Trail: The Reflection API
Lesson: Arrays and Enumerated Types
Section: Enumerated Types

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Getting and Setting Fields with Enum Types

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 SetTrace example shows how code can translate the 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

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