Подскажите в чем ошибка Help ????

for (int i = 0; i < firstArray.length; i++) {
     mass[i] = firstArray[i];
 }
 for (int i = 0; i < secondArray.length; i++) {
         mass[i+10] = secondArray[i];
     }

int t = firstArray.length + secondArray.length;
    int[] mass = new int[t];
    for (int i = 0; i < firstArray.length; i++) {
        mass[i] = firstArray[i];
    }
    int t1 = secondArray.length - 1;
    for (int i = 0; i < secondArray.length; i++) {
        mass[t1 + i] = secondArray[i];
    }
    return mass;
}

int t = firstArray.length + secondArray.length;
    int[] mass = new int[t];
    for (int i = 0; i < firstArray.length; i++) {
        mass[i] = firstArray[i];
    }
    int t1 = firstArray.length ;
    for (int i = 0; i < secondArray.length; i++) {
        mass[t1 + i] = secondArray[i];
    }
    return mass;
}

public static int[] mergeArrays(int[] firstArray, int[] secondArray) {
    int[] newArray = new int[firstArray.length + secondArray.length];
    System.arraycopy(firstArray, 0, newArray, 0, firstArray.length);
    System.arraycopy(secondArray, 0, newArray, firstArray.length, secondArray.length);
    return newArray;
}

public static int[] mergeArrays(int[] firstArray, int[] secondArray) {
        return IntStream.concat(Arrays.stream(firstArray), Arrays.stream(secondArray)).toArray();
    }
}

public static void main(String[] args) {
        int[] firstArray = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        int[] secondArray = {10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20};
        System.out.println(Arrays.toString(mergeArrays(firstArray, secondArray)));
    }

    public static int[] mergeArrays(int[] firstArray, int[] secondArray) {
    //напишите тут ваш код
        int t = firstArray.length + secondArray.length;
        int[] mass = new int[t];
        for (int i = 0; i < 10; i++) {
            mass[i] = firstArray[i];
        }
        int t1 = firstArray.length ;
        for (int i = 0; i < secondArray.length; i++) {
            mass[t1 + i] = secondArray[i];
        }
        return mass;
    }
}