Java by API/java.lang.reflect/Field

Field: Class<?> getType()

   <source lang="java">
 

import java.lang.reflect.Field; public class Main {

 public static void initIntFields(final Object obj) {
   try {
     Field[] fields = obj.getClass().getDeclaredFields();
     for (int idx = 0; idx < fields.length; idx++) {
       if (fields[idx].getType() == int.class) {
         fields[idx].setAccessible(true);
         fields[idx].setInt(obj, 0);
       }
     }
   } catch (final IllegalAccessException ex) {
     throw new RuntimeException(ex);
   }
 }
 public static final void main(final String[] args) {
   Integer value = new Integer("123");
   System.out.println("Before: " + value);
   initIntFields(value);
   System.out.println("After: " + value);
 }

}

 </source>
   
  
 
  

Field: getAnnotation(Class<DataField> annotationClass)

   <source lang="java">

import java.lang.annotation.Annotation; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import java.lang.reflect.Field; public class Main {

 public static void main(String[] args) {
   Class d = DataBean.class;
   Field fs[] = d.getFields();
   for (Field f : fs) {
     System.out.println(f);
     Annotation a = f.getAnnotation(DataField.class);
     if (a != null) {
       System.out.println(f.getName());
     }
   }
 }

} class DataBean {

 @DataField
 public String name;
 @DataField
 public String data;
 public String description;

} @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) @interface DataField { }

 </source>
   
  
 
  

Field: getDouble(Object obj)

   <source lang="java">

import java.lang.reflect.Field; public class Main {

 public static void main(String[] args) throws Exception {
   Object clazz = new TestClass();
   String lookingForValue = "firstValue";
   Field field = clazz.getClass().getField(lookingForValue);
   Class clazzType = field.getType();
   if (clazzType.toString().equals("double"))
     System.out.println(field.getDouble(clazz));
   else if (clazzType.toString().equals("int"))
     System.out.println(field.getInt(clazz));
   
   //System.out.println(field.get(clazz));
 }

} class TestClass {

 public double firstValue = 3.14;

}

 </source>
   
  
 
  

Field: getGenericType()

   <source lang="java">

/*

* Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Sun Microsystems nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import java.lang.reflect.Field; import java.util.List; public class Main<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("FieldSpy");
     Field f = c.getField("name");
     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();
   }
 }

}

 </source>
   
  
 
  

Field: getInt(Object obj)

   <source lang="java">

import java.lang.reflect.Field; public class Main {

 public static void main(String[] args) throws Exception {
   Object clazz = new TestClass();
   String lookingForValue = "firstValue";
   Field field = clazz.getClass().getField(lookingForValue);
   Class clazzType = field.getType();
   if (clazzType.toString().equals("double"))
     System.out.println(field.getDouble(clazz));
   else if (clazzType.toString().equals("int"))
     System.out.println(field.getInt(clazz));
   
   //System.out.println(field.get(clazz));
 }

} class TestClass {

 public double firstValue = 3.14;

}

 </source>
   
  
 
  

Field: getModifiers()

   <source lang="java">

import java.lang.reflect.Field; import java.lang.reflect.Modifier; public class Main {

 public static void main(String[] args) {
   String name = "java.util.Date";
   try {
     Class cl = Class.forName(name);
     System.out.println("class " + name);
     System.out.println("Its methods:");
     printFields(cl);
     System.out.println();
   } catch (ClassNotFoundException e) {
     System.out.println("Class not found.");
   }
 }
 public static void printFields(Class cl) {
   Field[] fields = cl.getDeclaredFields();
   for (int i = 0; i < fields.length; i++) {
     Field f = fields[i];
     Class type = f.getType();
     String name = f.getName();
     System.out.print(Modifier.toString(f.getModifiers()));
     System.out.println(" " + type.getName() + " " + name + ";");
   }
 }

}

 </source>
   
  
 
  

Field: get(Object obj)

