How to add value into Arraylist?

Question

I'm trying to add whatever value that scanner read from the file into an ArrayList<Integer>, but for some reason it doesn't work; it keeps complaining that "java: incompatible types: java.lang.String cannot be converted to java.lang.Integer."

Can anyone help me what is wrong with this? It works with the String type of Arraylist but not with Integer types of arraylist. So, I am guessing this is the problem, but not exactly know what is going on.

/**
 * readFromFile method will read from the database file and save all usernames, 
 * passwords, and keys in the corresponding ArrayLists.If the file cannot open,
* output error message: <dbName> cannot open!
 * @param dbName The file name to save all users, passwords, and keys
 * @param users
 * @param pwds
 * @param keys
 */
public static void readFromFile(String dbName, ArrayList<String> users, ArrayList<String> pwds, ArrayList<Integer> keys) {
    Path path = Paths.get("dbName");
    Scanner scan = null;
    try {
        scan = new Scanner(path);
    } catch (IOException e) {
        System.out.println(dbName +"cannot"+ " open!");
    }

    for (int i=0; i<users.size(); i++) {
        users.add(scan.next());
        pwds.add(scan.next());
        keys.add(scan.next()); // This is where the problem occurs
    }

keys.add( Integer.parseInt(scan.nextLine()); you need to convert from a string to a number. — Angel Koh, Apr 15 '21 at 12:17
A `String` is not an `Integer`. Why do you expect this to work? — Mark Rotteveel, Apr 15 '21 at 12:38

score 1 · Answer 1 · answered Apr 15 '21 at 12:28

The variable keys is defined as ArrayList<Integer>.
The output of scan.next() is a String.

The compiler recognizes this and so it will not allow you to insert a String into a List<Integer>. You need to parse the Integer out of the String prior to insertion or invoke the int-specific method on the Scanner.

keys.add(Integer.valueOf(scan.next()));
... or ....
keys.add(scan.nextInt());

score 0 · Answer 2 · answered Apr 15 '21 at 12:16

0

You need to use nextInt or to parse string value.

keys.add(scan.nextInt());

answered Apr 15 '21 at 12:16

Akif Hadziabdic

2,692
1
14
25

score 0 · Answer 3 · answered Apr 15 '21 at 12:28

0

correct to keys.add(Integer.parseInt(scan.nextLine())

public static void readFromFile(String dbName, ArrayList<String> users, ArrayList<String> pwds, ArrayList<Integer> keys) {
    Path path = Paths.get("dbName");
    Scanner scan = null;
    try {
        scan = new Scanner(path);
    } catch (IOException e) {
        System.out.println(dbName +"cannot"+ " open!");
    }

    for (int i=0; i<users.size(); i++) {
        users.add(scan.next());
        pwds.add(scan.next());
       keys.add(Integer.parseInt(scan.nextLine()) // This is where the problem occurs
    }

answered Apr 15 '21 at 12:28

SYED MUSTAFA HUSSAIN

469
3
13

Using `nexLine()` instead of `next()` in the original code will change the behaviour of the code. – Mark Rotteveel Apr 15 '21 at 12:39
@MarkRotteveel thankyou for correcting me then what we use? Next() function? – SYED MUSTAFA HUSSAIN Apr 15 '21 at 12:53
Given the OP is using `next()` (which returns a String), then you should as well (or switch to `nextInt()`) – Mark Rotteveel Apr 15 '21 at 13:11

How to add value into Arraylist?

3 Answers3