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.
A field may be either of primitive or reference type. There are eight primitive types: boolean
, byte
, short
, int
, long
, char
, float
, and double
. A reference type is anything that is a direct or indirect subclass of
java.lang.Object
including interfaces, arrays, and enumerated types.
The
example prints the field's type and generic type given a fully-qualified binary class name and field name.FieldSpy
import java.lang.reflect.Field; import java.util.List; public class FieldSpy<T> { public boolean[][] b = {{ false, false }, { true, true } }; public String name = "Alice"; public List<Integer> list; public T val; public static void main(String... args) { try { Class<?> c = Class.forName(args[0]); Field f = c.getField(args[1]); System.out.format("Type: %s%n", f.getType()); System.out.format("GenericType: %s%n", f.getGenericType()); // production code should handle these exceptions more gracefully } catch (ClassNotFoundException x) { x.printStackTrace(); } catch (NoSuchFieldException x) { x.printStackTrace(); } } }
Sample output to retrieve the type of the three public fields in this class (b
, name
, and the parameterized type list
), follows. User input is in italics.
$ java FieldSpy FieldSpy b Type: class [[Z GenericType: class [[Z $ java FieldSpy FieldSpy name Type: class java.lang.String GenericType: class java.lang.String $ java FieldSpy FieldSpy list Type: interface java.util.List GenericType: java.util.List<java.lang.Integer> $ java FieldSpy FieldSpy val Type: class java.lang.Object GenericType: T
The type for the field b
is two-dimensional array of boolean. The syntax for the type name is described in
Class.getName()
.
The type for the field val
is reported as java.lang.Object
because generics are implemented via type erasure which removes all information regarding generic types during compilation. Thus T
is replaced by the upper bound of the type variable, in this case, java.lang.Object
.
Field.getGenericType()
will consult the
Signature Attribute in the class file if it's present. If the attribute isn't available, it falls back on
Field.getType()
which was not changed by the introduction of generics. The other methods in reflection with name getGenericFoo
for some value of Foo are implemented similarly.