Java Reflection - Java Array Reflection

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We can use isArray() method from Class class to check if a class is an array.

We can create an array, read and modify its element's values using reflection through java.lang.reflect.Array class.

The getLength() method from Array class gets the length of an array.

All methods from the Array class are static.

To create an array, use the overloaded static method newInstance() from the Array class.

Object newInstance(Class<?> componentType,  int  arrayLength)
Object newInstance(Class<?> componentType,  int... dimensions)

The first method creates an array from the specified component type and array length.

The second version creates an array of the specified component type and dimensions.

The return type from the newInstance() method is Object and we need to cast it to the actual array type.

The following code creates an array of int of length 5.

int[] ids = (int[])Array.newInstance(int.class, 5);

To create an array of int of dimension 5 by 3.

int[][] matrix = (int[][])Array.newInstance(int.class, 5, 3);

Example

The following code shows how to create an array dynamically and manipulate its elements.

import java.lang.reflect.Array;
/*from ww w. j  av  a  2 s .co m*/
public class Main {
  public static void main(String[] args) {
    try {
      Object my = Array.newInstance(int.class, 2);

      int n1 = Array.getInt(my, 0);
      int n2 = Array.getInt(my, 1);
      System.out.println("n1 = " + n1 + ", n2=" + n2);

      Array.set(my, 0, 11);
      Array.set(my, 1, 12);

      n1 = Array.getInt(my, 0);
      n2 = Array.getInt(my, 1);
      System.out.println("n1 = " + n1 + ", n2=" + n2);
    } catch (NegativeArraySizeException | IllegalArgumentException
        | ArrayIndexOutOfBoundsException e) {
      System.out.println(e.getMessage());
    }
  }
}

The code above generates the following result.

Get the dimension of an array

Java supports an array of arrays.

The method getComponentType() from Class class returns the Class object for an array's element type.

The following code illustrates how to get the dimension of an array.

public class Main {
  public static void main(String[] args) {
    int[][][] intArray = new int[1][2][3];
/* www. j a  v a  2 s.c o m*/
    System.out.println("int[][][] dimension is " + getArrayDimension(intArray));
  }

  public static int getArrayDimension(Object array) {
    int dimension = 0;
    Class c = array.getClass();
    if (!c.isArray()) {
      throw new IllegalArgumentException("Object is not  an  array");
    }
    while (c.isArray()) {
      dimension++;
      c = c.getComponentType();
    }
    return dimension;
  }
}

The code above generates the following result.

Expanding an Array

Java array is a fixed-length data structure.

To enlarge array we can create an array of a bigger size and copy the old array elements to the new one

The following code shows how to expand an Array Using Reflection

import java.lang.reflect.Array;
import java.util.Arrays;
/*w w w  . jav a  2  s  .com*/
public class Main {
  public static void main(String[] args) {
    int[] ids = new int[2];
    System.out.println(ids.length);
    System.out.println(Arrays.toString(ids));

    ids = (int[]) expandBy(ids, 2);

    ids[2] = 3;
    System.out.println(ids.length);
    System.out.println(Arrays.toString(ids));
  }

  public static Object expandBy(Object oldArray, int increment) {
    Object newArray = null;
    int oldLength = Array.getLength(oldArray);
    int newLength = oldLength + increment;
    Class<?> c = oldArray.getClass();
    newArray = Array.newInstance(c.getComponentType(), newLength);
    System.arraycopy(oldArray, 0, newArray, 0, oldLength);
    return newArray;
  }
}

The code above generates the following result.

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