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.
Fields which store enums are set and retrieved as any other reference type, using
Field.set()
and
Field.get()
. For more information on accessing fields, see the Fields section of this trail.
Consider application which needs to dynamically modify the trace level in a server application which normally does not allow this change during runtime. Assume the instance of the server object is available. The
example shows how code can translate the
SetTrace
String
representation of an enum into an enum type and retrieve and set the value of a field storing an enum.
import java.lang.reflect.Field; import static java.lang.System.out; enum TraceLevel { OFF, LOW, MEDIUM, HIGH, DEBUG } class MyServer { private TraceLevel level = TraceLevel.OFF; } public class SetTrace { public static void main(String... args) { TraceLevel newLevel = TraceLevel.valueOf(args[0]); try { MyServer svr = new MyServer(); Class<?> c = svr.getClass(); Field f = c.getDeclaredField("level"); f.setAccessible(true); TraceLevel oldLevel = (TraceLevel)f.get(svr); out.format("Original trace level: %s%n", oldLevel); if (oldLevel != newLevel) { f.set(svr, newLevel); out.format(" New trace level: %s%n", f.get(svr)); } // production code should handle these exceptions more gracefully } catch (IllegalArgumentException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } catch (NoSuchFieldException x) { x.printStackTrace(); } } }
Since the enum constants are singletons, the ==
and !=
operators may be used to compare enum constants of the same type.
$ java SetTrace OFF Original trace level: OFF $ java SetTrace DEBUG Original trace level: OFF New trace level: DEBUG