Number is not added at back of existing array

Learning about Arrays. I am not able to figure out why a new number is not added to the back of my existing array. I read in two textfiles in file_1.txt are the numbers '1 2 3' and in file_2.txt is the number '91'. Basically without the method of Void addBack() the program does what I expect, however by adding the method it seems not make a new Array. Even when I go over the elements[i] = elements[i-1] it won't print it as a whole. I am expecting to print for the first part
The numbers are: 1 2 3 and the second part The numbers are: 1 2 3 91.

public class ExampleLecture {
IntRow readIntRow(Scanner input) {
IntRow result = new IntRow();
while (input.hasNext()) {
result.add(input.nextInt());
}
return result;
}
IntRow setBack(Scanner input) {
IntRow result = new IntRow();
while(input.hasNext()) {
result.addBack(input.nextInt());
System.out.println("here");
}
return result;
}
void print(IntRow row) {
for (int i = 0; i < row.numberOfElements; i++) {
System.out.printf("%d ", row.elements[i]);
}
System.out.printf("\n");
}
void start() {
Scanner in = UIAuxiliaryMethods.askUserForInput().getScanner();
Scanner in2 =UIAuxiliaryMethods.askUserForInput().getScanner();
IntRow row = readIntRow(in);
IntRow row2 = setBack(in2);
System.out.printf("the numbers are: ");
print (row);
System.out.printf("the new numbers are: ");
print (row2);
}
public static void main(String[] args) {
new ExampleLecture().start();
}
}

    package examplelecture;
class IntRow {
static final int MAX_NUMBER_OF_ELEMENTS = 250;
int[] elements;
int numberOfElements;
IntRow() {
elements = new int[MAX_NUMBER_OF_ELEMENTS];
numberOfElements = 0;
}
void add(int number) {
elements[numberOfElements] = number;
numberOfElements += 1;
}
void addBack(int number) {
for (int i = numberOfElements; i>0; i--) {
elements[i] = elements[i-1];
elements[i] = number;
}
}
}

You have 2 successive assignments which write to the same position:

elements[i] = elements[i-1];
elements[i] = number;

The value is alway overwritten with number, so the first statement has no effect.

Also in your addBack method your for cycle:

for (int i = numberOfElements; i>0; i--) {

What happens if numberOfElements is 0?

You call it addBack but it looks like a better name for the method is addFirst. Usually index 0 is considered the front, not the back.

Admittedly it is not entirely clear what you are trying to accomplish. But you may have several problems. The first is as follows:

	IntRow setBack(Scanner input) {
IntRow result = new IntRow();
while (input.hasNext()) {
result.addBack(input.nextInt());
System.out.println("here");
}
return result;
}

IntRow has nothing in it since it is new. So all you are doing is iterating over the new file which has just 91 in it. Remember, result has no items. So it won't even iterate once in addBack.

So just do the following:

Change your addBack method to just add the numbers. Why use a loop to cascade down the elements since you are doing this within the same instance of IntRow? Just add it on to the end using the numberofElements as the next index.

void addBack(int number) {
elements[numberOfElements++] = number;        
}

If you want to copy the contents of one IntRow object to another you would need another method in the IntRow class. Something like:

public void copy(IntRow r) {
for (int i = 0; i < r.numerOfElements; i++) {
elements[i] = r.elements[i];
}
numerOfElements = r.numberOfElements;
}

And keeping with good design it might be better to return numberOfElements in a method such as public int size();

First off, both the readIntRow() and setBack() methods create new IntRow objects row and row2. If you want the result to be appended to the first IntRow object created i.e. to row , you should call:

        IntRow row = readIntRow(in);
IntRow row2 = row.setBack(in2);

and setBack() needs to be modified to:

    IntRow setBack(Scanner input) {
while(input.hasNext()) {
this.add(input.nextInt());
System.out.println("here");
}
return this;
}

Note that in setBack(), if you are trying to append numbers to the end of the IntRow object, you should call add() instead of addBack() as above. If you are trying to add to the front, you should call addBack() [and it might be better to call it addFront() instead].

Also, in the implementation of addBack(), if you are trying to add to the front of the IntRow object, the element[i] = number operation should take place only once, after the loop. Otherwise all the values in indices <= numberOfElements would be overwritten with number.

    void addBack(int number) {
for (int i = numberOfElements; i>0; i--) {
elements[i] = elements[i-1];
}
elements[0] = number;
}