JAVA, comparing two strings

The problem is that now that you've told Scanner to use ; as a delimiter, it's not using whitespace as a delimiter anymore. So the token being tested against "45" isn't "45", it's "456\n45" (the end of the previous line, the newline, and the beginning of the next line), which isn't a match.

Change your useDelimiter line to use both semicolons and whitespace as your delimiters:

scanner.useDelimiter("[;\\s]");

...and then the scanner sees the "456" and the "45" separately, and matches the "45".

This code:

import java.util.*;
import java.io.*;

public class Parse {
    public static final void main(String[] args) {
        try {
            String result = test(45);
            System.out.println("result = " + result);
        }
        catch (Exception e) {
            System.out.println("Exception");
        }
    }

    public static String test(int numVol)throws Exception{
        File file = new File("test.csv");
        Scanner scanner = new Scanner(file);
        scanner.useDelimiter("[;\\s]"); // <==== Change is here
        String line = "";
        String sNumVol = ""+numVol;
        while (scanner.hasNext()){
            line = scanner.next();
            if(line.equals(sNumVol)){
                scanner.close();
                return line;
            }
        }
        scanner.close();
        return line;
    }
}

With this test.csv:

54;a;23;c;de;56
23;d;24;c;h;456
45;87;c;y;535
432;42;h;h;543

Shows this:

$ java Parse
result = 45

The way to find the answer to this problem was simply to walk through the code with a debugger and watch the value of line, or (if for some reason you don't have a debugger?!), insert a System.out.println("line = " + line); statement into the loop to see what was being compared. For instance, if you insert a System.out.println("line = " + line); above the line = scanner.next(); line above and you just use ";" as the delimiter:

import java.util.*;
import java.io.*;

public class Parse {
    public static final void main(String[] args) {
        try {
            String result = test(45);
            System.out.println("result = " + result);
        }
        catch (Exception e) {
            System.out.println("Exception");
        }
    }

    public static String test(int numVol)throws Exception{
        File file = new File("test.csv");
        Scanner scanner = new Scanner(file);
        scanner.useDelimiter(";"); // <== Just using ";"
        String line = "";
        String sNumVol = ""+numVol;
        while (scanner.hasNext()){
            line = scanner.next();
            System.out.println("line = [[" + line + "]]");
            if(line.equals(sNumVol)){
                scanner.close();
                return line;
            }
        }
        scanner.close();
        return line;
    }
}

You see this:

$ java Parse
line = [[54]]
line = [[a]]
line = [[23]]
line = [[c]]
line = [[de]]
line = [[56
23]]
line = [[d]]
line = [[24]]
line = [[c]]
line = [[h]]
line = [[456
45]]
line = [[87]]
line = [[c]]
line = [[y]]
line = [[535
432]]
line = [[42]]
line = [[h]]
line = [[h]]
line = [[543
]]
result = 543

...which helps visualize the problem.