Searching a list

Is a value in the list, and where? Go through the indexes and compare each value with the one you're looking for. Keep the index of the match in a variable that starts at -1: no real index is -1, so it means "not found yet".

import java.util.ArrayList;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        ArrayList<String> shelf = new ArrayList<>();
        shelf.add("atlas");
        shelf.add("cookbook");
        shelf.add("diary");
        shelf.add("novel");

        String wanted = scanner.nextLine();
        int found = -1;
        for (int i = 0; i < shelf.size(); i++) {
            if (shelf.get(i).equals(wanted)) {
                found = i;
                break;
            }
        }

        if (found == -1) {
            System.out.println("No " + wanted + " on the shelf");
        } else {
            System.out.println("The " + wanted + " is at index " + found);
        }
    }
}

diary

The diary is at index 2

• shelf.get(i) is a String, so it's compared with equals, as always for Strings.
• break stops the search at the first match. Without it, the loop would go on, and found would end up as the index of the last match.
• Only after the loop is it certain that the value isn't there: found is still -1.

For numbers, keep the value you're looking for in an int variable, and numbers.get(i) == wanted compares the numbers.

Every match

To find all the places a value appears, leave out the break, and act on each match inside the loop. Here each six is printed and counted:

ArrayList<Integer> rolls = new ArrayList<>();
rolls.add(3);
rolls.add(6);
rolls.add(2);
rolls.add(6);
rolls.add(6);
rolls.add(1);

int sixes = 0;
for (int i = 0; i < rolls.size(); i++) {
    if (rolls.get(i) == 6) {
        System.out.println("Six on roll " + (i + 1));
        sixes++;
    }
}
System.out.println("Sixes: " + sixes);

Six on roll 2
Six on roll 4
Six on roll 5
Sixes: 3

Lists have ready-made methods for some searches, such as indexOf and contains, and the next module introduces them. Writing the loop yourself shows how they work, and a loop can do more than they can, like finding every match.