How to concat two ArrayLists?

I have two ArrayLists of equal size. List 1 consists of 10 names and list 2 consists of their phone numbers.

I want to concat the names and number into one ArrayList. How do I do this?

You can use .addAll() to add the elements of the second list to the first:

array1.addAll(array2);

Edit: Based on your clarification above ("i want a single String in the new Arraylist which has both name and number."), you would want to loop through the first list and append the item from the second list to it.

Something like this:

int length = array1.size();
if (length != array2.size()) { // Too many names, or too many numbers
    // Fail
}
ArrayList<String> array3 = new ArrayList<String>(length); // Make a new list
for (int i = 0; i < length; i++) { // Loop through every name/phone number combo
    array3.add(array1.get(i) + " " + array2.get(i)); // Concat the two, and add it
}

If you put in:

array1 : ["a", "b", "c"]
array2 : ["1", "2", "3"]

You will get:

array3 : ["a 1", "b 2", "c 3"]

add one ArrayList to second ArrayList as:

Arraylist1.addAll(Arraylist2);

EDIT : if you want to Create new ArrayList from two existing ArrayList then do as:

ArrayList<String> arraylist3=new ArrayList<String>();

arraylist3.addAll(Arraylist1); // add first arraylist

arraylist3.addAll(Arraylist2); // add Second arraylist

If you want to do it one line and you do not want to change list1 or list2 you can do it using stream

List<String> list1 = Arrays.asList("London", "Paris");
List<String> list2 = Arrays.asList("Moscow", "Tver");

List<String> list = Stream.concat(list1.stream(),list2.stream()).collect(Collectors.toList());

ArrayList<String> resultList = new ArrayList<String>();
resultList.addAll(arrayList1);
resultList.addAll(arrayList2);

for a lightweight list that does not copy the entries, you may use sth like this:

List<Object> mergedList = new ConcatList<>(list1, list2);

here the implementation:

public class ConcatList<E> extends AbstractList<E> {

    private final List<E> list1;
    private final List<E> list2;

    public ConcatList(final List<E> list1, final List<E> list2) {
        this.list1 = list1;
        this.list2 = list2;
    }

    @Override
    public E get(final int index) {
        return getList(index).get(getListIndex(index));
    }

    @Override
    public E set(final int index, final E element) {
        return getList(index).set(getListIndex(index), element);
    }

    @Override
    public void add(final int index, final E element) {
        getList(index).add(getListIndex(index), element);
    }

    @Override
    public E remove(final int index) {
        return getList(index).remove(getListIndex(index));
    }

    @Override
    public int size() {
        return list1.size() + list2.size();
    }

    @Override
    public void clear() {
        list1.clear();
        list2.clear();
    }

    private int getListIndex(final int index) {
        final int size1 = list1.size();
        return index >= size1 ? index - size1 : index;
    }

    private List<E> getList(final int index) {
        return index >= list1.size() ? list2 : list1;
    }

}

One ArrayList1 add to data,

mArrayList1.add(data);

and Second ArrayList2 to add other data,

mArrayList2.addAll(mArrayList1);

var arr3 = new arraylist();
for(int i=0, j=0, k=0; i<arr1.size()+arr2.size(); i++){
	if(i&1)
		arr3.add(arr1[j++]);
	else
		arr3.add(arr2[k++]);
}

as you say, "the names and numbers beside each other".