   <source lang="java">

import java.lang.reflect.Field; import java.util.Date; public class Main {

 public static void main(String[] args) throws Exception {
   Bean demo = new Bean();
   Class clazz = demo.getClass();
   Field field = clazz.getField("id");
   field.set(demo, new Long(10));
   Object value = field.get(demo);
   System.out.println("Value = " + value);
   field = clazz.getField("now");
   field.set(null, new Date());
   value = field.get(null);
   System.out.println("Value = " + value);
 }

} class Bean {

 public static Date now;
 public Long id;
 public String name;

}

 </source>
   
  
 
  

Field: isEnumConstant()

   <source lang="java">

/*

* Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Sun Microsystems nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import static java.lang.System.out; import java.lang.reflect.Field; import java.lang.reflect.Modifier; enum Spy {

 BLACK, WHITE

} public class Main {

 volatile int share;
 int instance;
 class Inner {
 }
 public static void main(String... args) {
   try {
     Class<?> c = Class.forName(args[0]);
     int searchMods = 0x0;
     for (int i = 1; i < args.length; i++) {
       searchMods |= modifierFromString(args[i]);
     }
     Field[] flds = c.getDeclaredFields();
     out.format("Fields in Class "%s" containing modifiers:  %s%n", c
         .getName(), Modifier.toString(searchMods));
     boolean found = false;
     for (Field f : flds) {
       int foundMods = f.getModifiers();
       // Require all of the requested modifiers to be present
       if ((foundMods & searchMods) == searchMods) {
         out.format("%-8s [ synthetic=%-5b enum_constant=%-5b ]%n", f
             .getName(), f.isSynthetic(), f.isEnumConstant());
         found = true;
       }
     }
     if (!found) {
       out.format("No matching fields%n");
     }
     // production code should handle this exception more gracefully
   } catch (ClassNotFoundException x) {
     x.printStackTrace();
   }
 }
 private static int modifierFromString(String s) {
   int m = 0x0;
   if ("public".equals(s))
     m |= Modifier.PUBLIC;
   else if ("protected".equals(s))
     m |= Modifier.PROTECTED;
   else if ("private".equals(s))
     m |= Modifier.PRIVATE;
   else if ("static".equals(s))
     m |= Modifier.STATIC;
   else if ("final".equals(s))
     m |= Modifier.FINAL;
   else if ("transient".equals(s))
     m |= Modifier.TRANSIENT;
   else if ("volatile".equals(s))
     m |= Modifier.VOLATILE;
   return m;
 }

}

 </source>
   
  
 
  

Field: isSynthetic()

   <source lang="java">

/*

* Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Sun Microsystems nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import static java.lang.System.out; import java.lang.reflect.Field; import java.lang.reflect.Modifier; enum Spy {

 BLACK, WHITE

} public class Main {

 volatile int share;
 int instance;
 class Inner {
 }
 public static void main(String... args) {
   try {
     Class<?> c = Class.forName(args[0]);
     int searchMods = 0x0;
     for (int i = 1; i < args.length; i++) {
       searchMods |= modifierFromString(args[i]);
     }
     Field[] flds = c.getDeclaredFields();
     out.format("Fields in Class "%s" containing modifiers:  %s%n", c
         .getName(), Modifier.toString(searchMods));
     boolean found = false;
     for (Field f : flds) {
       int foundMods = f.getModifiers();
       // Require all of the requested modifiers to be present
       if ((foundMods & searchMods) == searchMods) {
         out.format("%-8s [ synthetic=%-5b enum_constant=%-5b ]%n", f
             .getName(), f.isSynthetic(), f.isEnumConstant());
         found = true;
       }
     }
     if (!found) {
       out.format("No matching fields%n");
     }
     // production code should handle this exception more gracefully
   } catch (ClassNotFoundException x) {
     x.printStackTrace();
   }
 }
 private static int modifierFromString(String s) {
   int m = 0x0;
   if ("public".equals(s))
     m |= Modifier.PUBLIC;
   else if ("protected".equals(s))
     m |= Modifier.PROTECTED;
   else if ("private".equals(s))
     m |= Modifier.PRIVATE;
   else if ("static".equals(s))
     m |= Modifier.STATIC;
   else if ("final".equals(s))
     m |= Modifier.FINAL;
   else if ("transient".equals(s))
     m |= Modifier.TRANSIENT;
   else if ("volatile".equals(s))
     m |= Modifier.VOLATILE;
   return m;
 }

