Troubleshooting
以下示例显示了使用枚举类型时可能遇到的问题。
IllegalArgumentException try实例化枚举类型时
如前所述,枚举类型的实例化是禁止的。 EnumTrouble示例try进行此操作。
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:
try显式实例化枚举是编译时错误,因为这将阻止定义的枚举常量唯一。反射代码中也强制执行此限制。try使用其默认构造函数实例化类的代码应首先调用Class.isEnum()以确定该类是否为枚举。
设置具有不兼容的枚举类型的字段时, IllegalArgumentException
存储具有适当枚举类型的枚举的字段。 (实际上,必须将* any *类型的字段设置为兼容类型.)EnumTroubleToo示例会产生预期的错误。
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:
严格来说,任何将X
类型的字段设置为Y
类型的值的try都只有在以下语句成立的情况下才能成功:
X.class.isAssignableFrom(Y.class) == true
可以修改代码以执行以下测试,以验证类型是否兼容:
if (f.getType().isAssignableFrom(E0.class))
// compatible
else
// expect IllegalArgumentException