Java supports enumeration types (in short enum) since Java 5. Enumeration is used to define a list of constants which can be later referenced in source code by its name. Enumeration type in Java is based on concept of the class (as opposed to C and C++ in which it is based on integer type). In short it means that enumeration type is actually a special kind of class and each constant is actually a singleton instance of this class. Such design opens up a lot possibilities that are described below.
Basics of enumeration type
Below is an example of a enumeration type which represents four quarters of the years:
public enum QuarterOfTheYear {
Q1, Q2, Q3, Q4
}
and its sample usage:
System.out.printf("Quarter1 : %s\n", QuarterOfTheYear.Q1);
for (QuarterOfTheYear quarter : QuarterOfTheYear.values()) {
System.out.println("Quarter: %s\n", quarter);
}
Available methods
All enums in Java implicitly extend classjava.lang.Enum. Each enumeration type contains following methods:
| Method | Description |
|---|---|
| Object clone() | Throws CloneNotSupportedException (to ensure that there is only one instance of each enum |
| int compareTo(Enum e) | Compares this enum with the specified object for order |
| boolean equals(Object other) | Returns true if the specified object is equal to this enum constant |
| void finalize() | Enum classes cannot have finalize methods |
| Class<E> getDeclaringClass() | Returns the Class object corresponding to this enum constant’s enum type |
| int hashCode() | Returns a hash code for this enum constant |
| String name() | Returns the name of this enum constant, exactly as declared in its enum declaration |
| int ordinal() | Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero) |
| String toString() | Returns the name of this enum constant, as contained in the declaration |
| static <T extends Enum<T>> T valueOf​(Class enumType, String name) | Returns the enum constant of the specified enum type with the specified name |
| static T valueOf(String name) | Returns enum fields with given name |
| T[] values() | Returns table of all enum constants |
Most of these methods are already declared in java.lang.Enum class but the last two ones are in fact implicitly declared and synthesized by the compiler.
Comparison
Each enum has equals() method which can be used to check if two enum constants are the same. Given the fact that there can be only one instance of each enum constants, it is also possible and correct to use equal sign == to perform the same check.
Fields and methods in enumeration types
Enumeration type is a Java class so it can contain its own fields, methods and constructors:
public enum QuarterOfTheYear {
Q1("Quarter One"),
Q2("Quarter Two"),
Q3("Quarter Three"),
Q4("Quarter Four");
private String description;
QuarterOfTheYear(String description) {
this.description = description;
}
public String getDescription() {
return description;
}
public static QuarterOfTheYear getQuarterByDate(LocalDate date) {
int quarterNo = date.get(IsoFields.QUARTER_OF_YEAR);
return QuarterOfTheYear.values()[quarterNo - 1];
}
}Differences from standard Java classes
There are however some differences compared to standard Java classes.
The constructor of enum (if defined) must have either private or package-private access. This is to prevent creation of duplicate enum constants. Also it is impossible to call enum constructor yourself.
Additionally, enum cannot extend any other class because it already extends (implicitly) java.lang.Enum class. Yet it can implement interfaces.
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