}

 </source>
   
  
 
  

Field: setAccessible(boolean flag)

   <source lang="java">

/*

* Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Sun Microsystems nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import java.lang.reflect.Field; public class Main {

 public final boolean b = true;
 public static void main(String... args) {
   Main ft = new Main();
   try {
     Class<?> c = ft.getClass();
     Field f = c.getDeclaredField("b");
      f.setAccessible(true); // solution
     f.setBoolean(ft, Boolean.FALSE); // IllegalAccessException
     // production code should handle these exceptions more gracefully
   } catch (NoSuchFieldException x) {
     x.printStackTrace();
   } catch (IllegalArgumentException x) {
     x.printStackTrace();
   } catch (IllegalAccessException x) {
     x.printStackTrace();
   }
 }

}

 </source>
   
  
 
  

Field: setBoolean(Object obj, boolean z)

   <source lang="java">

/*

* Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Sun Microsystems nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import java.lang.reflect.Field; public class Main {

 public final boolean b = true;
 public static void main(String... args) {
   Main ft = new Main();
   try {
     Class<?> c = ft.getClass();
     Field f = c.getDeclaredField("b");
      f.setAccessible(true); // solution
     f.setBoolean(ft, Boolean.FALSE); // IllegalAccessException
     // production code should handle these exceptions more gracefully
   } catch (NoSuchFieldException x) {
     x.printStackTrace();
   } catch (IllegalArgumentException x) {
     x.printStackTrace();
   } catch (IllegalAccessException x) {
     x.printStackTrace();
   }
 }

}

 </source>
   
  
 
  

Field: setInt(Object obj, int i)

   <source lang="java">
 

/* Before: [a=21.25, b=54.5, c=5665, d=2043, e=3121, f=1019] After: [a=21.25, b=54.5, c=0, d=0, e=0, f=0]

• /

/*

*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import java.lang.reflect.Field; /**

* Demonstrates how to set public field objects.
*
* @author 
* @version $Revision: 1.3 $
*/

public class HiddenFieldModification {

 /** 
  * Sets all int fields in an object to 0.
  *
  * @param obj The object to operate on.
  *
  * @throws RuntimeException If there is a reflection problem.
  */
 public static void initIntFields(final Object obj) {
   try {
     Field[] fields = obj.getClass()
                       .getDeclaredFields();
     for (int idx = 0; idx < fields.length; idx++) {
       if (fields[idx].getType() == int.class) {
         fields[idx].setAccessible(true);
         fields[idx].setInt(obj, 0);
       }
     }
   } catch (final IllegalAccessException ex) {
     throw new RuntimeException(ex);
   }
 }
 /** 
  * Demo Method.
  *
  * @param args Command line arguments.
  */
 public static final void main(final String[] args) {
   SomeNumbers value = new SomeNumbers();
   System.out.println("Before: " + value);
   initIntFields(value);
   System.out.println("After: " + value);
 }

} class SomeNumbers {

 /** A demo double. */
 public double a = 21.25d;
 /** A demo float. */
 public float b = 54.5f;
 /** A Demo int */
 public int c = 5665;
 /** Another demo int. */
 public int d = 2043;
 /** Another demo int. */
 protected int e = 3121;
 /** Another demo int. */
 private int f = 1019;
 /** 
  * @see java.lang.Object#toString()
  */
 public String toString() {
   return new String("[a=" + a + ", b=" + b + ", c=" + c + ", d=" + d + ", e=" + e
                     + ", f=" + f + "]");
 }

}

 </source>