我使用scan read的java程序不会停止执行

Question

I am a newbee in the Java world. I wrote this program which reads in string array... When I run it, it never stops?!! what shall I add /change to make it end scanning?

import java.util.*;

public class Ex21 {
public static void main(String[] args) {
    int i, n = 5;
    String c;
    ArrayList<String>words = new ArrayList<>();
    System.out.println("Enter multi strings: ");
    Scanner input = new Scanner(System.in);
    boolean loop = true;
    while(loop) {
        words.add(input.next());
        Collections.sort(words);
        System.out.println("The sorted list is: " + words);
    }
    }

}

Answer 1

A while loop by definition continues executing its body until its condition (in this case the variable loop ) evaluates to false . You never set loop to false in the body of the while -loop, hence the condition will never evaluate to false and the loop will never end.

Additionally, it seems like you want to sort a list of words entered by the user. I wouldn't advise calling Collections.sort on every iteration of the loop. Maybe look into using a data structure that keeps its elements sorted on its own, such as a TreeSet . Or, at least, only call the sort method once, directly after the loop.

Answer 2

while(condition) {
     /* do something */
}

means /* do something */ happens unless condition == false, in your case it alwats true, that is why it doesn't stop. So Java behaves ok in your case.

Answer 3

while(loop) with loop always having the value true in your program is a so called endless loop , as the name says it never ends and this is what you are experiencing.

To make a loop stop you have to set the value of loop to false if some condition is fulfilled or terminate the loop using the key word break .

A condition may be for example having a certain word witch lets your loop terminate when it is entered, something like "exit"

Here is an example how you could set loop to false

String word = input.next();
boolean loop = ! "exit".equalsIgnoreCase(word);
while (loop) {
    words.add(word);
    Collections.sort(words);
    System.out.println("The sorted list is: " + words);
    word = input.next();
    loop = ! "exit".equalsIgnoreCase(word);
}
System.out.println("Bye!");

Here is an other example how you could cancel the loop using break

while (true) {
    String word = input.next();
    if("exit".equalsIgnoreCase(word)) {
        break;
    }
    words.add(word);
    Collections.sort(words);
    System.out.println("The sorted list is: " + words);
}
System.out.println("Bye!");

Note that the word exit is banned from the set that your arraylist can contain, You can change the program though to have it saved too.