用Java创建Login，不会转到else语句吗？

Question

I'm making a login program, but it is getting some errors. If I put in a name that is not in the list, I get an error. It doesn't seem to reach the else statement also, and I'm not sure why.

import java.lang.*;
import java.util.*;
import java.io.*;
import java.math.*;
public class HelloWorld {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);

    String userName;
    int n = 10;
    int i = 0;

    String[] array = new String[n];
    array[0] = "John";
    array[1] = "Johny";
    array[2] = "ben";


    System.out.println("Enter your user name(Note:**Case Sensative**)");
    userName = input.nextLine();

    while (i <= array.length) {
      if (array[i].equals(userName)) {
        System.out.println("Your UserName is valid");
        break;
      }
      else if (!array[i].equals(userName)){
        i++;
      }
      else {
          System.out.println("Your UserName is not valid");
          break;
      }
    }
  }
}

Answer 1

First, I'd consider using a for-loop as it'll manage the i value for you, but also consider the logic you have

if element.equals(userName) {...} 
else if !element.equals(userName) {...} 
else {...}

How is it possible to ever get into the else block, either element will be or won't be equal to the userName , there is no other state. I'd consider having a flag which indicates if a user was found or not and checking that at the other end of the loop

As a general concept...

//...
userName = input.nextLine();

String validUser = null;
for (int index = 0; index < array.length; index++) {
    if (array[i].equals(userName)) {
        validUser = userName;
        break;
    }
}
if (validUser != null) {
    System.out.println("UserName is valid");
} else {
    System.out.println("Your UserName is not valid");
}

Because you probably don't care about the index so much, you can also do something like...

for (String element : array) {
    if (array[i].equals(userName)) {
        validUser = userName;
        break;
    }
}

which is shorthand version.

Have a look at The for statement for more details

Answer 2

First of all your if and else if are the only condition the java will run in your context and there are no other condition running afterwards. It is like you are doing TRUE and FALSE and expecting another condition. Make use of foreach in java much simpler way.

boolean found = false;
for(String name: array){
    if (name.equals(userName)) {
        found = true;
        break;
    }
}

if(!found){
    System.out.println("Your UserName is not valid");
}else{
    System.out.println("Your UserName is valid");
}

When you get mastered use the methods of Array , Collections and specially lambdas stream api for little bit lift up your java knowledge.

Answer 3

Your Statement will never go to the else part, because the the input( userName ) will either EQUAL array[i] , or not. Think about it...

if (array[i].equals(userName)) { //It will do this if userName is array[i]
    System.out.println("Your UserName is valid");
    break;
}
else if (!array[i].equals(userName)) { //It will do this if userName is NOT array[i]
    i++;
}

else { //this is impossible, unreachable code
    System.out.println("Your UserName is not valid");
    break;
}

The reason your else is not reachable is because it is an impossible scenario. I edited your program, and it works now. Here's the whole thing:

import java.util.Scanner;

public class Experiments {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String userName;
        boolean found = false;

        String[] array = new String[3];
        array[0] = "John";
        array[1] = "Johny";
        array[2] = "ben";


        System.out.println("Enter your user name:");
        userName = input.nextLine();

        for(int i=0; i<array.length; i++) {
            if (userName.equalsIgnoreCase(array[i])) { 
                System.out.println("Your UserName is valid");
                found = true;
                break;
            }
            if (!found && i == array.length-1) {
                System.out.println("Your UserName is not valid");
            }
        }
        input.close();
    } 
}

If the input (userName) is in the array at any spot, it will say "Your UserName is valid" If it isn't in the array, it will say "Your UserName is not valid" .

Here is an example of a valid userName:

Enter your user name:
John
Your UserName is valid

And if the UserName is invalid:

Enter your user name:
Jimmy
Your UserName is not valid

Please comment if you have any questions about this, and I will explain.