Creating New Arrays

Just as in non-reflective code, reflection supports the ability to dynamically create arrays of arbitrary type and dimensions via java.lang.reflect.Array.newInstance(). Consider ArrayCreator, a basic interpreter capable of dynamically creating arrays. The syntax that will be parsed is as follows:

fully_qualified_class_name variable_name[] = 
 { val1, val2, val3, ... }

Assume that the fully_qualified_class_name represents a class that has a constructor with a single String argument. The dimensions of the array are determined by the number of values provided. The following example will construct an instance of an array of fully_qualified_class_name and populate its values with instances given by val1, val2, etc. (This example assumes familiarity with Class.getConstructor() and java.lang.reflect.Constructor.newInstance(). For a discussion of the reflection APIs for Constructor see the Creating New Class Instances section of this trail.)

import java.lang.reflect.Array;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.util.Arrays;
import static java.lang.System.out;
public class ArrayCreator {
 private static String s = "java.math.BigInteger bi[] = { 123, 234, 345 }";
 private static Pattern p = Pattern.compile("^\\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();
 }
 }
 }
}

$ java ArrayCreator
java.math.BigInteger [] = [123, 234, 345]

The above example shows one case where it may be desirable to create an array via reflection; namely if the component type is not known until runtime. In this case, the code uses Class.forName() to get a class for the desired component type and then calls a specific constructor to initialize each component of the array before setting the corresponding array value.