Java Tutorial/Reflection/Array

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Array reflection and two dimensional array

   <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.Array; public class CreateMatrix { 

 public static void main(String... args) {
   Object matrix = Array.newInstance(int.class, 2, 2);
   Object row0 = Array.get(matrix, 0);
   Object row1 = Array.get(matrix, 1);
   Array.setInt(row0, 0, 1);
   Array.setInt(row0, 1, 2);
   Array.setInt(row1, 0, 3);
   Array.setInt(row1, 1, 4);
   for (int i = 0; i < 2; i++)
     for (int j = 0; j < 2; j++)
       out.format("matrix[%d][%d] = %d%n", i, j, ((int[][]) matrix)[i][j]);
 }

}</source>





class name for double and float array

   <source lang="java">

class ClassInfoDemo2 { 

 public static void main(String[] args) {
   Class c = String.class;
   System.out.println(c.getName());
   c = ClassInfoDemo2.class;
   System.out.println(c.getName());
   c = double.class;
   System.out.println(c.getName());
   c = float[].class;
   System.out.println(c.getName());
 }

}</source>





Create array with Array.newInstance

   <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.Array; import java.lang.reflect.Constructor; import java.lang.reflect.InvocationTargetException; import java.util.Arrays; import java.util.regex.Matcher; import java.util.regex.Pattern; public class ArrayCreator { 

 private static String s = "java.math.BigInteger bi[] = { 123, 234, 345 }";
 private static Pattern p = Pattern
     .rupile("^\\s*(\\S+)\\s*\\w+\\[\\].*\\{\\s*([^}]+)\\s*\\}");
 public static void main(String... args) {
   Matcher m = p.matcher(s);
   if (m.find()) {
     String cName = m.group(1);
     String[] cVals = m.group(2).split("[\\s,]+");
     int n = cVals.length;
     try {
       Class<?> c = Class.forName(cName);
       Object o = Array.newInstance(c, n);
       for (int i = 0; i < n; i++) {
         String v = cVals[i];
         Constructor ctor = c.getConstructor(String.class);
         Object val = ctor.newInstance(v);
         Array.set(o, i, val);
       }
       Object[] oo = (Object[]) o;
       out.format("%s[] = %s%n", cName, Arrays.toString(oo));
       // production code should handle these exceptions more gracefully
     } catch (ClassNotFoundException x) {
       x.printStackTrace();
     } catch (NoSuchMethodException x) {
       x.printStackTrace();
     } catch (IllegalAccessException x) {
       x.printStackTrace();
     } catch (InstantiationException x) {
       x.printStackTrace();
     } catch (InvocationTargetException x) {
       x.printStackTrace();
     }
   }
 }

}</source>





Create integer array with Array.newInstance

   <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.Array; public class ArrayTrouble { 

 public static void main(String... args) {
   Object o = Array.newInstance(int.class, 0);
   int[] i = (int[]) o;
   int[] j = new int[0];
   out.format("i.length = %d, j.length = %d, args.length = %d%n", i.length,
       j.length, args.length);
   Array.getInt(o, 0); // ArrayIndexOutOfBoundsException
 }

}</source>





Demonstrates the use of the Array class

   <source lang="java">

/* 

*     file: ArrayDemo.java
*  package: oreilly.hcj.reflection
*
* 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. DO NOT EDIT ABOVE THIS LINE ########## */

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

* Demonstrates the use of the Array class.
*
* @author 
* @version $Revision$
*/

public class ArrayDemo { 

 /** 
  * Copy an array and return the copy.
  *
  * @param input The array to copy.
  *
  * @return The coppied array.
  *
  * @throws IllegalArgumentException If input is not an array.
  */
 public static Object copyArray(final Object input) {
   final Class type = input.getClass();
   if (!type.isArray()) {
     throw new IllegalArgumentException();
   }
   final int length = Array.getLength(input);
   final Class componentType = type.getComponentType();
   final Object result = Array.newInstance(componentType, length);
   for (int idx = 0; idx < length; idx++) {
     Array.set(result, idx, Array.get(input, idx));
   }
   return result;
 }
 /** 
  * Run the demo.
  *
  * @param args Command line arguments (ignored).
  */
 public static void main(final String[] args) {
   try {
     int[] x = new int[] { 2, 3, 8, 7, 5 };
     char[] y = new char[] { "a", "z", "e" };
     String[] z = new String[] { "Jim", "John", "Joe" };
     System.out.println(" -- x and y --");
     outputArrays(x, y);
     System.out.println(" -- x and copy of x --");
     outputArrays(x, copyArray(x));
     System.out.println(" -- y and copy of y --");
     outputArrays(y, copyArray(y));
     System.out.println(" -- z and copy of z --");
     outputArrays(z, copyArray(z));
   } catch (final Exception ex) {
     ex.printStackTrace();
   }
 }
 /** 
  * Print out 2 arrays in columnar format.
  *
  * @param first The array for the first column.
  * @param second The array for the second column.
  *
  * @throws IllegalArgumentException __UNDOCUMENTED__
  */
 public static void outputArrays(final Object first, final Object second) {
   if (!first.getClass()
             .isArray()) {
     throw new IllegalArgumentException("first is not an array.");
   }
   if (!second.getClass()
              .isArray()) {
     throw new IllegalArgumentException("second is not an array.");
   }
   final int lengthFirst = Array.getLength(first);
   final int lengthSecond = Array.getLength(second);
   final int length = Math.max(lengthFirst, lengthSecond);
   for (int idx = 0; idx < length; idx++) {
     System.out.print("[" + idx + "]\t");
     if (idx < lengthFirst) {
       System.out.print(Array.get(first, idx) + "\t\t");
     } else {
       System.out.print("\t\t");
     }
     if (idx < lengthSecond) {
       System.out.print(Array.get(second, idx) + "\t");
     }
     System.out.println();
   }
 }

