如何检查文件中是否包含用户输入的特定数字？ （Java）的

Question

Hi I'm trying to write a program that will check a designated file for a number that a user has input, however if the number is not present in the file I want the program to loop back to the beginning, how could I do this?

Here is the mess I've made so far:

import java.util.Scanner;

public class Classtest2 {
    public static void main(String[] args) {

    //    //1. class
    Scanner sc = new Scanner(Classtest2.class.getResourceAsStream("trombones.txt"));
    Scanner myScanner = new Scanner(System.in);

    System.out.println("What number would you like to check for?");
    String number = myScanner.nextLine() ;
    String keyWord = number, word;
    int lineNum = 0;

    while(sc.hasNextLine()) {
        word = sc.next();

        if(word.equals(myScanner.nextLine().trim())) {
            System.out.println("The number "+number+ " is there");

            break;
        } else {
            // not found
        }
    } // main
} // class

Answer 1

all you need to do is take input from user and then check is it exist in file .you shouldn't use myScanner.nextLine().trim() inside while loop because then it waits to get user input every loop time .ones you get a number from user ,you should check it in the file .what you did is get user input and then again wait for user to input value again and again

fixed code

   import java.util.Scanner;

    public class Classtest2 {
        public static void main(String[] args) {

        //    //1. class
        Scanner sc = new Scanner(Classtest2.class.getResourceAsStream("trombones.txt"));
        Scanner myScanner = new Scanner(System.in);

        System.out.println("What number would you like to check for?");
        String number = myScanner.nextLine() ;
        int lineNum = 0;

        while(sc.hasNextLine()) {
            word = sc.next();

            if(word.equals(number.trim())) { //here was the problem
                System.out.println("The number "+number+ " is there");

                return;
            } else {

            }
        } 
        System.out.println("not found");  // not found
    } 

the best approach

import java.util.Scanner;

public class Classtest2 {

    public static void main(String[] args) {

        Scanner myScanner = new Scanner(System.in);
        System.out.println("What number would you like to check for?");
        String number = myScanner.nextLine();
        if(isFound(number)){
            System.out.println("The number "+number+ " is there");
        }else{
            System.out.println("The number "+number+ " doesn't exist");
        }
    }

    public static boolean isFound(String number) {

        Scanner sc = new Scanner(Classtest2.class.getResourceAsStream("trombones.txt"));
        String word="";
        while (sc.hasNextLine()) {
            word = sc.next();

            if (word.equals(number.trim())) { 
                return true;
            }
        }
        return false;
    }
}

Answer 2

This works for me, hope is useful for you:

package test;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;

public class Classtest2 {

    public static void main(String[] args) throws IOException {

        File f = new File("trombones.txt");
        f.createNewFile();
        Scanner myScanner = new Scanner(System.in);
        boolean numExiste = menu(new Scanner(f), myScanner);
        while (!(numExiste)){
            numExiste = menu(new Scanner(f), myScanner);
        }
        myScanner.close();

    }

    private static boolean menu(Scanner fileScanner, Scanner myScanner) throws FileNotFoundException{
        String word = "";
        System.out.println("What number would you like to check for?");
        String number = myScanner.nextLine().trim();

        while(fileScanner.hasNextLine()){
            word = fileScanner.nextLine();
            if(word.trim().equals(number)){
                System.out.println("The number "+number+ " is there");
                return true;
            }else{
                //Not the number
            }
        }
        System.out.println("The number "+ number+ " is not in the file\n\n");
        return false;

    }
} // main } // class