The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Reflection provides three enum-specific APIs:
Class.isEnum()
Class.getEnumConstants()
java.lang.reflect.Field.isEnumConstant()
Sometimes it is necessary to dynamically retrieve the list of enum constants; in non-reflective code this is accomplished by invoking the implicitly declared static method
values()
on the enum. If an instance of an enum type is not available the only way to get a list of the possible values is to invoke
Class.getEnumConstants()
since it is impossible to instantiate an enum type.
Given a fully qualified name, the
example shows how to retrieve an ordered list of constants in an enum using
EnumConstants
Class.getEnumConstants()
.
import java.util.Arrays; import static java.lang.System.out; enum Eon { HADEAN, ARCHAEAN, PROTEROZOIC, PHANEROZOIC } public class EnumConstants { public static void main(String... args) { try { Class<?> c = (args.length == 0 ? Eon.class : Class.forName(args[0])); out.format("Enum name: %s%nEnum constants: %s%n", c.getName(), Arrays.asList(c.getEnumConstants())); if (c == Eon.class) out.format(" Eon.values(): %s%n", Arrays.asList(Eon.values())); // production code should handle this exception more gracefully } catch (ClassNotFoundException x) { x.printStackTrace(); } } }
Samples of the output follows. User input is in italics.
$ java EnumConstants java.lang.annotation.RetentionPolicy Enum name: java.lang.annotation.RetentionPolicy Enum constants: [SOURCE, CLASS, RUNTIME]
$ java EnumConstants java.util.concurrent.TimeUnit Enum name: java.util.concurrent.TimeUnit Enum constants: [NANOSECONDS, MICROSECONDS, MILLISECONDS, SECONDS, MINUTES, HOURS, DAYS]
This example also shows that value returned by
Class.getEnumConstants()
is identical to the value returned by invoking values()
on an enum type.
$ java EnumConstants Enum name: Eon Enum constants: [HADEAN, ARCHAEAN, PROTEROZOIC, PHANEROZOIC] Eon.values(): [HADEAN, ARCHAEAN, PROTEROZOIC, PHANEROZOIC]
Since enums are classes, other information may be obtained using the same Reflection APIs described in the Fields, Methods, and Constructors sections of this trail. The
code illustrates how to use these APIs to get additional information about the enum's declaration. The example uses
EnumSpy
Class.isEnum()
to restrict the set of classes examined. It also uses
Field.isEnumConstant()
to distinguish enum constants from other fields in the enum declaration (not all fields are enum constants).
import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Method; import java.lang.reflect.Member; import java.util.List; import java.util.ArrayList; import static java.lang.System.out; public class EnumSpy { private static final String fmt = " %11s: %s %s%n"; public static void main(String... args) { try { Class<?> c = Class.forName(args[0]); if (!c.isEnum()) { out.format("%s is not an enum type%n", c); return; } out.format("Class: %s%n", c); Field[] flds = c.getDeclaredFields(); List<Field> cst = new ArrayList<Field>(); // enum constants List<Field> mbr = new ArrayList<Field>(); // member fields for (Field f : flds) { if (f.isEnumConstant()) cst.add(f); else mbr.add(f); } if (!cst.isEmpty()) print(cst, "Constant"); if (!mbr.isEmpty()) print(mbr, "Field"); Constructor[] ctors = c.getDeclaredConstructors(); for (Constructor ctor : ctors) { out.format(fmt, "Constructor", ctor.toGenericString(), synthetic(ctor)); } Method[] mths = c.getDeclaredMethods(); for (Method m : mths) { out.format(fmt, "Method", m.toGenericString(), synthetic(m)); } // production code should handle this exception more gracefully } catch (ClassNotFoundException x) { x.printStackTrace(); } } private static void print(List<Field> lst, String s) { for (Field f : lst) { out.format(fmt, s, f.toGenericString(), synthetic(f)); } } private static String synthetic(Member m) { return (m.isSynthetic() ? "[ synthetic ]" : ""); } }
$ java EnumSpy java.lang.annotation.RetentionPolicy Class: class java.lang.annotation.RetentionPolicy Constant: public static final java.lang.annotation.RetentionPolicy java.lang.annotation.RetentionPolicy.SOURCE Constant: public static final java.lang.annotation.RetentionPolicy java.lang.annotation.RetentionPolicy.CLASS Constant: public static final java.lang.annotation.RetentionPolicy java.lang.annotation.RetentionPolicy.RUNTIME Field: private static final java.lang.annotation.RetentionPolicy[] java.lang.annotation.RetentionPolicy. [ synthetic ] Constructor: private java.lang.annotation.RetentionPolicy() Method: public static java.lang.annotation.RetentionPolicy[] java.lang.annotation.RetentionPolicy.values() Method: public static java.lang.annotation.RetentionPolicy java.lang.annotation.RetentionPolicy.valueOf(java.lang.String)
The output shows that declaration of
java.lang.annotation.RetentionPolicy
only contains the three enum constants. The enum constants are exposed as public static final
fields. The field, constructor, and methods are compiler generated. The $VALUES
field is related to the implementation of the values()
method.
