Enum represent a fixed set of constants
Eg:
Common examples include compass directions (values of NORTH, SOUTH, EAST, and WEST) and the days of the week.
A data sets where you know all possible values at compile time
public enum Directions {
NORTH, SOUTH, EAST, WEST
}
How to represent enumerable value without Java enum
Since Enum in Java is only available from Java 1.5 its worth to discuss how we used to represent enumerable values in Java prior JDK 1.5 and without it. I use public static final constant to replicate enum like behavior. Let’s see an Enum example in Java to understand the concept better. In this example we will use US Currency Coin as enumerable which has values like PENNY (1) NICKLE (5), DIME (10), and QUARTER (25).
class CurrencyDenom {
public static final int PENNY = 1;
public static final int NICKLE = 5;
public static final int DIME = 10;
public static final int QUARTER = 25;
}
class Currency {
int currency; //CurrencyDenom.PENNY,CurrencyDenom.NICKLE,
// CurrencyDenom.DIME,CurrencyDenom.QUARTER
}
Though this can server our purpose it has some serious limitations:
1) No Type-Safety: First of all it’s not type-safe; you can assign any valid int value to currency e.g. 99 though there is no coin to represent that value.
2) No Meaningful Printing: printing value of any of these constant will print its numeric value instead of meaningful name of coin e.g. when you print NICKLE it will print "5" instead of "NICKLE"
3) No namespace: to access the currencyDenom constant we need to prefix class name e.g. CurrencyDenom.PENNY instead of just using PENNY though this can also be achieved by using static import in JDK 1.5
Java Enum is answer of all this limitation. Enum in Java is type-safe, provides meaningful String names and has there own namespace. Now let's see same example using Enum in Java:
public enum Currency {PENNY, NICKLE, DIME, QUARTER};
Here Currency is our enum and PENNY, NICKLE, DIME, QUARTER are enum constants. Notice curly braces around enum constants because Enum are type like class and interface in Java. Also we have followed similar naming convention for enum like class and interface (first letter in Caps) and since Enum constants are implicitly static final we have used all caps to specify them like Constants in Java.
What is Enum in Java
Enum is a keyword in java and on more detail term Java Enum is type like class and interface and can be used to define a set of Enum constants. Enum constants are implicitly static and final and you can not change there value once created. Enum in Java provides type-safety and can be used inside switch statment like int variables. Since enum is a keyword you can not use as variable name and since its only introduced in JDK 1.5 all your previous code which has enum as variable name will not work and needs to be re-factored.
Benefits of Enums in Java:
1) Enum is type-safe you can not assign anything else other than predefined Enum constants to an Enum variable. It is compiler error to assign something else unlike the public static final variables used in Enum int pattern and Enum String pattern.
2) Enum has its own name-space.
3) Best feature of Enum is you can use Enum in Java inside Switch statement like int or char primitive data type.we will also see example of using java enum in switch statement in this java enum tutorial.
4) Adding new constants on Enum in Java is easy and you can add new constants without breaking existing code.
Important points about Enum in Java
1) Enums in Java are type-safe and has there own name-space. It means your enum will have a type for example "Currency" in below example and you can not assign any value other than specified in Enum Constants.
public enum Currency {PENNY, NICKLE, DIME, QUARTER};
Currency coin = Currency.PENNY;
coin = 1; //compilation error
2) Enum in Java are reference type like class or interface and you can define constructor, methods and variables inside java Enum which makes it more powerful than Enum in C and C++ as shown in next example of Java Enum type.
3) You can specify values of enum constants at the creation time as shown in below example:
public enum Currency {PENNY(1), NICKLE(5), DIME(10), QUARTER(25)};
But for this to work you need to define a member variable and a constructor because PENNY (1) is actually calling a constructor which accepts int value , see below example.
public enum Currency {
PENNY(1), NICKLE(5), DIME(10), QUARTER(25);
private int value;
private Currency(int value) {
this.value = value;
}
};
Constructor of enum in java must be private any other access modifier will result in compilation error. Now to get the value associated with each coin you can define a public getValue() method inside java enum like any normal java class. Also semi colon in the first line is optional.
