Java Enumerations explained

Just like the primitive data types, there is another data type supported by Java which is the enumerated type. In this blog post, I will be explaining Java enumerations in detail.

Enum definition

An enumerated type is a type whose values consist of a fixed set of constants. Common examples are days of the week which take the values Sunday,Monday, Tuesday,Wednesday,Thursday,Friday and Saturday. Another example could be Colors which could take values like Red,Blue,Yellow, Green corresponding to the colors.

In Java, you can define an enumerated type by using the enum keyword. The syntax is as follows:

enum NameOfEnum {value1,value2,value3.......,valuen}

You can have as many values as you wish. For example, you can define an enum for the days of the week as follows:

enum Days {Sunday,Monday, Tuesday,Wednesday,Thursday,Friday,Saturday}

The values Sunday,Monday, etc are called enumeration constants. For the enum Days, these are the only values that are allowed.

You should use enumerated types when you need to represent a fixed set of constants where you know all possible values at compile time.

The values of an enum are static values and you can retrieve them using the dot operator. So to fetch a value from the Days enum, you can write Days.Sunday;

Enum as DataType

An enum is a datatype. So the way you require an int variable to store an int value, you require an enum of type Days to store a value of Enum days. So for example, to retrieve and store the value “Saturday” from the enum Days, you need to declare a variable of type Days as:

Days today = Days.Saturday;

To be specific, you cannot assign an enum variable of one type with a value of another type, it will result in a compilation error. So if you declare another enum called Colors and try to write something like this:

Days today = Colors.BLUE;

This will result in a compilation error because the left side enum is Days while the right side is Colors.

Values as Numbers

The enum internally stores the enumeration constant as numerical values. So in the Days example above, the enum Days will have the values 0,1,2,3,4,5,6 internally. However, while using the enumeration constants, they are output as descriptive values.

Enum declaration scope

You cannot declare an enum  in any method of a class. So you need to declare an enum outside the main method.