The following examples show problems which may be encountered when using enumerated types.

IllegalArgumentException When Attempting to Instantiate an Enum Type

As has been mentioned, instantiation of enum types is forbidden. The EnumTrouble example attempts this.

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

enum Charge {
    POSITIVE, NEGATIVE, NEUTRAL;
    Charge() {
	out.format("under construction%n");
    }
}

public class EnumTrouble {

    public static void main(String... args) {
	try {
	    Class<?> c = Charge.class;

 	    Constructor[] ctors = c.getDeclaredConstructors();
 	    for (Constructor ctor : ctors) {
		out.format("Constructor: %s%n",  ctor.toGenericString());
 		ctor.setAccessible(true);
 		ctor.newInstance();
 	    }

        // production code should handle these exceptions more gracefully
	} catch (InstantiationException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	} catch (InvocationTargetException x) {
	    x.printStackTrace();
	}
    }
}

$ java EnumTrouble
Constructor: private Charge()
Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
        at java.lang.reflect.Constructor.newInstance(Constructor.java:511)
        at EnumTrouble.main(EnumTrouble.java:22)

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Tip: It is a compile-time error to attempt to explicitly instantiate an enum because that would prevent the defined enum constants from being unique. This restriction is also enforced in reflective code. Code which attempts to instantiate classes using their default constructors should invoke Class.isEnum() first to determine if the class is an enum.

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IllegalArgumentException when Setting a Field with an Incompatible Enum Type

Fields storing enums set with the appropriate enum type. (Actually, fields of any type must be set with compatible types.) The EnumTroubleToo example produces the expected error.

import java.lang.reflect.Field;

enum E0 { A, B }
enum E1 { A, B }

class ETest {
    private E0 fld = E0.A;
}

public class EnumTroubleToo {
    public static void main(String... args) {
	try {
	    ETest test = new ETest();
	    Field f = test.getClass().getDeclaredField("fld");
	    f.setAccessible(true);
 	    f.set(test, E1.A);  // IllegalArgumentException

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}

$ java EnumTroubleToo
Exception in thread "main" java.lang.IllegalArgumentException: Can not set E0 field ETest.fld to E1
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:146)
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:150)
        at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:63)
        at java.lang.reflect.Field.set(Field.java:657)
        at EnumTroubleToo.main(EnumTroubleToo.java:16)

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Tip:  Strictly speaking, any attempt to set a field of type X to a value of type Y can only succeed if the following statement holds:

X.class.isAssignableFrom(Y.class) == true

The code could be modified to perform the following test to verify whether the types are compatible:

if (f.getType().isAssignableFrom(E0.class))
    // compatible
else
    // expect IllegalArgumentException

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