4) Enum constants are implicitly static and final and can not be changed once created. For example below code of java enum will result in compilation error:
Currency.PENNY = Currency.DIME;
The final field EnumExamples.Currency.PENNY cannot be re assigned.
5) Enum in java can be used as an argument on switch statment and with "case:" like int or char primitive type. This feature of java enum makes them very useful for switch operations. Let’s see an example of how to use java enum inside switch statement:
Currency usCoin = Currency.DIME;
switch (usCoin) {
case PENNY:
System.out.println("Penny coin");
break;
case NICKLE:
System.out.println("Nickle coin");
break;
case DIME:
System.out.println("Dime coin");
break;
case QUARTER:
System.out.println("Quarter coin");
}
from JDK 7 onwards you can also String in Switch case in Java code.
6) Since constants defined inside Enum in Java are final you can safely compare them using "==" equality operator as shown in following example of Java Enum:
Currency usCoin = Currency.DIME;
if(usCoin == Currency.DIME){
System.out.println("enum in java can be compared using ==");
}
By the way comparing objects using == operator is not recommended, Always use equals() method or compareTo() method to compare Objects.
7) Java compiler automatically generates static values() method for every enum in java. Values() method returns array of Enum constants in the same order they have listed in Enum and you can use values() to iterate over values of Enum in Java as shown in below example:
for(Currency coin: Currency.values()){
System.out.println("coin: " + coin);
}
And it will print:
coin: PENNY
coin: NICKLE
coin: DIME
coin: QUARTER
Notice the order its exactly same with defined order in enums.
8) In Java Enum can override methods also. Let’s see an example of overriding toString() method inside Enum in Java to provide meaningful description for enums constants.
public enum Currency {
........
@Override
public String toString() {
switch (this) {
case PENNY:
System.out.println("Penny: " + value);
break;
case NICKLE:
System.out.println("Nickle: " + value);
break;
case DIME:
System.out.println("Dime: " + value);
break;
case QUARTER:
System.out.println("Quarter: " + value);
}
return super.toString();
}
};
And here is how it looks like when displayed:
Currency usCoin = Currency.DIME;
System.out.println(usCoin);
output:
Dime: 10
9) Two new collection classes EnumMap and EnumSet are added into collection package to support Java Enum. These classes are high performance implementation of Map and Set interface in Java and we should use this whenever there is any opportunity.
10) You can not create instance of enums by using new operator in Java because constructor of Enum in Java can only be private and Enums constants can only be created inside Enums itself.
11) Instance of Enum in Java is created when any Enum constants are first called or referenced in code.
12) Enum in Java can implement the interface and override any method like normal class It’s also worth noting that Enum in java implicitly implement both Serializable and Comparable interface. Let's see and example of how to implement interface using Java Enum:
public enum Currency implements Runnable{
PENNY(1), NICKLE(5), DIME(10), QUARTER(25);
private int value;
............
@Override
public void run() {
System.out.println("Enum in Java implement interfaces");
}
}
13) You can define abstract methods inside Enum in Java and can also provide different implementation for different instances of enum in java. Let’s see an example of using abstract method inside enum in java
public enum Currency implements Runnable{
PENNY(1) {
@Override
public String color() {
return "copper";
}
}, NICKLE(5) {
@Override
public String color() {
return "bronze";
}
}, DIME(10) {
@Override
public String color() {
return "silver";
}
}, QUARTER(25) {
@Override
public String color() {
return "silver";
}
};
private int value;
public abstract String color();
private Currency(int value) {
this.value = value;
}
..............
}
In this example since every coin will have different color we made the color() method abstract and let each instance of Enum to define there own color. You can get color of any coin by just calling color() method as shown in below example of java enum:
System.out.println("Color: " + Currency.DIME.color());
Ref: javarevisited
