEnums.west);
}
}
Above constatnts can be defined with enums as below,
enum Directions
{
NORTH, SOUTH, EAST, WEST;
}
public class EnumsExample
{
public static void main(String[] args)
{
Directions d1 = Directions.EAST;
System.out.println(d1);
Directions d2 = Directions.NORTH;
System.out.println(d2);
System.out.println(Directions.SOUTH);
System.out.println(Directions.WEST);
}
}
Let’s see some things-to-remember about java enums.
- Enums in java are declared with enum keyword and constants in enums must be valid java identifier. It is good practice to declare constants with UPPERCASE letters.
enum Days
{
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
}
- Duplicate enum constants are not allowed.
enum Directions
{
NORTH, NORTH, SOUTH, EAST, WEST; //Compile Time Error : Duplicate Constants
}
- Every constant of enum is public, static and final by default. As every constant is static, they can be accessed directly using enum name.
enum enums
{
A, B, C;
}
public class EnumsExample
{
public static void main(String[] args)
{
enums en = enums.A; //accessing constant A through enum name
enums en1 = enums.B; //accessing constant B through enum name
enums en2 = enums.C; //accessing constant C through enum name
}
}
- Enums can have any number of fields. methods and constructors and Each constant will have their own copy of fields and methods.
enum enums
{
A, B, C;
int i; //enums can have fields
private enums()
{
//enums can have Constructor
}
void methodOfEnum()
{
//enums can have methods
}
}
public class EnumsExample
{
public static void main(String[] args)
{
enums en = enums.A;
System.out.println(en.i); //Constant A has field i
en.methodOfEnum(); //Constant A has methodOfEnum()
enums en1 = enums.B;
System.out.println(en1.i); //Constant B has field i
en1.methodOfEnum(); //Constant B has methodOfEnum()
enums en2 = enums.C;
System.out.println(en2.i); //Constant C has field i
en2.methodOfEnum(); //Constant C has methodOfEnum()
}
}
- If enum has only constants, then semicolon (;) at the end of constant declaration is not mandatory. But, if enum has other members, then semicolon is mandatory.
enum enums
{
A, B, C, D //semicolon at the end of this statement is not mandatory
}
enum enums
{
A, B, C; //semocolon at the end of this statement is mandatory, because it has other members
int i; //enums has a field
void methodOfEnum()
{
//enums has a method
}
}
- Every enum in java extends java.lang.Enum class. Enum class is an abstract class in java.lang package. As every enum extends Enum class, it should not extend any other class. Because, Multiple inheritance is not allowed in java. But enums can implement any number of interfaces.
interface AnyInterface
{
abstract void methodOfInterface();
}
enum enums implements AnyInterface
{
A, B, C;
@Override
public void methodOfInterface()
{
//MethodOfInterface is implemented
}
}
- Enums can be declared inside a class. If declared inside a class, they are static by default and can be accessed directly by Class name.
class ClassContainingEnum
{
enum enums
{
A, B, C
}
}
public class MainClass
{
public static void main(String[] args)
{
System.out.println(ClassContainingEnum.enums.A); //Accessing enums directly using class name
}
}
- Enum constants can override generalized method defined in the enum body.
enum enums
{
FIRST
{
@Override
void commonMethod()
{
System.out.println("Common method Overridden in FIRST");
}
},
SECOND
{
@Override
void commonMethod()
{
System.out.println("Common method Overridden in SECOND");
}
},
THIRD
{
@Override
void commonMethod()
{
System.out.println("Common method Overridden in THIRD");
}
};
void commonMethod()
{
System.out.println("Generalized method, Common to all constants");
}
}
public class EnumsExample
{
public static void main(String[] args)
{
enums.FIRST.commonMethod(); //Output : Common method Overridden in FIRST
enums.SECOND.commonMethod(); //Output : Common method Overridden in SECOND
enums.THIRD.commonMethod(); //Output : Common method Overridden in THIRD
}
}
- Enum can have abstract method declared in it’s body provided each enum constants must implement it.
enum enums
{
FIRST
{
@Override
void abstractMethod()
{
//Abstract Method Implemented
}
},
SECOND
{
@Override
void abstractMethod()
{
//Abstract Method Implemented
}
},
THIRD
{
@Override
void abstractMethod()
{
//Abstract Method Implemented
}
};
abstract void abstractMethod();
}
- Enum Constants can have their own fields and method defined in their body, but these fields and methods are visible only within the constant body.
enum enums
{
FIRST
{
int j = 10; // Field of FIRST
void methodOfFirst()
{
System.out.println(j); //Field j can be used within the constant body
}
@Override
void abstractMethod()
{
methodOfFirst(); //methodOfFirst() can be used within the constant body
}
},
SECOND
{
int k = 20; //Field of SECOND
void methodOfSecond()
{
System.out.println(k); //Field k can be used within the constant body
}
@Override
void abstractMethod()
{
methodOfSecond(); //methodOfSecond() can be used within the constant body
}
};
int i; //this field is available in all constants
abstract void abstractMethod(); //this method is available in all constants
}
public class EnumsExample
{
public static void main(String[] args)
{
System.out.println(enums.FIRST.j); //Compile time error : Field j is not visible here
enums.FIRST.methodOfFirst(); //Compile time error : methodOfFirst() is not visible here
enums.FIRST.abstractMethod();
System.out.println(enums.SECOND.k); //Compile time error : Field k is not visible here
enums.SECOND.methodOfSecond(); //Compile time error : methodOfSecond() is not visible here
enums.SECOND.abstractMethod();
}
}
- After observing all the above features of enums, we come to know that enums can have constuctors, fields and methods. Enums can implement interface. Enums extend Enum class. That means they have all features of classes. Therefore they are special type of classes. Moreover, after compilation, .class files are generated for all enums. Then what enum constants are?…. You can treat them as static inner classes of enums as they can be referred directly using enum name and they can hold fields and methods in them.
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