} /* ########## End of File ########## */</source>





Determining If an Object Is an Array

   <source lang="java">

public class Main { 

 public static void main(String[] argv) throws Exception {
   boolean b = "".getClass().isArray();
   if (b) {
     System.out.println("object is an array");
   }
 }

}</source>





Getting the Component Type of an Array Object

   <source lang="java">

public class Main { 

 public static void main(String[] argv) throws Exception {
   Object o = new int[1][2][3];
   o.getClass().getComponentType();
 }

}</source>





Getting the Length and Dimensions of an Array Object

   <source lang="java">

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

 public static void main(String[] argv) throws Exception {
   Object o = new int[1][2][3];
   int len = Array.getLength(o); // 1
   System.out.println(len);
   int dim = getDim(o); // 3
   System.out.println(dim);
 }
 public static int getDim(Object array) {
   int dim = 0;
   Class cls = array.getClass();
   while (cls.isArray()) {
     dim++;
     cls = cls.getComponentType();
   }
   return dim;
 }

}</source>





Is field an array

   <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; public class ArrayFind { 

 public static void main(String... args) {
   boolean found = false;
   try {
     Class<?> cls = Class.forName(args[0]);
     Field[] flds = cls.getDeclaredFields();
     for (Field f : flds) {
       Class<?> c = f.getType();
       if (c.isArray()) {
         found = true;
         out.format("%s%n" + "           Field: %s%n"
             + "            Type: %s%n" + "  Component Type: %s%n", f, f
             .getName(), c, c.getComponentType());
       }
     }
     if (!found) {
       out.format("No array fields%n");
     }
     // production code should handle this exception more gracefully
   } catch (ClassNotFoundException x) {
     x.printStackTrace();
   }
 }

}</source>





Returns the length of the specified array, can deal with Object arrays and with primitive arrays.

   <source lang="java">

/* Copyright 2004 The Apache Software Foundation 

*
*   Licensed under the Apache License, Version 2.0 (the "License");
*   you may not use this file except in compliance with the License.
*   You may obtain a copy of the License at
*
*       http://www.apache.org/licenses/LICENSE-2.0
*
*   Unless required by applicable law or agreed to in writing, software
*   distributed under the License is distributed on an "AS IS" BASIS,
*   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
*   See the License for the specific language governing permissions and
*  limitations under the License.
*/

import java.lang.reflect.Array;

/** 

* Operations on arrays, primitive arrays (like int[]) and
* primitive wrapper arrays (like Integer[]).
* 
* This class tries to handle null input gracefully.
* An exception will not be thrown for a null
* array input. However, an Object array that contains a null
* element may throw an exception. Each method documents its behaviour.
*
* @author Stephen Colebourne
* @author Moritz Petersen
* @author 
* @author Maarten Coene
* @since 2.0
* @version $Id: ArrayUtils.java 632503 2008-03-01 00:21:52Z ggregory $
*/

public class Main { 

 //-----------------------------------------------------------------------
 /**
  * Returns the length of the specified array.
  * This method can deal with Object arrays and with primitive arrays.
  *
  * If the input array is null, 0 is returned.
  *
* 
   * ArrayUtils.getLength(null)            = 0
   * ArrayUtils.getLength([])              = 0
   * ArrayUtils.getLength([null])          = 1
   * ArrayUtils.getLength([true, false])   = 2
   * ArrayUtils.getLength([1, 2, 3])       = 3
   * ArrayUtils.getLength(["a", "b", "c"]) = 3
   * 
  *
  * @param array  the array to retrieve the length from, may be null
  * @return The length of the array, or 0 if the array is null
  * @throws IllegalArgumentException if the object arguement is not an array.
  * @since 2.1
  */
 public static int getLength(Object array) {
     if (array == null) {
         return 0;
     }
     return Array.getLength(array);
 }

}</source>





Use Array.setInt to fill an array

   <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.err; import java.lang.reflect.Array; public class ArrayTroubleAgain { 

 public static void main(String... args) {
   Integer[] ary = new Integer[2];
   try {
     Array.setInt(ary, 0, 1); // IllegalArgumentException
     // production code should handle these exceptions more gracefully
   } catch (IllegalArgumentException x) {
     err.format("Unable to box%n");
   } catch (ArrayIndexOutOfBoundsException x) {
     x.printStackTrace();
   }
 }

}</source>





Use Array.setShort and Array.setLong

   <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.Array; public class ArrayTroubleToo { 

 public static void main(String... args) {
   Object o = new int[2];
   Array.setShort(o, 0, (short) 2); // widening, succeeds
   Array.setLong(o, 1, 2L); // narrowing, fails
 }

}</source>


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