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

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~~The Java Tutorials have been written for JDK 8.~~Java教程是为JDK 8编写的。~~Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.~~本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。
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~~See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.~~有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。

Troubleshooting

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)

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.

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)

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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