FlattenArrayConverter.java

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.Objects;

/**
 * This class contains various methods for manipulating arrays
 *
 * @author Vipul Shah
 */
public class FlattenArrayConverter {

    /**
     *
     * @param objects Array of Objects which needs to be flatten
     * @return flatten array of objects
     */
    public static Object[] flattenArray(Object[] objects) {

        if(objects == null){
            return null;
        }

        List<Object> outputList = new LinkedList<>();

        for (Object i : objects) {
            if(i!=null) {
                if (i instanceof Object[]) {
                    outputList.addAll(asListFromArray(flattenArray((Object[]) i)));
                } else {
                    outputList.add(i);
                }
            }
        }

        return toArrayFromList(outputList);
    }

    /**
     *
     * @param objectArray target Array which needs to be converted into {@link List}
     * @return list of items from array
     */
    private static List<Object> asListFromArray(Object[] objectArray){
        if(objectArray==null)
            return null;

        return Arrays.asList(objectArray);
    }

    /**
     *
     * @param objectList target {@link List} which needs to be converted into Array
     * @return array of items from list
     */
    private static Object[] toArrayFromList(List<Object> objectList){
        if(objectList==null)
            return null;

        return objectList.toArray(new Object[objectList.size()]);
    }
}

Main.java

import org.junit.Assert;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {
    @Test
    public void flattenTest() {
        Object[] integerArray1 = {1};
        Object[] integerArray2 = {new Object[]{1}, 2};
        Object[] integerArray3 = {new Object[]{new Object[]{1}, 2}, 3};
        Object[] integerArray4 = {new Object[]{new Object[]{new Object[]{1}, 2}, 3}, 4};
        Object[] integerArray5 = {new Object[]{new Object[]{null, 2}, 3}, 4};

        Assert.assertTrue(Arrays.equals(null, FlattenArrayConverter.flattenArray(null)));
        Assert.assertTrue(Arrays.equals(new Object[]{1}, FlattenArrayConverter.flattenArray(integerArray1)));
        Assert.assertTrue(Arrays.equals(new Object[]{1, 2}, FlattenArrayConverter.flattenArray(integerArray2)));
        Assert.assertTrue(Arrays.equals(new Object[]{1, 2, 3}, FlattenArrayConverter.flattenArray(integerArray3)));
        Assert.assertTrue(Arrays.equals(new Object[]{1, 2, 3, 4}, FlattenArrayConverter.flattenArray(integerArray4)));
        Assert.assertTrue(Arrays.equals(new Object[]{2, 3, 4}, FlattenArrayConverter.flattenArray(integerArray5)));

    }
}