int[] array3 = new int[array1.length + array2.length];

int[] array1 = ...;
int[] array2 = ...;
int[] result = new int[array1.length + array2.length];
for (int i = 0; i < array1.length; ++i) {
    result[i]  = array1[i];
}
for (int i = 0; i < array2.length; ++i) {
    result[array1.length + i] = array2[i];
}

int size = arr1.length + arr2.length;
int indexval=0;
int[] arr3 = new int[size];

for(int i=0;i<arr1.length;i++)
{
 arr3[i] = arr1[i];
 indexval++;
}

for(int i=0,i<arr2.length;i++)
{
  arr3[indexval] = arr2[i];
  indexval++;
}

int [] array1 = {1,2,3, 4};
int [] array2 = {5,6,7};
int [] array3 = ArrayUtils.addAll(array1, array2);

int[] array1 = { 1, 2, 3, 4, 5 };
int[] array2 = { 6, 7, 8, 9, 10 };

int[] array3 = ArrayUtils.addAll(array1, array2);