Adding an element to the begining of an ArrayList in java

Question

Just wondering what's the proper concept to add an element at the first position of a list ?

For example :

The primary list has these elements :

1 3 5 6

And id like to add this element at the beginning (position 0) : 7

So it would be like this at the final :

7 1 3 5 6

Do i need to copy all the elements in a temporary Arraylist and re-insert everything one at a time ?

ArrayList class will take care of rearranging the elements after an insert (in whichever position you want). You'd have to do it yourself if you were using static arrays. — Baderous
– Baderous, Commented Oct 17, 2014 at 14:01

Elliott Frisch · Accepted Answer · 2014-10-17 14:01:11Z

6

You could use List.add(0, E) like

List<Integer> al = new ArrayList<Integer>(Arrays.asList(1, 3, 5, 6));
al.add(0, 7);
System.out.println(al);

Output is (as requested)

[7, 1, 3, 5, 6]

answered Oct 17, 2014 at 14:01

Elliott Frisch

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4 Comments

Cyberflow Over a year ago

Oh ok ! So if i use the method and tell it to add at the 0 position it will bump all the other element of 1 space in the liste ? I tought it wouldve replace the element

Elliott Frisch Over a year ago

Correct, it will shift the other elements. No, that's List#set(int,E).

Cyberflow Over a year ago

So List# set (int, E) will replace the element at the given position ? Ok ... sounds good. I tought i had to copy everything in the list and move in another to insert everything again.

yshavit Over a year ago

@Cyberflow The docs for List.add(int, E) explicitly mention the bumping: "Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices)."

Blair Nangle · Accepted Answer · 2022-11-30 09:01:03Z

0

Functionally:

Integer toPrepend = 7;
List<Integer> initial = List.of(1,  3,  5,  6);

Stream.concat(Stream.of(toPrepend), initial.stream()).collect(Collectors.toList())

edited Nov 30, 2022 at 9:01

answered Nov 30, 2022 at 8:44

Blair Nangle

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