为什么我不能在while（java）循环中退出？

Question

I am new here and i got into trouble about my coding, i hope someone could help me break this loop, cause no matter what i type, it always in the loop.My code is as follow:

    boolean flag = true;
    while(flag)
    {
        System.out.println("Please enter the movie's title:");
        title = s.nextLine().trim();
        if(title.trim().equals(""))
        {
            System.out.println("Please do not enter an empty title!");
        }
        else 
        {
            for(Movie m: movies)
            {
                if(title.equals(m.getTitle().trim()))
                {
                    System.out.println("Title already exists!");
                }
                else
                {
                    m.setTitle(title);
                    flag = false;
                    break;
                }
            }
        }
    }

Answer 1

该中断将留给循环，但是您仍然需要再中断一次才能在while循环中退出。

Answer 2

You could use an little known feature of Java labeled breaks.

Example from the officials docs :

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

        int[][] arrayOfInts = { 
            { 32, 87, 3, 589 },
            { 12, 1076, 2000, 8 },
            { 622, 127, 77, 955 }
        };
        int searchfor = 12;

        int i;
        int j = 0;
        boolean foundIt = false;

    search:
        for (i = 0; i < arrayOfInts.length; i++) {
            for (j = 0; j < arrayOfInts[i].length;
                 j++) {
                if (arrayOfInts[i][j] == searchfor) {
                    foundIt = true;
                    break search;
                }
            }
        }

        if (foundIt) {
            System.out.println("Found " + searchfor + " at " + i + ", " + j);
        } else {
            System.out.println(searchfor + " not in the array");
        }
    }
}

So you basically have to put a label, eg, outer: before the actual loop and then you can use break outer; to break out of the outer loop instead of the inner loop. Note this also works with continue .

Answer 3

if(title.trim().equals(""))
    {
        System.out.println("Please do not enter an empty title!");
         flag=flase;
    }

and here also

 if(title.equals(m.getTitle().trim()))
            {System.out.println("Title already exists!"); 
flag=false;
break;

 }

Answer 4

There's a small logical problem. When you start entering the movie names, you are gonna stop input/entering the values at some point, right?. The program must also know that so add a condition check title.trim().equals("break")

boolean flag = true;
        while(flag)
        {
            System.out.println("Please enter the movie's title:");
            title = s.nextLine().trim();
            if(title.trim().equals(""))
            {
                System.out.println("Please do not enter an empty title!");
            }
            else if(!title.trim().equals("break")) 
            {
                for(Movie m: movies)
                {
                    if(title.equals(m.getTitle().trim()))
                    {
                        System.out.println("Title already exists!");
                    }
                    else
                    {
                        m.setTitle(title);
                        flag = false;
                        break;
                    }
                }
            }else{
                flag = false;
                break;
            }
   }

The above code should do the trick. Let me know if the problem still persists @Adam.

Answer 5

initially when you enter your first title there are no objects inside "movies" sequence which "for" loop can go through, thus I believe your code initially never enters the "for" loop and continues.

I recommend use a if(movies == null) statement before for loop to check for empty sequence.