java.lang.reflect

Class Field

Implemented Interfaces:
AnnotatedElement, Member

public final class Field
extends AccessibleObject
implements Member

The Field class represents a member variable of a class. It also allows dynamic access to a member, via reflection. This works for both static and instance fields. Operations on Field objects know how to do widening conversions, but throw IllegalArgumentException if a narrowing conversion would be necessary. You can query for information on this Field regardless of location, but get and set access may be limited by Java language access controls. If you can't do it in the compiler, you can't normally do it here either.

Note: This class returns and accepts types as Classes, even primitive types; there are Class types defined that represent each different primitive type. They are java.lang.Boolean.TYPE, java.lang.Byte.TYPE,, also available as boolean.class, byte.class, etc. These are not to be confused with the classes java.lang.Boolean, java.lang.Byte, etc., which are real classes.

Also note that this is not a serializable class. It is entirely feasible to make it serializable using the Externalizable interface, but this is on Sun, not me.

Since:
1.1
See Also:
Member, Class, Class.getField(String), Class.getDeclaredField(String), Class.getFields(), Class.getDeclaredFields()

Fields inherited from interface java.lang.reflect.Member

DECLARED, PUBLIC

Method Summary

boolean
equals(Object o)
Compare two objects to see if they are semantically equivalent.
extends Annotation> T getAnnotation(Class annotationClass)
Returns the element's annotation for the specified annotation type, or null if no such annotation exists.
Object
get(Object o)
Get the value of this Field.
boolean
getBoolean(Object o)
Get the value of this boolean Field.
byte
getByte(Object o)
Get the value of this byte Field.
char
getChar(Object o)
Get the value of this Field as a char.
Annotation[]
getDeclaredAnnotations()
Returns all annotations directly defined by the element.
Class
getDeclaringClass()
Gets the class that declared this field, or the class where this field is a non-inherited member.
double
getDouble(Object o)
Get the value of this Field as a double.
float
getFloat(Object o)
Get the value of this Field as a float.
Type
getGenericType()
Return the generic type of the field.
int
getInt(Object o)
Get the value of this Field as an int.
long
getLong(Object o)
Get the value of this Field as a long.
int
getModifiers()
Gets the modifiers this field uses.
String
getName()
Gets the name of this field.
short
getShort(Object o)
Get the value of this Field as a short.
Class
getType()
Gets the type of this field.
int
hashCode()
Get the hash code for the Field.
boolean
isEnumConstant()
Return true if this field represents an enum constant, false otherwise.
boolean
isSynthetic()
Return true if this field is synthetic, false otherwise.
void
set(Object o, Object value)
Set the value of this Field.
void
setBoolean(Object o, boolean value)
Set this boolean Field.
void
setByte(Object o, byte value)
Set this byte Field.
void
setChar(Object o, char value)
Set this char Field.
void
setDouble(Object o, double value)
Set this double Field.
void
setFloat(Object o, float value)
Set this float Field.
void
setInt(Object o, int value)
Set this int Field.
void
setLong(Object o, long value)
Set this long Field.
void
setShort(Object o, short value)
Set this short Field.
String
toGenericString()
String
toString()
Get a String representation of the Field.

Methods inherited from class java.lang.reflect.AccessibleObject

extends Annotation> T getAnnotation, getAnnotations, getDeclaredAnnotations, isAccessible, isAnnotationPresent, setAccessible, setAccessible

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

equals

public boolean equals(Object o)
Compare two objects to see if they are semantically equivalent. Two Fields are semantically equivalent if they have the same declaring class, name, and type. Since you can't creat a Field except through the VM, this is just the == relation.
Overrides:
equals in interface Object
Parameters:
o - the object to compare to
Returns:
true if they are equal; false if not

extends Annotation> T getAnnotation

public extends Annotation> T getAnnotation(Class annotationClass)
Returns the element's annotation for the specified annotation type, or null if no such annotation exists.
Specified by:
extends Annotation> T getAnnotation in interface AnnotatedElement
Overrides:
extends Annotation> T getAnnotation in interface AccessibleObject
Parameters:
annotationClass - the type of annotation to look for.
Returns:
this element's annotation for the specified type, or null if no such annotation exists.
Throws:
NullPointerException - if the annotation class is null.

get

public Object get(Object o)
            throws IllegalAccessException
Get the value of this Field. If it is primitive, it will be wrapped in the appropriate wrapper type (boolean = java.lang.Boolean).

If the field is static, o will be ignored. Otherwise, if o is null, you get a NullPointerException, and if it is incompatible with the declaring class of the field, you get an IllegalArgumentException.

Next, if this Field enforces access control, your runtime context is evaluated, and you may have an IllegalAccessException if you could not access this field in similar compiled code. If the field is static, and its class is uninitialized, you trigger class initialization, which may end in a ExceptionInInitializerError.

Finally, the field is accessed, and primitives are wrapped (but not necessarily in new objects). This method accesses the field of the declaring class, even if the instance passed in belongs to a subclass which declares another field to hide this one.

Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if o is not an instance of the class or interface declaring this field
NullPointerException - if o is null and this field requires an instance

getBoolean

public boolean getBoolean(Object o)
            throws IllegalAccessException
Get the value of this boolean Field. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a boolean field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getByte

public byte getByte(Object o)
            throws IllegalAccessException
Get the value of this byte Field. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getChar

public char getChar(Object o)
            throws IllegalAccessException
Get the value of this Field as a char. If the field is static, o will be ignored.
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a char field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getDeclaredAnnotations

public Annotation[] getDeclaredAnnotations()
Returns all annotations directly defined by the element. If there are no annotations directly associated with the element, then a zero-length array will be returned. The returned array may be modified by the client code, but this will have no effect on the annotation content of this class, and hence no effect on the return value of this method for future callers.
Specified by:
getDeclaredAnnotations in interface AnnotatedElement
Overrides:
getDeclaredAnnotations in interface AccessibleObject
Returns:
the annotations directly defined by the element.
Since:
1.5

getDeclaringClass

public Class getDeclaringClass()
Gets the class that declared this field, or the class where this field is a non-inherited member.
Specified by:
getDeclaringClass in interface Member
Returns:
the class that declared this member

getDouble

public double getDouble(Object o)
            throws IllegalAccessException
Get the value of this Field as a double. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte, short, char, int, long, float, or double field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getFloat

public float getFloat(Object o)
            throws IllegalAccessException
Get the value of this Field as a float. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte, short, char, int, long, or float field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getGenericType

public Type getGenericType()
Return the generic type of the field. If the field type is not a generic type, the method returns the same as getType().
Since:
1.5

getInt

public int getInt(Object o)
            throws IllegalAccessException
Get the value of this Field as an int. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte, short, char, or int field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getLong

public long getLong(Object o)
            throws IllegalAccessException
Get the value of this Field as a long. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte, short, char, int, or long field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getModifiers

public int getModifiers()
Gets the modifiers this field uses. Use the Modifier class to interpret the values. A field can only have a subset of the following modifiers: public, private, protected, static, final, transient, and volatile.
Specified by:
getModifiers in interface Member
Returns:
an integer representing the modifiers to this Member
See Also:
Modifier

getName

public String getName()
Gets the name of this field.
Specified by:
getName in interface Member
Returns:
the name of this field

getShort

public short getShort(Object o)
            throws IllegalAccessException
Get the value of this Field as a short. If the field is static, o will be ignored.
Parameters:
o - the object to get the value of this Field from
Returns:
the value of the Field
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte or short field of o, or if o is not an instance of the declaring class of this field
NullPointerException - if o is null and this field requires an instance
See Also:
get(Object)

getType

public Class getType()
Gets the type of this field.
Returns:
the type of this field

hashCode

public int hashCode()
Get the hash code for the Field. The Field hash code is the hash code of its name XOR'd with the hash code of its class name.
Overrides:
hashCode in interface Object
Returns:
the hash code for the object.

isEnumConstant

public boolean isEnumConstant()
Return true if this field represents an enum constant, false otherwise.
Since:
1.5

isSynthetic

public boolean isSynthetic()
Return true if this field is synthetic, false otherwise.
Specified by:
isSynthetic in interface Member
Since:
1.5

set

public void set(Object o,
                Object value)
            throws IllegalAccessException
Set the value of this Field. If it is a primitive field, the value will be unwrapped from the passed object (boolean = java.lang.Boolean).

If the field is static, o will be ignored. Otherwise, if o is null, you get a NullPointerException, and if it is incompatible with the declaring class of the field, you get an IllegalArgumentException.

Next, if this Field enforces access control, your runtime context is evaluated, and you may have an IllegalAccessException if you could not access this field in similar compiled code. This also occurs whether or not there is access control if the field is final. If the field is primitive, and unwrapping your argument fails, you will get an IllegalArgumentException; likewise, this error happens if value cannot be cast to the correct object type. If the field is static, and its class is uninitialized, you trigger class initialization, which may end in a ExceptionInInitializerError.

Finally, the field is set with the widened value. This method accesses the field of the declaring class, even if the instance passed in belongs to a subclass which declares another field to hide this one.

Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if value cannot be converted by a widening conversion to the underlying type of the Field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setBoolean

public void setBoolean(Object o,
                       boolean value)
            throws IllegalAccessException
Set this boolean Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a boolean field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setByte

public void setByte(Object o,
                    byte value)
            throws IllegalAccessException
Set this byte Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a byte, short, int, long, float, or double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setChar

public void setChar(Object o,
                    char value)
            throws IllegalAccessException
Set this char Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a char, int, long, float, or double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setDouble

public void setDouble(Object o,
                      double value)
            throws IllegalAccessException
Set this double Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setFloat

public void setFloat(Object o,
                     float value)
            throws IllegalAccessException
Set this float Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a float or long field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setInt

public void setInt(Object o,
                   int value)
            throws IllegalAccessException
Set this int Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not an int, long, float, or double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setLong

public void setLong(Object o,
                    long value)
            throws IllegalAccessException
Set this long Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a long, float, or double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

setShort

public void setShort(Object o,
                     short value)
            throws IllegalAccessException
Set this short Field. If the field is static, o will be ignored.
Parameters:
o - the object to set this Field on
value - the value to set this Field to
Throws:
IllegalAccessException - if you could not normally access this field (i.e. it is not public)
IllegalArgumentException - if this is not a short, int, long, float, or double field, or if o is not an instance of the class declaring this field
NullPointerException - if o is null and this field requires an instance

toGenericString

public String toGenericString()

toString

public String toString()
Get a String representation of the Field. A Field's String representation is "<modifiers> <type> <class>.<fieldname>".
Example: public transient boolean gnu.parse.Parser.parseComplete
Overrides:
toString in interface Object
Returns:
the String representation of the Field

java.lang.reflect.Field - reflection of Java fields Copyright (C) 1998, 2001, 2005, 2008 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.