Class.getFields()
and
Class.getDeclaredFields()
do not make any guarantee that the order of the returned values matches the order in the declaring source code. If ordering is required by an application, use
Class.getEnumConstants()
.
The output for
java.util.concurrent.TimeUnit
shows that much more complicated enums are possible. This class includes several methods as well as additional fields declared static final
which are not enum constants.
$ java EnumSpy java.util.concurrent.TimeUnit Class: class java.util.concurrent.TimeUnit Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.NANOSECONDS Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.MICROSECONDS Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.MILLISECONDS Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.SECONDS Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.MINUTES Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.HOURS Constant: public static final java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.DAYS Field: static final long java.util.concurrent.TimeUnit.C0 Field: static final long java.util.concurrent.TimeUnit.C1 Field: static final long java.util.concurrent.TimeUnit.C2 Field: static final long java.util.concurrent.TimeUnit.C3 Field: static final long java.util.concurrent.TimeUnit.C4 Field: static final long java.util.concurrent.TimeUnit.C5 Field: static final long java.util.concurrent.TimeUnit.C6 Field: static final long java.util.concurrent.TimeUnit.MAX Field: private static final java.util.concurrent.TimeUnit[] java.util.concurrent.TimeUnit. [ synthetic ] Constructor: private java.util.concurrent.TimeUnit() Constructor: java.util.concurrent.TimeUnit (java.lang.String,int,java.util.concurrent.TimeUnit) [ synthetic ] Method: public static java.util.concurrent.TimeUnit java.util.concurrent.TimeUnit.valueOf(java.lang.String) Method: public static java.util.concurrent.TimeUnit[] java.util.concurrent.TimeUnit.values() Method: public void java.util.concurrent.TimeUnit.sleep(long) throws java.lang.InterruptedException Method: public long java.util.concurrent.TimeUnit.toNanos(long) Method: public long java.util.concurrent.TimeUnit.convert (long,java.util.concurrent.TimeUnit) Method: abstract int java.util.concurrent.TimeUnit.excessNanos (long,long) Method: public void java.util.concurrent.TimeUnit.timedJoin (java.lang.Thread,long) throws java.lang.InterruptedException Method: public void java.util.concurrent.TimeUnit.timedWait (java.lang.Object,long) throws java.lang.InterruptedException Method: public long java.util.concurrent.TimeUnit.toDays(long) Method: public long java.util.concurrent.TimeUnit.toHours(long) Method: public long java.util.concurrent.TimeUnit.toMicros(long) Method: public long java.util.concurrent.TimeUnit.toMillis(long) Method: public long java.util.concurrent.TimeUnit.toMinutes(long) Method: public long java.util.concurrent.TimeUnit.toSeconds(long) Method: static long java.util.concurrent.TimeUnit.x(long,long,long)