else语句不会在if else语句中执行

Question

For some reason, no matter what I do, every time I make an if else statement in my Java programs, it either only ever executes the if statement, or both the if and else statement.

import java.util.Scanner;

public class Week06_NelsonPimentel_Assignment {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int[] validnum;
        int i = 0;
        validnum = new int[5];

        System.out.println("Please enter a number between 50 and 100");

        while (i < validnum.length) {
            validnum[i] = input.nextInt();

            if (validnum[i] <= 101 || validnum[i] >= 49) {
                System.out.println("yes");
                i++;
            } else {
                System.out.println("no");
            }
        }

    }

}

Answer 1

if (validnum[i] <= 101 || validnum[i] >= 49) { will always evaluate to true , and so "yes" will be printed and i incremented, unless of course i is outside the bounds of the array.

Replace || with && ?

Also consider writing int[] validnum = new int[5]; rather than having two separate steps. That way, validnum is never in an uninitialised state which tends to result in stabler programs.

Answer 2

You have an endless loop in your code, when validnum[i] doesn't meet your condition you don't advance your loop index. Take i++; out of the if or replace your while loops with ( NOTE: You don't neet i++; if you are using for loop):

for (int i = 0; i < validnum.length; i++) {
        validnum[i] = input.nextInt();

        if (validnum[i] <= 101 || validnum[i] >= 49) {
            System.out.println("yes");
        } else {
            System.out.println("no");
        }
}

Why do you use the array? in the code you posted it looks like a waste of memory.

Answer 3

As per your business logic the if statement will always returns true.

My Observation: Please enter a number between 50 and 100.

validnum[i] = input.nextInt();
if (validnum[i] <= 101 || validnum[i] >= 49) {
}

Whether you are enter between 50 to 100 or less than 50 or greater than 100 the if statement will always satisfied either one of the below condition

validnum[i] <= 101 
validnum[i] >= 49